From 25c241e0aaef2a7d0f4a7398d30cd5f08f368fad Mon Sep 17 00:00:00 2001
From: Ben Cochran <bc@squareup.com>
Date: Wed, 4 Mar 2020 10:25:15 -0800
Subject: [PATCH 1/2] Add ViewEnvironment (#739)

ViewEnvironment is a container of strongly-typed key/value pairs that can be
passed down the view-side of a workflow tree. This allows containers to provide
view-oriented hinting to their children as well as allows for view-side changes
to view-only environment without re-rendering the workflow tree (for example,
runtime theme changes).
---
 .../Sources/BackStackContainer.swift          | 10 +-
 .../Sources/BackStackScreen.swift             |  4 +-
 .../Sources/ScreenWrapperViewController.swift | 10 +-
 .../Sources/CrossFadeContainer.swift          | 16 +--
 .../SampleApp/Sources/DemoScreen.swift        | 10 +-
 .../SampleApp/Sources/WelcomeScreen.swift     | 10 +-
 .../DemoApp/BarScreen.swift                   | 12 +--
 .../DemoApp/FooScreen.swift                   | 12 +--
 ...itScreenContainerScreenSnapshotTests.swift | 15 +--
 .../Sources/Environment+SplitScreen.swift     | 41 ++++++++
 .../Sources/SplitScreenContainerScreen.swift  |  4 +-
 .../SplitScreenContainerViewController.swift  | 30 ++++--
 .../Authentication/LoadingScreen.swift        |  8 +-
 .../Sources/Authentication/LoginScreen.swift  |  2 +-
 .../Authentication/TwoFactorScreen.swift      | 10 +-
 .../Sources/Game/GamePlayScreen.swift         | 10 +-
 .../Sources/Game/NewGameScreen.swift          | 10 +-
 .../Sources/Welcome/WelcomeScreen.swift       | 10 +-
 .../Sources/Todo/List/TodoListScreen.swift    | 10 +-
 .../Sources/Welcome/WelcomeScreen.swift       | 10 +-
 .../Sources/Todo/Edit/TodoEditScreen.swift    | 10 +-
 .../Sources/Todo/List/TodoListScreen.swift    | 10 +-
 .../Sources/Welcome/WelcomeScreen.swift       | 10 +-
 .../Sources/Todo/Edit/TodoEditScreen.swift    | 10 +-
 .../Sources/Todo/List/TodoListScreen.swift    | 10 +-
 .../Sources/Welcome/WelcomeScreen.swift       | 10 +-
 .../Sources/Todo/Edit/TodoEditScreen.swift    | 10 +-
 .../Sources/Todo/List/TodoListScreen.swift    | 10 +-
 .../Sources/Welcome/WelcomeScreen.swift       | 10 +-
 .../Container/ContainerViewController.swift   | 24 +++--
 .../Sources/Screen/AnyScreen/AnyScreen.swift  | 11 ++-
 .../UntypedScreenViewController.swift         | 38 --------
 swift/WorkflowUI/Sources/Screen/Screen.swift  |  2 +-
 .../Sources/Screen/ScreenViewController.swift | 31 +++---
 .../ViewEnvironment/ViewEnvironment.swift     | 97 +++++++++++++++++++
 .../ViewEnvironment/ViewEnvironmentKey.swift  | 46 +++++++++
 .../DescribedViewController.swift             |  8 +-
 .../Tests/ContainerViewControllerTests.swift  |  8 +-
 .../ViewControllerDescriptionTests.swift      |  6 +-
 39 files changed, 393 insertions(+), 212 deletions(-)
 create mode 100644 swift/Samples/SplitScreenContainer/Sources/Environment+SplitScreen.swift
 delete mode 100644 swift/WorkflowUI/Sources/Screen/AnyScreen/UntypedScreenViewController.swift
 create mode 100644 swift/WorkflowUI/Sources/Screen/ViewEnvironment/ViewEnvironment.swift
 create mode 100644 swift/WorkflowUI/Sources/Screen/ViewEnvironment/ViewEnvironmentKey.swift

diff --git a/swift/Samples/BackStackContainer/Sources/BackStackContainer.swift b/swift/Samples/BackStackContainer/Sources/BackStackContainer.swift
index 67f25f319..3dbec5532 100644
--- a/swift/Samples/BackStackContainer/Sources/BackStackContainer.swift
+++ b/swift/Samples/BackStackContainer/Sources/BackStackContainer.swift
@@ -19,10 +19,10 @@ import WorkflowUI
 public final class BackStackContainer: ScreenViewController<BackStackScreen>, UINavigationControllerDelegate {
     private let navController: UINavigationController
 
-    public required init(screen: BackStackScreen) {
+    public required init(screen: BackStackScreen, environment: ViewEnvironment) {
         self.navController = UINavigationController()
 
-        super.init(screen: screen)
+        super.init(screen: screen, environment: environment)
 
         update(with: screen)
     }
@@ -42,7 +42,7 @@ public final class BackStackContainer: ScreenViewController<BackStackScreen>, UI
         navController.view.frame = view.bounds
     }
 
-    public override func screenDidChange(from previousScreen: BackStackScreen) {
+    public override func screenDidChange(from previousScreen: BackStackScreen, previousEnvironment: ViewEnvironment) {
         update(with: screen)
     }
 
@@ -63,10 +63,10 @@ public final class BackStackContainer: ScreenViewController<BackStackScreen>, UI
                 viewController.matches(item: item)
             }) {
                 let existingViewController = existingViewControllers.remove(at: idx)
-                existingViewController.update(item: item)
+                existingViewController.update(item: item, environment: environment)
                 updatedViewControllers.append(existingViewController)
             } else {
-                updatedViewControllers.append(ScreenWrapperViewController(item: item))
+                updatedViewControllers.append(ScreenWrapperViewController(item: item, environment: environment))
             }
 
         }
diff --git a/swift/Samples/BackStackContainer/Sources/BackStackScreen.swift b/swift/Samples/BackStackContainer/Sources/BackStackScreen.swift
index 95b33b13a..0c8f670a4 100644
--- a/swift/Samples/BackStackContainer/Sources/BackStackScreen.swift
+++ b/swift/Samples/BackStackContainer/Sources/BackStackScreen.swift
@@ -23,8 +23,8 @@ public struct BackStackScreen: Screen {
         self.items = items
     }
 
-    public var viewControllerDescription: ViewControllerDescription {
-        return BackStackContainer.description(for: self)
+    public func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return BackStackContainer.description(for: self, environment: environment)
     }
 }
 
diff --git a/swift/Samples/BackStackContainer/Sources/ScreenWrapperViewController.swift b/swift/Samples/BackStackContainer/Sources/ScreenWrapperViewController.swift
index 8eaeac4ae..ef1c707da 100644
--- a/swift/Samples/BackStackContainer/Sources/ScreenWrapperViewController.swift
+++ b/swift/Samples/BackStackContainer/Sources/ScreenWrapperViewController.swift
@@ -22,13 +22,15 @@ import WorkflowUI
 final class ScreenWrapperViewController: UIViewController {
     let key: AnyHashable
     let screenType: Any.Type
+    let environment: ViewEnvironment
 
     let contentViewController: DescribedViewController
 
-    init(item: BackStackScreen.Item) {
+    init(item: BackStackScreen.Item, environment: ViewEnvironment) {
         self.key = item.key
         self.screenType = item.screenType
-        self.contentViewController = DescribedViewController(screen: item.screen)
+        self.environment = environment
+        self.contentViewController = DescribedViewController(screen: item.screen, environment: environment)
 
         super.init(nibName: nil, bundle: nil)
 
@@ -51,8 +53,8 @@ final class ScreenWrapperViewController: UIViewController {
         contentViewController.view.frame = view.bounds
     }
 
-    func update(item: BackStackScreen.Item) {
-        contentViewController.update(screen: item.screen)
+    func update(item: BackStackScreen.Item, environment: ViewEnvironment) {
+        contentViewController.update(screen: item.screen, environment: environment)
         update(barVisibility: item.barVisibility)
     }
 
diff --git a/swift/Samples/SampleApp/Sources/CrossFadeContainer.swift b/swift/Samples/SampleApp/Sources/CrossFadeContainer.swift
index 6accd032e..b04d0f150 100644
--- a/swift/Samples/SampleApp/Sources/CrossFadeContainer.swift
+++ b/swift/Samples/SampleApp/Sources/CrossFadeContainer.swift
@@ -39,8 +39,8 @@ struct CrossFadeScreen: Screen {
         return self.key == otherScreen.key
     }
 
-    var viewControllerDescription: ViewControllerDescription {
-        return CrossFadeContainerViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return CrossFadeContainerViewController.description(for: self, environment: environment)
     }
 }
 
@@ -48,9 +48,9 @@ struct CrossFadeScreen: Screen {
 fileprivate final class CrossFadeContainerViewController: ScreenViewController<CrossFadeScreen> {
     var childViewController: DescribedViewController
 
-    required init(screen: CrossFadeScreen) {
-        childViewController = DescribedViewController(screen: screen.baseScreen)
-        super.init(screen: screen)
+    required init(screen: CrossFadeScreen, environment: ViewEnvironment) {
+        childViewController = DescribedViewController(screen: screen.baseScreen, environment: environment)
+        super.init(screen: screen, environment: environment)
     }
 
     override func viewDidLoad() {
@@ -67,13 +67,13 @@ fileprivate final class CrossFadeContainerViewController: ScreenViewController<C
         childViewController.view.frame = view.bounds
     }
 
-    override func screenDidChange(from previousScreen: CrossFadeScreen) {
+    override func screenDidChange(from previousScreen: CrossFadeScreen, previousEnvironment: ViewEnvironment) {
         if screen.isEquivalent(to: previousScreen) {
-            childViewController.update(screen: screen.baseScreen)
+            childViewController.update(screen: screen.baseScreen, environment: environment)
         } else {
             // The new screen is different than the previous. Animate the transition.
             let oldChild = childViewController
-            childViewController = DescribedViewController(screen: screen.baseScreen)
+            childViewController = DescribedViewController(screen: screen.baseScreen, environment: environment)
             addChild(childViewController)
             view.addSubview(childViewController.view)
             UIView.transition(
diff --git a/swift/Samples/SampleApp/Sources/DemoScreen.swift b/swift/Samples/SampleApp/Sources/DemoScreen.swift
index 074a8b867..e140c3fba 100644
--- a/swift/Samples/SampleApp/Sources/DemoScreen.swift
+++ b/swift/Samples/SampleApp/Sources/DemoScreen.swift
@@ -29,8 +29,8 @@ struct DemoScreen: Screen {
     let isRefreshEnabled: Bool
     let onRefreshTap: () -> Void
 
-    var viewControllerDescription: ViewControllerDescription {
-        return DemoViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return DemoViewController.description(for: self, environment: environment)
     }
 }
 
@@ -42,12 +42,12 @@ fileprivate final class DemoViewController: ScreenViewController<DemoScreen> {
     private let statusLabel: UILabel
     private let refreshButton: UIButton
 
-    required init(screen: DemoScreen) {
+    required init(screen: DemoScreen, environment: ViewEnvironment) {
         titleButton = UIButton(frame: .zero)
         subscribeButton = UIButton(frame: .zero)
         statusLabel = UILabel(frame: .zero)
         refreshButton = UIButton(frame: .zero)
-        super.init(screen: screen)
+        super.init(screen: screen, environment: environment)
 
         update(with: screen)
     }
@@ -106,7 +106,7 @@ fileprivate final class DemoViewController: ScreenViewController<DemoScreen> {
             height: height)
     }
 
-    override func screenDidChange(from previousScreen: DemoScreen) {
+    override func screenDidChange(from previousScreen: DemoScreen, previousEnvironment: ViewEnvironment) {
         update(with: screen)
     }
 
diff --git a/swift/Samples/SampleApp/Sources/WelcomeScreen.swift b/swift/Samples/SampleApp/Sources/WelcomeScreen.swift
index 933de898b..310a017ca 100644
--- a/swift/Samples/SampleApp/Sources/WelcomeScreen.swift
+++ b/swift/Samples/SampleApp/Sources/WelcomeScreen.swift
@@ -22,8 +22,8 @@ struct WelcomeScreen: Screen {
     var onNameChanged: (String) -> Void
     var onLoginTapped: () -> Void
 
-    var viewControllerDescription: ViewControllerDescription {
-        return WelcomeViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return WelcomeViewController.description(for: self, environment: environment)
     }
 }
 
@@ -33,11 +33,11 @@ fileprivate final class WelcomeViewController: ScreenViewController<WelcomeScree
     let nameField: UITextField
     let button: UIButton
 
-    required init(screen: WelcomeScreen) {
+    required init(screen: WelcomeScreen, environment: ViewEnvironment) {
         welcomeLabel = UILabel(frame: .zero)
         nameField = UITextField(frame: .zero)
         button = UIButton(frame: .zero)
-        super.init(screen: screen)
+        super.init(screen: screen, environment: environment)
 
         update(with: screen)
     }
@@ -89,7 +89,7 @@ fileprivate final class WelcomeViewController: ScreenViewController<WelcomeScree
             .insetBy(dx: inset, dy: 0.0)
     }
 
-    override func screenDidChange(from previousScreen: WelcomeScreen) {
+    override func screenDidChange(from previousScreen: WelcomeScreen, previousEnvironment: ViewEnvironment) {
         update(with: screen)
     }
 
diff --git a/swift/Samples/SplitScreenContainer/DemoApp/BarScreen.swift b/swift/Samples/SplitScreenContainer/DemoApp/BarScreen.swift
index 7d9bd2c08..59f81d50c 100644
--- a/swift/Samples/SplitScreenContainer/DemoApp/BarScreen.swift
+++ b/swift/Samples/SplitScreenContainer/DemoApp/BarScreen.swift
@@ -22,8 +22,8 @@ struct BarScreen: Screen {
     let backgroundColors: [UIColor]
     let viewTapped: () -> Void
 
-    var viewControllerDescription: ViewControllerDescription {
-        return BarScreenViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return BarScreenViewController.description(for: self, environment: environment)
     }
 }
 
@@ -34,8 +34,8 @@ fileprivate final class BarScreenViewController: ScreenViewController<BarScreen>
     private lazy var tapGestureRecognizer: UITapGestureRecognizer = .init()
     private var gradientLayer: CAGradientLayer?
     
-    required init(screen: BarScreen) {
-        super.init(screen: screen)
+    required init(screen: BarScreen, environment: ViewEnvironment) {
+        super.init(screen: screen, environment: environment)
         
         update(with: screen)
     }
@@ -58,8 +58,8 @@ fileprivate final class BarScreenViewController: ScreenViewController<BarScreen>
         
         updateGradient(for: view, colors: screen.backgroundColors)
     }
-    
-    override func screenDidChange(from previousScreen: BarScreen) {
+
+    override func screenDidChange(from previousScreen: BarScreen, previousEnvironment: ViewEnvironment) {
         update(with: screen)
     }
     
diff --git a/swift/Samples/SplitScreenContainer/DemoApp/FooScreen.swift b/swift/Samples/SplitScreenContainer/DemoApp/FooScreen.swift
index 370378f1d..3d2e62152 100644
--- a/swift/Samples/SplitScreenContainer/DemoApp/FooScreen.swift
+++ b/swift/Samples/SplitScreenContainer/DemoApp/FooScreen.swift
@@ -22,8 +22,8 @@ struct FooScreen: Screen {
     let backgroundColor: UIColor
     let viewTapped: () -> Void
 
-    var viewControllerDescription: ViewControllerDescription {
-        return FooScreenViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return FooScreenViewController.description(for: self, environment: environment)
     }
 }
 
@@ -33,8 +33,8 @@ fileprivate final class FooScreenViewController: ScreenViewController<FooScreen>
     private lazy var titleLabel: UILabel = .init()
     private lazy var tapGestureRecognizer: UITapGestureRecognizer = .init()
     
-    required init(screen: FooScreen) {
-        super.init(screen: screen)
+    required init(screen: FooScreen, environment: ViewEnvironment) {
+        super.init(screen: screen, environment: environment)
         
         update(with: screen)
     }
@@ -54,8 +54,8 @@ fileprivate final class FooScreenViewController: ScreenViewController<FooScreen>
             titleLabel.centerYAnchor.constraint(equalTo: view.centerYAnchor),
         ])
     }
-    
-    override func screenDidChange(from previousScreen: FooScreen) {
+
+    override func screenDidChange(from previousScreen: FooScreen, previousEnvironment: ViewEnvironment) {
         update(with: screen)
     }
     
diff --git a/swift/Samples/SplitScreenContainer/SnapshotTests/SplitScreenContainerScreenSnapshotTests.swift b/swift/Samples/SplitScreenContainer/SnapshotTests/SplitScreenContainerScreenSnapshotTests.swift
index 61d24aa97..53d0cabe7 100644
--- a/swift/Samples/SplitScreenContainer/SnapshotTests/SplitScreenContainerScreenSnapshotTests.swift
+++ b/swift/Samples/SplitScreenContainer/SnapshotTests/SplitScreenContainerScreenSnapshotTests.swift
@@ -29,7 +29,8 @@ class SplitScreenContainerScreenSnapshotTests: FBSnapshotTestCase {
             )
 
             let viewController = SplitScreenContainerViewController(
-                screen: splitScreenContainerScreen
+                screen: splitScreenContainerScreen,
+                environment: .empty
             )
             viewController.view.layoutIfNeeded()
 
@@ -44,8 +45,8 @@ fileprivate struct FooScreen: Screen {
     let backgroundColor: UIColor
     let viewTapped: () -> Void
 
-    var viewControllerDescription: ViewControllerDescription {
-        return FooScreenViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return FooScreenViewController.description(for: self, environment: environment)
     }
 }
 
@@ -55,8 +56,8 @@ fileprivate final class FooScreenViewController: ScreenViewController<FooScreen>
     private lazy var titleLabel: UILabel = .init()
     private lazy var tapGestureRecognizer: UITapGestureRecognizer = .init()
     
-    required init(screen: FooScreen) {
-        super.init(screen: screen)
+    required init(screen: FooScreen, environment: ViewEnvironment) {
+        super.init(screen: screen, environment: environment)
         
         update(with: screen)
     }
@@ -76,8 +77,8 @@ fileprivate final class FooScreenViewController: ScreenViewController<FooScreen>
             titleLabel.centerYAnchor.constraint(equalTo: view.centerYAnchor),
             ])
     }
-    
-    override func screenDidChange(from previousScreen: FooScreen) {
+
+    override func screenDidChange(from previousScreen: FooScreen, previousEnvironment: ViewEnvironment) {
         update(with: screen)
     }
     
diff --git a/swift/Samples/SplitScreenContainer/Sources/Environment+SplitScreen.swift b/swift/Samples/SplitScreenContainer/Sources/Environment+SplitScreen.swift
new file mode 100644
index 000000000..d489b840a
--- /dev/null
+++ b/swift/Samples/SplitScreenContainer/Sources/Environment+SplitScreen.swift
@@ -0,0 +1,41 @@
+/*
+* Copyright 2020 Square Inc.
+*
+* Licensed under the Apache License, Version 2.0 (the "License");
+* you may not use this file except in compliance with the License.
+* You may obtain a copy of the License at
+*
+*     http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+import WorkflowUI
+
+public enum SplitScreenPosition {
+    /// Not appearing in a split screen context
+    case none
+
+    /// Appearing in the leading position in a split screen
+    case leading
+
+    /// Appearing in the trailing position in a split screen
+    case trailing
+}
+
+extension ViewEnvironment {
+
+    internal(set) public var splitScreenPosition: SplitScreenPosition {
+        get { return self[SplitScreenPositionKey.self] }
+        set { self[SplitScreenPositionKey.self] = newValue }
+    }
+
+}
+
+private enum SplitScreenPositionKey: ViewEnvironmentKey {
+    static var defaultValue: SplitScreenPosition = .none
+}
diff --git a/swift/Samples/SplitScreenContainer/Sources/SplitScreenContainerScreen.swift b/swift/Samples/SplitScreenContainer/Sources/SplitScreenContainerScreen.swift
index 511f86d14..5f5b765e6 100644
--- a/swift/Samples/SplitScreenContainer/Sources/SplitScreenContainerScreen.swift
+++ b/swift/Samples/SplitScreenContainer/Sources/SplitScreenContainerScreen.swift
@@ -48,8 +48,8 @@ public struct SplitScreenContainerScreen<LeadingScreenType: Screen, TrailingScre
         self.separatorWidth = separatorWidth
     }
 
-    public var viewControllerDescription: ViewControllerDescription {
-        return SplitScreenContainerViewController.description(for: self)
+    public func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return SplitScreenContainerViewController.description(for: self, environment: environment)
     }
 
 }
diff --git a/swift/Samples/SplitScreenContainer/Sources/SplitScreenContainerViewController.swift b/swift/Samples/SplitScreenContainer/Sources/SplitScreenContainerViewController.swift
index d3798700e..725f98e25 100644
--- a/swift/Samples/SplitScreenContainer/Sources/SplitScreenContainerViewController.swift
+++ b/swift/Samples/SplitScreenContainer/Sources/SplitScreenContainerViewController.swift
@@ -29,13 +29,21 @@ internal final class SplitScreenContainerViewController<LeadingScreenType: Scree
     
     private var needsAnimatedLayout = false
 
-    required init(screen: ContainerScreen) {
-        leadingContentViewController = DescribedViewController(screen: screen.leadingScreen)
-        trailingContentViewController = DescribedViewController(screen: screen.trailingScreen)
-        super.init(screen: screen)
+    required init(screen: ContainerScreen, environment: ViewEnvironment) {
+        leadingContentViewController = DescribedViewController(
+            screen: screen.leadingScreen,
+            environment: environment
+                .setting(keyPath: \.splitScreenPosition, to: .leading)
+        )
+        trailingContentViewController = DescribedViewController(
+            screen: screen.trailingScreen,
+            environment: environment
+                .setting(keyPath: \.splitScreenPosition, to: .trailing)
+        )
+        super.init(screen: screen, environment: environment)
     }
 
-    override internal func screenDidChange(from previousScreen: ContainerScreen) {
+    override internal func screenDidChange(from previousScreen: ContainerScreen, previousEnvironment: ViewEnvironment) {
         if screen.ratio != previousScreen.ratio {
             needsAnimatedLayout = true
         }
@@ -48,8 +56,16 @@ internal final class SplitScreenContainerViewController<LeadingScreenType: Scree
     private func update(with screen: ContainerScreen) {
         separatorView.backgroundColor = screen.separatorColor
         
-        leadingContentViewController.update(screen: screen.leadingScreen)
-        trailingContentViewController.update(screen: screen.trailingScreen)
+        leadingContentViewController.update(
+            screen: screen.leadingScreen,
+            environment: environment
+                .setting(keyPath: \.splitScreenPosition, to: .leading)
+        )
+        trailingContentViewController.update(
+            screen: screen.trailingScreen,
+            environment: environment
+                .setting(keyPath: \.splitScreenPosition, to: .trailing)
+        )
         
         //Intentional force of layout pass after updating the child view controllers
         view.layoutIfNeeded()
diff --git a/swift/Samples/TicTacToe/Sources/Authentication/LoadingScreen.swift b/swift/Samples/TicTacToe/Sources/Authentication/LoadingScreen.swift
index 175da7a0c..68ceed15d 100644
--- a/swift/Samples/TicTacToe/Sources/Authentication/LoadingScreen.swift
+++ b/swift/Samples/TicTacToe/Sources/Authentication/LoadingScreen.swift
@@ -17,8 +17,8 @@ import WorkflowUI
 
 
 struct LoadingScreen: Screen {
-    var viewControllerDescription: ViewControllerDescription {
-        return LoadingScreenViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return LoadingScreenViewController.description(for: self, environment: environment)
     }
 }
 
@@ -26,10 +26,10 @@ struct LoadingScreen: Screen {
 fileprivate final class LoadingScreenViewController: ScreenViewController<LoadingScreen> {
     let loadingLabel: UILabel
 
-    required init(screen: LoadingScreen) {
+    required init(screen: LoadingScreen, environment: ViewEnvironment) {
         self.loadingLabel = UILabel(frame: .zero)
 
-        super.init(screen: screen)
+        super.init(screen: screen, environment: environment)
     }
 
     override func viewDidLoad() {
diff --git a/swift/Samples/TicTacToe/Sources/Authentication/LoginScreen.swift b/swift/Samples/TicTacToe/Sources/Authentication/LoginScreen.swift
index 62cddbfd4..07c0a9cb0 100644
--- a/swift/Samples/TicTacToe/Sources/Authentication/LoginScreen.swift
+++ b/swift/Samples/TicTacToe/Sources/Authentication/LoginScreen.swift
@@ -25,7 +25,7 @@ struct LoginScreen: Screen {
     var onPasswordChanged: (String) -> Void
     var onLoginTapped: () -> Void
 
-    var viewControllerDescription: ViewControllerDescription {
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
         return ViewControllerDescription(
             build: { LoginViewController() },
             update: { $0.update(with: self) })
diff --git a/swift/Samples/TicTacToe/Sources/Authentication/TwoFactorScreen.swift b/swift/Samples/TicTacToe/Sources/Authentication/TwoFactorScreen.swift
index 1156dc383..f09b9724a 100644
--- a/swift/Samples/TicTacToe/Sources/Authentication/TwoFactorScreen.swift
+++ b/swift/Samples/TicTacToe/Sources/Authentication/TwoFactorScreen.swift
@@ -20,8 +20,8 @@ struct TwoFactorScreen: Screen {
     var title: String
     var onLoginTapped: (String) -> Void
 
-    var viewControllerDescription: ViewControllerDescription {
-        return TwoFactorViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return TwoFactorViewController.description(for: self, environment: environment)
     }
 }
 
@@ -31,12 +31,12 @@ fileprivate final class TwoFactorViewController: ScreenViewController<TwoFactorS
     let twoFactorField: UITextField
     let button: UIButton
 
-    required init(screen: TwoFactorScreen) {
+    required init(screen: TwoFactorScreen, environment: ViewEnvironment) {
         self.titleLabel = UILabel(frame: .zero)
         self.twoFactorField = UITextField(frame: .zero)
         self.button = UIButton(frame: .zero)
 
-        super.init(screen: screen)
+        super.init(screen: screen, environment: environment)
 
         update(with: screen)
     }
@@ -89,7 +89,7 @@ fileprivate final class TwoFactorViewController: ScreenViewController<TwoFactorS
             .insetBy(dx: inset, dy: 0.0)
     }
 
-    override func screenDidChange(from previousScreen: TwoFactorScreen) {
+    override func screenDidChange(from previousScreen: TwoFactorScreen, previousEnvironment: ViewEnvironment) {
         update(with: screen)
     }
 
diff --git a/swift/Samples/TicTacToe/Sources/Game/GamePlayScreen.swift b/swift/Samples/TicTacToe/Sources/Game/GamePlayScreen.swift
index 436eadb1e..f0dd42ff4 100644
--- a/swift/Samples/TicTacToe/Sources/Game/GamePlayScreen.swift
+++ b/swift/Samples/TicTacToe/Sources/Game/GamePlayScreen.swift
@@ -23,8 +23,8 @@ struct GamePlayScreen: Screen {
     var board: [[Board.Cell]]
     var onSelected: (Int, Int) -> Void
 
-    var viewControllerDescription: ViewControllerDescription {
-        return GamePlayViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return GamePlayViewController.description(for: self, environment: environment)
     }
 }
 
@@ -33,7 +33,7 @@ final class GamePlayViewController: ScreenViewController<GamePlayScreen> {
     let titleLabel: UILabel
     let cells: [[UIButton]]
 
-    required init(screen: GamePlayScreen) {
+    required init(screen: GamePlayScreen, environment: ViewEnvironment) {
         self.titleLabel = UILabel(frame: .zero)
         var cells: [[UIButton]] = []
 
@@ -46,7 +46,7 @@ final class GamePlayViewController: ScreenViewController<GamePlayScreen> {
         }
 
         self.cells = cells
-        super.init(screen: screen)
+        super.init(screen: screen, environment: environment)
         update(with: screen)
     }
 
@@ -106,7 +106,7 @@ final class GamePlayViewController: ScreenViewController<GamePlayScreen> {
         }
     }
 
-    override func screenDidChange(from previousScreen: GamePlayScreen) {
+    override func screenDidChange(from previousScreen: GamePlayScreen, previousEnvironment: ViewEnvironment) {
         update(with: screen)
     }
 
diff --git a/swift/Samples/TicTacToe/Sources/Game/NewGameScreen.swift b/swift/Samples/TicTacToe/Sources/Game/NewGameScreen.swift
index 0533dd6cf..3da552b94 100644
--- a/swift/Samples/TicTacToe/Sources/Game/NewGameScreen.swift
+++ b/swift/Samples/TicTacToe/Sources/Game/NewGameScreen.swift
@@ -27,8 +27,8 @@ struct NewGameScreen: Screen {
         case startGame
     }
 
-    var viewControllerDescription: ViewControllerDescription {
-        return NewGameViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return NewGameViewController.description(for: self, environment: environment)
     }
 }
 
@@ -40,14 +40,14 @@ final class NewGameViewController: ScreenViewController<NewGameScreen> {
     let playerOField: UITextField
     let startGameButton: UIButton
 
-    required init(screen: NewGameScreen) {
+    required init(screen: NewGameScreen, environment: ViewEnvironment) {
         self.playerXLabel = UILabel(frame: .zero)
         self.playerXField = UITextField(frame: .zero)
         self.playerOLabel = UILabel(frame: .zero)
         self.playerOField = UITextField(frame: .zero)
         self.startGameButton = UIButton(frame: .zero)
 
-        super.init(screen: screen)
+        super.init(screen: screen, environment: environment)
         update(with: screen)
     }
 
@@ -129,7 +129,7 @@ final class NewGameViewController: ScreenViewController<NewGameScreen> {
 
     }
 
-    override func screenDidChange(from previousScreen: NewGameScreen) {
+    override func screenDidChange(from previousScreen: NewGameScreen, previousEnvironment: ViewEnvironment) {
         update(with: screen)
     }
 
diff --git a/swift/Samples/Tutorial/Frameworks/Tutorial1Complete/Sources/Welcome/WelcomeScreen.swift b/swift/Samples/Tutorial/Frameworks/Tutorial1Complete/Sources/Welcome/WelcomeScreen.swift
index faf89c3d1..a57f71373 100644
--- a/swift/Samples/Tutorial/Frameworks/Tutorial1Complete/Sources/Welcome/WelcomeScreen.swift
+++ b/swift/Samples/Tutorial/Frameworks/Tutorial1Complete/Sources/Welcome/WelcomeScreen.swift
@@ -26,8 +26,8 @@ struct WelcomeScreen: Screen {
     /// Callback when the login button is tapped.
     var onLoginTapped: () -> Void
 
-    var viewControllerDescription: ViewControllerDescription {
-        return WelcomeViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return WelcomeViewController.description(for: self, environment: environment)
     }
 }
 
@@ -36,9 +36,9 @@ final class WelcomeViewController: ScreenViewController<WelcomeScreen> {
 
     var welcomeView: WelcomeView
 
-    required init(screen: WelcomeScreen) {
+    required init(screen: WelcomeScreen, environment: ViewEnvironment) {
         self.welcomeView = WelcomeView(frame: .zero)
-        super.init(screen: screen)
+        super.init(screen: screen, environment: environment)
         update(with: screen)
     }
 
@@ -54,7 +54,7 @@ final class WelcomeViewController: ScreenViewController<WelcomeScreen> {
         welcomeView.frame = view.bounds.inset(by: view.safeAreaInsets)
     }
 
-    override func screenDidChange(from previousScreen: WelcomeScreen) {
+    override func screenDidChange(from previousScreen: WelcomeScreen, previousEnvironment: ViewEnvironment) {
         update(with: screen)
     }
 
diff --git a/swift/Samples/Tutorial/Frameworks/Tutorial2Complete/Sources/Todo/List/TodoListScreen.swift b/swift/Samples/Tutorial/Frameworks/Tutorial2Complete/Sources/Todo/List/TodoListScreen.swift
index a9ec59408..fc519745f 100644
--- a/swift/Samples/Tutorial/Frameworks/Tutorial2Complete/Sources/Todo/List/TodoListScreen.swift
+++ b/swift/Samples/Tutorial/Frameworks/Tutorial2Complete/Sources/Todo/List/TodoListScreen.swift
@@ -25,8 +25,8 @@ struct TodoListScreen: Screen {
     // Callback when a todo is selected
     var onTodoSelected: (Int) -> Void
 
-    var viewControllerDescription: ViewControllerDescription {
-        return TodoListViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return TodoListViewController.description(for: self, environment: environment)
     }
 }
 
@@ -34,9 +34,9 @@ struct TodoListScreen: Screen {
 final class TodoListViewController: ScreenViewController<TodoListScreen> {
     let todoListView: TodoListView
 
-    required init(screen: TodoListScreen) {
+    required init(screen: TodoListScreen, environment: ViewEnvironment) {
         self.todoListView = TodoListView(frame: .zero)
-        super.init(screen: screen)
+        super.init(screen: screen, environment: environment)
         update(with: screen)
     }
 
@@ -52,7 +52,7 @@ final class TodoListViewController: ScreenViewController<TodoListScreen> {
         todoListView.frame = view.bounds.inset(by: view.safeAreaInsets)
     }
 
-    override func screenDidChange(from previousScreen: TodoListScreen) {
+    override func screenDidChange(from previousScreen: TodoListScreen, previousEnvironment: ViewEnvironment) {
         update(with: screen)
     }
 
diff --git a/swift/Samples/Tutorial/Frameworks/Tutorial2Complete/Sources/Welcome/WelcomeScreen.swift b/swift/Samples/Tutorial/Frameworks/Tutorial2Complete/Sources/Welcome/WelcomeScreen.swift
index faf89c3d1..a57f71373 100644
--- a/swift/Samples/Tutorial/Frameworks/Tutorial2Complete/Sources/Welcome/WelcomeScreen.swift
+++ b/swift/Samples/Tutorial/Frameworks/Tutorial2Complete/Sources/Welcome/WelcomeScreen.swift
@@ -26,8 +26,8 @@ struct WelcomeScreen: Screen {
     /// Callback when the login button is tapped.
     var onLoginTapped: () -> Void
 
-    var viewControllerDescription: ViewControllerDescription {
-        return WelcomeViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return WelcomeViewController.description(for: self, environment: environment)
     }
 }
 
@@ -36,9 +36,9 @@ final class WelcomeViewController: ScreenViewController<WelcomeScreen> {
 
     var welcomeView: WelcomeView
 
-    required init(screen: WelcomeScreen) {
+    required init(screen: WelcomeScreen, environment: ViewEnvironment) {
         self.welcomeView = WelcomeView(frame: .zero)
-        super.init(screen: screen)
+        super.init(screen: screen, environment: environment)
         update(with: screen)
     }
 
@@ -54,7 +54,7 @@ final class WelcomeViewController: ScreenViewController<WelcomeScreen> {
         welcomeView.frame = view.bounds.inset(by: view.safeAreaInsets)
     }
 
-    override func screenDidChange(from previousScreen: WelcomeScreen) {
+    override func screenDidChange(from previousScreen: WelcomeScreen, previousEnvironment: ViewEnvironment) {
         update(with: screen)
     }
 
diff --git a/swift/Samples/Tutorial/Frameworks/Tutorial3Complete/Sources/Todo/Edit/TodoEditScreen.swift b/swift/Samples/Tutorial/Frameworks/Tutorial3Complete/Sources/Todo/Edit/TodoEditScreen.swift
index c36d013c2..7d5ab6cef 100644
--- a/swift/Samples/Tutorial/Frameworks/Tutorial3Complete/Sources/Todo/Edit/TodoEditScreen.swift
+++ b/swift/Samples/Tutorial/Frameworks/Tutorial3Complete/Sources/Todo/Edit/TodoEditScreen.swift
@@ -28,8 +28,8 @@ struct TodoEditScreen: Screen {
     var onTitleChanged: (String) -> Void
     var onNoteChanged: (String) -> Void
 
-    var viewControllerDescription: ViewControllerDescription {
-        return TodoEditViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return TodoEditViewController.description(for: self, environment: environment)
     }
 }
 
@@ -38,10 +38,10 @@ final class TodoEditViewController: ScreenViewController<TodoEditScreen> {
     // The `todoEditView` has all the logic for displaying the todo and editing.
     let todoEditView: TodoEditView
 
-    required init(screen: TodoEditScreen) {
+    required init(screen: TodoEditScreen, environment: ViewEnvironment) {
         self.todoEditView = TodoEditView(frame: .zero)
 
-        super.init(screen: screen)
+        super.init(screen: screen, environment: environment)
         update(with: screen)
     }
 
@@ -57,7 +57,7 @@ final class TodoEditViewController: ScreenViewController<TodoEditScreen> {
         todoEditView.frame = view.bounds.inset(by: view.safeAreaInsets)
     }
 
-    override func screenDidChange(from previousScreen: TodoEditScreen) {
+    override func screenDidChange(from previousScreen: TodoEditScreen, previousEnvironment: ViewEnvironment) {
         update(with: screen)
     }
 
diff --git a/swift/Samples/Tutorial/Frameworks/Tutorial3Complete/Sources/Todo/List/TodoListScreen.swift b/swift/Samples/Tutorial/Frameworks/Tutorial3Complete/Sources/Todo/List/TodoListScreen.swift
index a9ec59408..fc519745f 100644
--- a/swift/Samples/Tutorial/Frameworks/Tutorial3Complete/Sources/Todo/List/TodoListScreen.swift
+++ b/swift/Samples/Tutorial/Frameworks/Tutorial3Complete/Sources/Todo/List/TodoListScreen.swift
@@ -25,8 +25,8 @@ struct TodoListScreen: Screen {
     // Callback when a todo is selected
     var onTodoSelected: (Int) -> Void
 
-    var viewControllerDescription: ViewControllerDescription {
-        return TodoListViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return TodoListViewController.description(for: self, environment: environment)
     }
 }
 
@@ -34,9 +34,9 @@ struct TodoListScreen: Screen {
 final class TodoListViewController: ScreenViewController<TodoListScreen> {
     let todoListView: TodoListView
 
-    required init(screen: TodoListScreen) {
+    required init(screen: TodoListScreen, environment: ViewEnvironment) {
         self.todoListView = TodoListView(frame: .zero)
-        super.init(screen: screen)
+        super.init(screen: screen, environment: environment)
         update(with: screen)
     }
 
@@ -52,7 +52,7 @@ final class TodoListViewController: ScreenViewController<TodoListScreen> {
         todoListView.frame = view.bounds.inset(by: view.safeAreaInsets)
     }
 
-    override func screenDidChange(from previousScreen: TodoListScreen) {
+    override func screenDidChange(from previousScreen: TodoListScreen, previousEnvironment: ViewEnvironment) {
         update(with: screen)
     }
 
diff --git a/swift/Samples/Tutorial/Frameworks/Tutorial3Complete/Sources/Welcome/WelcomeScreen.swift b/swift/Samples/Tutorial/Frameworks/Tutorial3Complete/Sources/Welcome/WelcomeScreen.swift
index faf89c3d1..a57f71373 100644
--- a/swift/Samples/Tutorial/Frameworks/Tutorial3Complete/Sources/Welcome/WelcomeScreen.swift
+++ b/swift/Samples/Tutorial/Frameworks/Tutorial3Complete/Sources/Welcome/WelcomeScreen.swift
@@ -26,8 +26,8 @@ struct WelcomeScreen: Screen {
     /// Callback when the login button is tapped.
     var onLoginTapped: () -> Void
 
-    var viewControllerDescription: ViewControllerDescription {
-        return WelcomeViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return WelcomeViewController.description(for: self, environment: environment)
     }
 }
 
@@ -36,9 +36,9 @@ final class WelcomeViewController: ScreenViewController<WelcomeScreen> {
 
     var welcomeView: WelcomeView
 
-    required init(screen: WelcomeScreen) {
+    required init(screen: WelcomeScreen, environment: ViewEnvironment) {
         self.welcomeView = WelcomeView(frame: .zero)
-        super.init(screen: screen)
+        super.init(screen: screen, environment: environment)
         update(with: screen)
     }
 
@@ -54,7 +54,7 @@ final class WelcomeViewController: ScreenViewController<WelcomeScreen> {
         welcomeView.frame = view.bounds.inset(by: view.safeAreaInsets)
     }
 
-    override func screenDidChange(from previousScreen: WelcomeScreen) {
+    override func screenDidChange(from previousScreen: WelcomeScreen, previousEnvironment: ViewEnvironment) {
         update(with: screen)
     }
 
diff --git a/swift/Samples/Tutorial/Frameworks/Tutorial4Complete/Sources/Todo/Edit/TodoEditScreen.swift b/swift/Samples/Tutorial/Frameworks/Tutorial4Complete/Sources/Todo/Edit/TodoEditScreen.swift
index c36d013c2..7d5ab6cef 100644
--- a/swift/Samples/Tutorial/Frameworks/Tutorial4Complete/Sources/Todo/Edit/TodoEditScreen.swift
+++ b/swift/Samples/Tutorial/Frameworks/Tutorial4Complete/Sources/Todo/Edit/TodoEditScreen.swift
@@ -28,8 +28,8 @@ struct TodoEditScreen: Screen {
     var onTitleChanged: (String) -> Void
     var onNoteChanged: (String) -> Void
 
-    var viewControllerDescription: ViewControllerDescription {
-        return TodoEditViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return TodoEditViewController.description(for: self, environment: environment)
     }
 }
 
@@ -38,10 +38,10 @@ final class TodoEditViewController: ScreenViewController<TodoEditScreen> {
     // The `todoEditView` has all the logic for displaying the todo and editing.
     let todoEditView: TodoEditView
 
-    required init(screen: TodoEditScreen) {
+    required init(screen: TodoEditScreen, environment: ViewEnvironment) {
         self.todoEditView = TodoEditView(frame: .zero)
 
-        super.init(screen: screen)
+        super.init(screen: screen, environment: environment)
         update(with: screen)
     }
 
@@ -57,7 +57,7 @@ final class TodoEditViewController: ScreenViewController<TodoEditScreen> {
         todoEditView.frame = view.bounds.inset(by: view.safeAreaInsets)
     }
 
-    override func screenDidChange(from previousScreen: TodoEditScreen) {
+    override func screenDidChange(from previousScreen: TodoEditScreen, previousEnvironment: ViewEnvironment) {
         update(with: screen)
     }
 
diff --git a/swift/Samples/Tutorial/Frameworks/Tutorial4Complete/Sources/Todo/List/TodoListScreen.swift b/swift/Samples/Tutorial/Frameworks/Tutorial4Complete/Sources/Todo/List/TodoListScreen.swift
index a9ec59408..fc519745f 100644
--- a/swift/Samples/Tutorial/Frameworks/Tutorial4Complete/Sources/Todo/List/TodoListScreen.swift
+++ b/swift/Samples/Tutorial/Frameworks/Tutorial4Complete/Sources/Todo/List/TodoListScreen.swift
@@ -25,8 +25,8 @@ struct TodoListScreen: Screen {
     // Callback when a todo is selected
     var onTodoSelected: (Int) -> Void
 
-    var viewControllerDescription: ViewControllerDescription {
-        return TodoListViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return TodoListViewController.description(for: self, environment: environment)
     }
 }
 
@@ -34,9 +34,9 @@ struct TodoListScreen: Screen {
 final class TodoListViewController: ScreenViewController<TodoListScreen> {
     let todoListView: TodoListView
 
-    required init(screen: TodoListScreen) {
+    required init(screen: TodoListScreen, environment: ViewEnvironment) {
         self.todoListView = TodoListView(frame: .zero)
-        super.init(screen: screen)
+        super.init(screen: screen, environment: environment)
         update(with: screen)
     }
 
@@ -52,7 +52,7 @@ final class TodoListViewController: ScreenViewController<TodoListScreen> {
         todoListView.frame = view.bounds.inset(by: view.safeAreaInsets)
     }
 
-    override func screenDidChange(from previousScreen: TodoListScreen) {
+    override func screenDidChange(from previousScreen: TodoListScreen, previousEnvironment: ViewEnvironment) {
         update(with: screen)
     }
 
diff --git a/swift/Samples/Tutorial/Frameworks/Tutorial4Complete/Sources/Welcome/WelcomeScreen.swift b/swift/Samples/Tutorial/Frameworks/Tutorial4Complete/Sources/Welcome/WelcomeScreen.swift
index faf89c3d1..a57f71373 100644
--- a/swift/Samples/Tutorial/Frameworks/Tutorial4Complete/Sources/Welcome/WelcomeScreen.swift
+++ b/swift/Samples/Tutorial/Frameworks/Tutorial4Complete/Sources/Welcome/WelcomeScreen.swift
@@ -26,8 +26,8 @@ struct WelcomeScreen: Screen {
     /// Callback when the login button is tapped.
     var onLoginTapped: () -> Void
 
-    var viewControllerDescription: ViewControllerDescription {
-        return WelcomeViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return WelcomeViewController.description(for: self, environment: environment)
     }
 }
 
@@ -36,9 +36,9 @@ final class WelcomeViewController: ScreenViewController<WelcomeScreen> {
 
     var welcomeView: WelcomeView
 
-    required init(screen: WelcomeScreen) {
+    required init(screen: WelcomeScreen, environment: ViewEnvironment) {
         self.welcomeView = WelcomeView(frame: .zero)
-        super.init(screen: screen)
+        super.init(screen: screen, environment: environment)
         update(with: screen)
     }
 
@@ -54,7 +54,7 @@ final class WelcomeViewController: ScreenViewController<WelcomeScreen> {
         welcomeView.frame = view.bounds.inset(by: view.safeAreaInsets)
     }
 
-    override func screenDidChange(from previousScreen: WelcomeScreen) {
+    override func screenDidChange(from previousScreen: WelcomeScreen, previousEnvironment: ViewEnvironment) {
         update(with: screen)
     }
 
diff --git a/swift/Samples/Tutorial/Frameworks/Tutorial5Complete/Sources/Todo/Edit/TodoEditScreen.swift b/swift/Samples/Tutorial/Frameworks/Tutorial5Complete/Sources/Todo/Edit/TodoEditScreen.swift
index c36d013c2..7d5ab6cef 100644
--- a/swift/Samples/Tutorial/Frameworks/Tutorial5Complete/Sources/Todo/Edit/TodoEditScreen.swift
+++ b/swift/Samples/Tutorial/Frameworks/Tutorial5Complete/Sources/Todo/Edit/TodoEditScreen.swift
@@ -28,8 +28,8 @@ struct TodoEditScreen: Screen {
     var onTitleChanged: (String) -> Void
     var onNoteChanged: (String) -> Void
 
-    var viewControllerDescription: ViewControllerDescription {
-        return TodoEditViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return TodoEditViewController.description(for: self, environment: environment)
     }
 }
 
@@ -38,10 +38,10 @@ final class TodoEditViewController: ScreenViewController<TodoEditScreen> {
     // The `todoEditView` has all the logic for displaying the todo and editing.
     let todoEditView: TodoEditView
 
-    required init(screen: TodoEditScreen) {
+    required init(screen: TodoEditScreen, environment: ViewEnvironment) {
         self.todoEditView = TodoEditView(frame: .zero)
 
-        super.init(screen: screen)
+        super.init(screen: screen, environment: environment)
         update(with: screen)
     }
 
@@ -57,7 +57,7 @@ final class TodoEditViewController: ScreenViewController<TodoEditScreen> {
         todoEditView.frame = view.bounds.inset(by: view.safeAreaInsets)
     }
 
-    override func screenDidChange(from previousScreen: TodoEditScreen) {
+    override func screenDidChange(from previousScreen: TodoEditScreen, previousEnvironment: ViewEnvironment) {
         update(with: screen)
     }
 
diff --git a/swift/Samples/Tutorial/Frameworks/Tutorial5Complete/Sources/Todo/List/TodoListScreen.swift b/swift/Samples/Tutorial/Frameworks/Tutorial5Complete/Sources/Todo/List/TodoListScreen.swift
index a9ec59408..fc519745f 100644
--- a/swift/Samples/Tutorial/Frameworks/Tutorial5Complete/Sources/Todo/List/TodoListScreen.swift
+++ b/swift/Samples/Tutorial/Frameworks/Tutorial5Complete/Sources/Todo/List/TodoListScreen.swift
@@ -25,8 +25,8 @@ struct TodoListScreen: Screen {
     // Callback when a todo is selected
     var onTodoSelected: (Int) -> Void
 
-    var viewControllerDescription: ViewControllerDescription {
-        return TodoListViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return TodoListViewController.description(for: self, environment: environment)
     }
 }
 
@@ -34,9 +34,9 @@ struct TodoListScreen: Screen {
 final class TodoListViewController: ScreenViewController<TodoListScreen> {
     let todoListView: TodoListView
 
-    required init(screen: TodoListScreen) {
+    required init(screen: TodoListScreen, environment: ViewEnvironment) {
         self.todoListView = TodoListView(frame: .zero)
-        super.init(screen: screen)
+        super.init(screen: screen, environment: environment)
         update(with: screen)
     }
 
@@ -52,7 +52,7 @@ final class TodoListViewController: ScreenViewController<TodoListScreen> {
         todoListView.frame = view.bounds.inset(by: view.safeAreaInsets)
     }
 
-    override func screenDidChange(from previousScreen: TodoListScreen) {
+    override func screenDidChange(from previousScreen: TodoListScreen, previousEnvironment: ViewEnvironment) {
         update(with: screen)
     }
 
diff --git a/swift/Samples/Tutorial/Frameworks/Tutorial5Complete/Sources/Welcome/WelcomeScreen.swift b/swift/Samples/Tutorial/Frameworks/Tutorial5Complete/Sources/Welcome/WelcomeScreen.swift
index faf89c3d1..a57f71373 100644
--- a/swift/Samples/Tutorial/Frameworks/Tutorial5Complete/Sources/Welcome/WelcomeScreen.swift
+++ b/swift/Samples/Tutorial/Frameworks/Tutorial5Complete/Sources/Welcome/WelcomeScreen.swift
@@ -26,8 +26,8 @@ struct WelcomeScreen: Screen {
     /// Callback when the login button is tapped.
     var onLoginTapped: () -> Void
 
-    var viewControllerDescription: ViewControllerDescription {
-        return WelcomeViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return WelcomeViewController.description(for: self, environment: environment)
     }
 }
 
@@ -36,9 +36,9 @@ final class WelcomeViewController: ScreenViewController<WelcomeScreen> {
 
     var welcomeView: WelcomeView
 
-    required init(screen: WelcomeScreen) {
+    required init(screen: WelcomeScreen, environment: ViewEnvironment) {
         self.welcomeView = WelcomeView(frame: .zero)
-        super.init(screen: screen)
+        super.init(screen: screen, environment: environment)
         update(with: screen)
     }
 
@@ -54,7 +54,7 @@ final class WelcomeViewController: ScreenViewController<WelcomeScreen> {
         welcomeView.frame = view.bounds.inset(by: view.safeAreaInsets)
     }
 
-    override func screenDidChange(from previousScreen: WelcomeScreen) {
+    override func screenDidChange(from previousScreen: WelcomeScreen, previousEnvironment: ViewEnvironment) {
         update(with: screen)
     }
 
diff --git a/swift/WorkflowUI/Sources/Container/ContainerViewController.swift b/swift/WorkflowUI/Sources/Container/ContainerViewController.swift
index 90e9d9e5c..73cd7d826 100644
--- a/swift/WorkflowUI/Sources/Container/ContainerViewController.swift
+++ b/swift/WorkflowUI/Sources/Container/ContainerViewController.swift
@@ -35,11 +35,19 @@ public final class ContainerViewController<Output, ScreenType>: UIViewController
 
     private let (lifetime, token) = Lifetime.make()
 
-    private init(workflowHost: Any, rendering: Property<ScreenType>, output: Signal<Output, Never>) {
+    public var rootViewEnvironment: ViewEnvironment {
+        didSet {
+            // Re-render the current rendering with the new environment
+            render(screen: rendering.value, environment: rootViewEnvironment)
+        }
+    }
+
+    private init(workflowHost: Any, rendering: Property<ScreenType>, output: Signal<Output, Never>, rootViewEnvironment: ViewEnvironment) {
         self.workflowHost = workflowHost
-        self.rootViewController = DescribedViewController(screen: rendering.value)
+        self.rootViewController = DescribedViewController(screen: rendering.value, environment: rootViewEnvironment)
         self.rendering = rendering
         self.output = output
+        self.rootViewEnvironment = rootViewEnvironment
 
         super.init(nibName: nil, bundle: nil)
 
@@ -47,24 +55,26 @@ public final class ContainerViewController<Output, ScreenType>: UIViewController
             .signal
             .take(during: lifetime)
             .observeValues { [weak self] screen in
-                self?.render(screen: screen)
+                guard let self = self else { return }
+                self.render(screen: screen, environment: self.rootViewEnvironment)
             }
     }
 
-    public convenience init<W: Workflow>(workflow: W) where W.Rendering == ScreenType, W.Output == Output {
+    public convenience init<W: Workflow>(workflow: W, rootViewEnvironment: ViewEnvironment = .empty) where W.Rendering == ScreenType, W.Output == Output {
         let host = WorkflowHost(workflow: workflow)
         self.init(
             workflowHost: host,
             rendering: host.rendering,
-            output: host.output)
+            output: host.output,
+            rootViewEnvironment: rootViewEnvironment)
     }
 
     required public init?(coder aDecoder: NSCoder) {
         fatalError("init(coder:) has not been implemented")
     }
 
-    private func render(screen: ScreenType) {
-        rootViewController.update(screen: screen)
+    private func render(screen: ScreenType, environment: ViewEnvironment) {
+        rootViewController.update(screen: screen, environment: environment)
     }
 
     override public func viewDidLoad() {
diff --git a/swift/WorkflowUI/Sources/Screen/AnyScreen/AnyScreen.swift b/swift/WorkflowUI/Sources/Screen/AnyScreen/AnyScreen.swift
index 3ebb99957..926a992f6 100644
--- a/swift/WorkflowUI/Sources/Screen/AnyScreen/AnyScreen.swift
+++ b/swift/WorkflowUI/Sources/Screen/AnyScreen/AnyScreen.swift
@@ -23,8 +23,8 @@ public struct AnyScreen: Screen {
     /// The original screen, retained for debugging
     internal let wrappedScreen: Screen
 
-    /// The wrapped screen’s view controller description is passed straight through
-    public let viewControllerDescription: ViewControllerDescription
+    /// Stored getter for the wrapped screen’s view controller description
+    private let _viewControllerDescription: (ViewEnvironment) -> ViewControllerDescription
 
     public init<T: Screen>(_ screen: T) {
         if let anyScreen = screen as? AnyScreen {
@@ -32,7 +32,12 @@ public struct AnyScreen: Screen {
             return
         }
         self.wrappedScreen = screen
-        self.viewControllerDescription = screen.viewControllerDescription
+        self._viewControllerDescription = screen.viewControllerDescription(environment:)
+    }
+
+    public func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        // Passed straight through
+        return _viewControllerDescription(environment)
     }
 
 }
diff --git a/swift/WorkflowUI/Sources/Screen/AnyScreen/UntypedScreenViewController.swift b/swift/WorkflowUI/Sources/Screen/AnyScreen/UntypedScreenViewController.swift
deleted file mode 100644
index 8fe20a403..000000000
--- a/swift/WorkflowUI/Sources/Screen/AnyScreen/UntypedScreenViewController.swift
+++ /dev/null
@@ -1,38 +0,0 @@
-/*
- * Copyright 2019 Square Inc.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-#if canImport(UIKit)
-
-// Internal API for working with a screen view controller when the specific screen type is unknown.
-protocol UntypedScreenViewController {
-    var screenType: Screen.Type { get }
-    func update(untypedScreen: Screen)
-}
-
-extension ScreenViewController: UntypedScreenViewController {
-
-    // `var screenType: Screen.Type` is already present in ScreenViewController
-
-    func update(untypedScreen: Screen) {
-        guard let typedScreen = untypedScreen as? ScreenType else {
-            fatalError("Screen type mismatch: \(self) expected to receive a screen of type \(ScreenType.self), but instead received a screen of type \(type(of: screen))")
-        }
-        update(screen: typedScreen)
-    }
-
-}
-
-#endif
diff --git a/swift/WorkflowUI/Sources/Screen/Screen.swift b/swift/WorkflowUI/Sources/Screen/Screen.swift
index 0cffcdb6c..d62ecfc87 100644
--- a/swift/WorkflowUI/Sources/Screen/Screen.swift
+++ b/swift/WorkflowUI/Sources/Screen/Screen.swift
@@ -24,7 +24,7 @@ public protocol Screen {
 
     /// A view controller description that acts as a recipe to either build
     /// or update a previously-built view controller to match this screen.
-    var viewControllerDescription: ViewControllerDescription { get }
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription
 
 }
 
diff --git a/swift/WorkflowUI/Sources/Screen/ScreenViewController.swift b/swift/WorkflowUI/Sources/Screen/ScreenViewController.swift
index b2500d66a..9d565a90c 100644
--- a/swift/WorkflowUI/Sources/Screen/ScreenViewController.swift
+++ b/swift/WorkflowUI/Sources/Screen/ScreenViewController.swift
@@ -38,18 +38,17 @@ import UIKit
 /// ```
 open class ScreenViewController<ScreenType: Screen>: UIViewController {
 
-    private var currentScreen: ScreenType
-
-    public final var screen: ScreenType {
-        return currentScreen
-    }
+    private(set) public final var screen: ScreenType
 
     public final var screenType: Screen.Type {
         return ScreenType.self
     }
 
-    public required init(screen: ScreenType) {
-        self.currentScreen = screen
+    private(set) public final var environment: ViewEnvironment
+
+    public required init(screen: ScreenType, environment: ViewEnvironment) {
+        self.screen = screen
+        self.environment = environment
         super.init(nibName: nil, bundle: nil)
     }
 
@@ -58,14 +57,16 @@ open class ScreenViewController<ScreenType: Screen>: UIViewController {
         fatalError("init(coder:) has not been implemented")
     }
 
-    public final func update(screen: ScreenType) {
-        let previousScreen = currentScreen
-        currentScreen = screen
-        screenDidChange(from: previousScreen)
+    public final func update(screen: ScreenType, environment: ViewEnvironment) {
+        let previousScreen = self.screen
+        self.screen = screen
+        let previousEnvironment = self.environment
+        self.environment = environment
+        screenDidChange(from: previousScreen, previousEnvironment: previousEnvironment)
     }
 
     /// Subclasses should override this method in order to update any relevant UI bits when the screen model changes.
-    open func screenDidChange(from previousScreen: ScreenType) {
+    open func screenDidChange(from previousScreen: ScreenType, previousEnvironment: ViewEnvironment) {
 
     }
 
@@ -76,11 +77,11 @@ extension ScreenViewController {
     /// Convenience to create a view controller description for the given screen
     /// value. See the example on the comment for ScreenViewController for
     /// usage.
-    public final class func description(for screen: ScreenType) -> ViewControllerDescription {
+    public final class func description(for screen: ScreenType, environment: ViewEnvironment) -> ViewControllerDescription {
         return ViewControllerDescription(
             type: self,
-            build: { self.init(screen: screen) },
-            update: { $0.update(screen: screen) })
+            build: { self.init(screen: screen, environment: environment) },
+            update: { $0.update(screen: screen, environment: environment) })
     }
 
 }
diff --git a/swift/WorkflowUI/Sources/Screen/ViewEnvironment/ViewEnvironment.swift b/swift/WorkflowUI/Sources/Screen/ViewEnvironment/ViewEnvironment.swift
new file mode 100644
index 000000000..74faa9c28
--- /dev/null
+++ b/swift/WorkflowUI/Sources/Screen/ViewEnvironment/ViewEnvironment.swift
@@ -0,0 +1,97 @@
+/*
+* Copyright 2012 Square Inc.
+*
+* Licensed under the Apache License, Version 2.0 (the "License");
+* you may not use this file except in compliance with the License.
+* You may obtain a copy of the License at
+*
+*     http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+
+/// ViewEnvironment acts as a container for values to flow down the view-side
+/// of a rendering tree (as opposed to being passed down through Workflows).
+///
+/// This will often be used by containers to let their children know in what
+/// context they’re appearing (for example, a split screen container may set
+/// the environment of its two children according to which position they’re
+/// appearing in).
+public struct ViewEnvironment {
+
+    /// An empty view environment. This should only be used when setting up a
+    /// root workflow into a root ContainerViewController or when writing tests.
+    /// In other scenarios, containers should pass down the ViewEnvironment
+    /// value they get from above.
+    public static let empty: ViewEnvironment = ViewEnvironment()
+
+    /// Storage of [K.Type: K.Value] where K: ViewEnvironmentKey
+    private var storage: [ObjectIdentifier: Any]
+
+    /// Private empty initializer to make the `empty` environment explicit.
+    private init() {
+        storage = [:]
+    }
+
+    /// Get or set for the given ViewEnvironmentKey.
+    ///
+    /// This will typically only be used by the module that provides the
+    /// environment value. See documentation for ViewEnvironmentKey for a
+    /// usage example.
+    public subscript<Key>(key: Key.Type) -> Key.Value where Key: ViewEnvironmentKey {
+
+        get {
+            if let value = storage[ObjectIdentifier(key)] as? Key.Value {
+                return value
+            } else {
+                return Key.defaultValue
+            }
+        }
+
+        set {
+            storage[ObjectIdentifier(key)] = newValue
+        }
+
+    }
+
+    /// Returns a new ViewEnvironment with the given value set for the given
+    /// environment key.
+    ///
+    /// This is provided as a convenience for modifying the environment while
+    /// passing it down to children screens without the need for an intermediate
+    /// mutable value. It is functionally equivalent to the subscript setter.
+    public func setting<Key>(key: Key.Type, to value: Key.Value) -> ViewEnvironment where Key: ViewEnvironmentKey {
+        var newEnvironment = self
+        newEnvironment[key] = value
+        return newEnvironment
+    }
+
+    /// Returns a new ViewEnvironment with the given value set for the given
+    /// key path.
+    ///
+    /// This is provided as a convenience for modifying the environment while
+    /// passing it down to children screens.
+    ///
+    /// The following are functionally equivalent:
+    /// ```
+    /// var newEnvironment = environment
+    /// newEnvironment.someProperty = 42
+    /// ```
+    /// and
+    /// ```
+    /// let newEnvironment = environment.setting(\.someProperty, to: 42)
+    /// ```
+    ///
+    ///
+    public func setting<Value>(keyPath: WritableKeyPath<ViewEnvironment, Value>, to value: Value) -> ViewEnvironment {
+        var newEnvironment = self
+        newEnvironment[keyPath: keyPath] = value
+        return newEnvironment
+    }
+
+}
diff --git a/swift/WorkflowUI/Sources/Screen/ViewEnvironment/ViewEnvironmentKey.swift b/swift/WorkflowUI/Sources/Screen/ViewEnvironment/ViewEnvironmentKey.swift
new file mode 100644
index 000000000..9917189a4
--- /dev/null
+++ b/swift/WorkflowUI/Sources/Screen/ViewEnvironment/ViewEnvironmentKey.swift
@@ -0,0 +1,46 @@
+/*
+* Copyright 2012 Square Inc.
+*
+* Licensed under the Apache License, Version 2.0 (the "License");
+* you may not use this file except in compliance with the License.
+* You may obtain a copy of the License at
+*
+*     http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+
+/// A key into the ViewEnvironment.
+///
+/// Environment keys are associated with a specific type of value (`Value`) and
+/// must declare a default value.
+///
+/// Typically the key conforming to `ViewEnvironmentKey` will be private, and
+/// you are encouraged to provide a convenience accessor on `ViewEnvironment`
+/// as in the following example:
+///
+/// ```
+/// private enum ThemeKey: ViewEnvironmentKey {
+///     typealias Value = Theme
+///     var defaultValue: Theme
+/// }
+///
+/// extension ViewEnvironment {
+///     public var theme: Theme {
+///         get { self[ThemeKey.self] }
+///         set { self[ThemeKey.self] = newValue }
+///     }
+/// }
+/// ```
+public protocol ViewEnvironmentKey {
+
+    associatedtype Value
+
+    static var defaultValue: Value { get }
+
+}
diff --git a/swift/WorkflowUI/Sources/ViewControllerDescription/DescribedViewController.swift b/swift/WorkflowUI/Sources/ViewControllerDescription/DescribedViewController.swift
index b60016786..6224475ed 100644
--- a/swift/WorkflowUI/Sources/ViewControllerDescription/DescribedViewController.swift
+++ b/swift/WorkflowUI/Sources/ViewControllerDescription/DescribedViewController.swift
@@ -28,8 +28,8 @@ public final class DescribedViewController: UIViewController {
         super.init(nibName: nil, bundle: nil)
     }
 
-    public convenience init<S: Screen>(screen: S) {
-        self.init(description: screen.viewControllerDescription)
+    public convenience init<S: Screen>(screen: S, environment: ViewEnvironment) {
+        self.init(description: screen.viewControllerDescription(environment: environment))
     }
 
     @available(*, unavailable)
@@ -56,8 +56,8 @@ public final class DescribedViewController: UIViewController {
         }
     }
 
-    public func update<S: Screen>(screen: S) {
-        update(description: screen.viewControllerDescription)
+    public func update<S: Screen>(screen: S, environment: ViewEnvironment) {
+        update(description: screen.viewControllerDescription(environment: environment))
     }
 
     public override func viewDidLoad() {
diff --git a/swift/WorkflowUI/Tests/ContainerViewControllerTests.swift b/swift/WorkflowUI/Tests/ContainerViewControllerTests.swift
index 07b7bdaca..af7cb373e 100644
--- a/swift/WorkflowUI/Tests/ContainerViewControllerTests.swift
+++ b/swift/WorkflowUI/Tests/ContainerViewControllerTests.swift
@@ -26,16 +26,16 @@ import Workflow
 fileprivate struct TestScreen: Screen {
     var string: String
 
-    var viewControllerDescription: ViewControllerDescription {
-        return TestScreenViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return TestScreenViewController.description(for: self, environment: environment)
     }
 }
 
 fileprivate final class TestScreenViewController: ScreenViewController<TestScreen> {
     var onScreenChange: (() -> Void)? = nil
 
-    override func screenDidChange(from previousScreen: TestScreen) {
-        super.screenDidChange(from: previousScreen)
+    override func screenDidChange(from previousScreen: TestScreen, previousEnvironment: ViewEnvironment) {
+        super.screenDidChange(from: previousScreen, previousEnvironment: previousEnvironment)
         onScreenChange?()
     }
 }
diff --git a/swift/WorkflowUI/Tests/ViewControllerDescriptionTests.swift b/swift/WorkflowUI/Tests/ViewControllerDescriptionTests.swift
index b1d19ba00..7147daacc 100644
--- a/swift/WorkflowUI/Tests/ViewControllerDescriptionTests.swift
+++ b/swift/WorkflowUI/Tests/ViewControllerDescriptionTests.swift
@@ -90,8 +90,8 @@ class ViewControllerDescriptionTests: XCTestCase {
         // description
 
         struct MyScreen: Screen {
-            var viewControllerDescription: ViewControllerDescription {
-                return MyScreenViewController.description(for: self)
+            func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+                return MyScreenViewController.description(for: self, environment: environment)
             }
         }
 
@@ -100,7 +100,7 @@ class ViewControllerDescriptionTests: XCTestCase {
 
 
         let screen = MyScreen()
-        let description = screen.viewControllerDescription
+        let description = screen.viewControllerDescription(environment: .empty)
 
         let viewController = description.buildViewController()
         XCTAssertTrue(type(of: viewController) == MyScreenViewController.self)

From 6a46f0e7245b692758aec579853cc221bebde5cc Mon Sep 17 00:00:00 2001
From: Ben Cochran <bc@squareup.com>
Date: Wed, 4 Mar 2020 15:15:00 -0800
Subject: [PATCH 2/2] Update docs and template for ViewEnvironment

---
 .../building-a-view-controller-from-screen.md    | 16 ++++++++++------
 swift/Samples/Tutorial/Tutorial1.md              |  4 ++--
 swift/Samples/Tutorial/Tutorial3.md              |  4 ++--
 .../___FILEBASENAME___Screen.swift               | 16 ++++++++--------
 .../Sources/Screen/ScreenViewController.swift    |  4 ++--
 5 files changed, 24 insertions(+), 20 deletions(-)

diff --git a/docs/tutorial/building-a-view-controller-from-screen.md b/docs/tutorial/building-a-view-controller-from-screen.md
index 766a5b601..a3d35258e 100644
--- a/docs/tutorial/building-a-view-controller-from-screen.md
+++ b/docs/tutorial/building-a-view-controller-from-screen.md
@@ -11,6 +11,10 @@ controller from a view model update.
 struct DemoScreen: Screen {
     let title: String
     let onTap: () -> Void
+
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return DemoScreenViewController.description(for: self, environment: environment)
+    }
 }
 
 
@@ -18,9 +22,9 @@ class DemoScreenViewController: ScreenViewController<DemoScreen> {
 
     private let button: UIButton
 
-    required init(screen: DemoScreen) {
+    required init(screen: DemoScreen, environment: ViewEnvironment) {
         button = UIButton()
-        super.init(screen: screen)
+        super.init(screen: screen, environment: environment)
 
         update(screen: screen)
     }
@@ -39,8 +43,8 @@ class DemoScreenViewController: ScreenViewController<DemoScreen> {
         button.frame = view.bounds
     }
 
-    override func screenDidChange(from previousScreen: DemoScreen) {
-        super.screenDidChange(from: previousScreen)
+    override func screenDidChange(from previousScreen: DemoScreen, previousEnvironment: ViewEnvironment) {
+        super.screenDidChange(from: previousScreen, previousEnvironment: previousEnvironment)
         update(screen: screen)
     }
 
@@ -64,5 +68,5 @@ class DemoScreenViewController: ScreenViewController<DemoScreen> {
 1. The button is tapped. When the callback is called, we call the `onTap` closure passed into the
    screen. The workflow will handle this event, update its state, and a new screen will be rendered.
 1. The updated screen is passed to the view controller via the
-   `screenDidChange(from previousScreen:)` method. Again, the view controller updates the title of
-   the button based on what was passed in the screen.
+   `screenDidChange(from previousScreen: previousEnvironment: previousEnvironment:)` method. Again,
+   the view controller updates the title of the button based on what was passed in the screen.
diff --git a/swift/Samples/Tutorial/Tutorial1.md b/swift/Samples/Tutorial/Tutorial1.md
index b88cb75a8..b5a32284a 100644
--- a/swift/Samples/Tutorial/Tutorial1.md
+++ b/swift/Samples/Tutorial/Tutorial1.md
@@ -44,8 +44,8 @@ struct WelcomeScreen: Screen {
     /// Callback when the login button is tapped.
     var onLoginTapped: () -> Void
 
-    var viewControllerDescription: ViewControllerDescription {
-        return WelcomeViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return WelcomeViewController.description(for: self, environment: environment)
     }
 }
 ```
diff --git a/swift/Samples/Tutorial/Tutorial3.md b/swift/Samples/Tutorial/Tutorial3.md
index 8d0207113..33f5284a9 100644
--- a/swift/Samples/Tutorial/Tutorial3.md
+++ b/swift/Samples/Tutorial/Tutorial3.md
@@ -89,8 +89,8 @@ The `Screen` protocol also requires a `viewControllerDescription` property. This
 
 ```swift
 extension TodoEditScreen {
-    var viewControllerDescription: ViewControllerDescription {
-        TodoEditViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        TodoEditViewController.description(for: self, environment: environment)
     }
 }
 ```
diff --git a/swift/Tooling/Templates/Screen (View Controller).xctemplate/___FILEBASENAME___Screen.swift b/swift/Tooling/Templates/Screen (View Controller).xctemplate/___FILEBASENAME___Screen.swift
index 51be7c9b0..6ff8a4182 100755
--- a/swift/Tooling/Templates/Screen (View Controller).xctemplate/___FILEBASENAME___Screen.swift	
+++ b/swift/Tooling/Templates/Screen (View Controller).xctemplate/___FILEBASENAME___Screen.swift	
@@ -10,24 +10,24 @@ struct ___VARIABLE_productName___Screen: Screen {
     // It should also contain callbacks for any UI events, for example:
     // var onButtonTapped: () -> Void
 
-    var viewControllerDescription: ViewControllerDescription {
-        return ___VARIABLE_productName___ViewController.description(for: self)
+    func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
+        return ___VARIABLE_productName___ViewController.description(for: self, environment: environment)
     }
 }
 
 
 final class ___VARIABLE_productName___ViewController: ScreenViewController<___VARIABLE_productName___Screen> {
 
-    required init(screen: ___VARIABLE_productName___Screen) {
-        super.init(screen: screen)
-        update(with: screen)
+    required init(screen: ___VARIABLE_productName___Screen, environment: ViewEnvironment) {
+        super.init(screen: screen, environment: environment)
+        update(with: screen, environment: environment)
     }
 
-    override func screenDidChange(from previousScreen: ___VARIABLE_productName___Screen) {
-        update(with: screen)
+    override func screenDidChange(from previousScreen: ___VARIABLE_productName___Screen, previousEnvironment: ViewEnvironment) {
+        update(with: screen, environment: environment)
     }
 
-    private func update(with screen: ___VARIABLE_productName___Screen) {
+    private func update(with screen: ___VARIABLE_productName___Screen, environment: ViewEnvironment) {
         /// Update UI
     }
 
diff --git a/swift/WorkflowUI/Sources/Screen/ScreenViewController.swift b/swift/WorkflowUI/Sources/Screen/ScreenViewController.swift
index 9d565a90c..356753bef 100644
--- a/swift/WorkflowUI/Sources/Screen/ScreenViewController.swift
+++ b/swift/WorkflowUI/Sources/Screen/ScreenViewController.swift
@@ -25,13 +25,13 @@ import UIKit
 /// Using this base class, a screen can be implemented as:
 /// ```
 /// struct MyScreen: Screen {
-///     var viewControllerDescription: ViewControllerDescription {
+///     func viewControllerDescription(environment: ViewEnvironment) -> ViewControllerDescription {
 ///         return MyScreenViewController.description(for: self)
 ///     }
 /// }
 ///
 /// private class MyScreenViewController: ScreenViewController<MyScreen> {
-///     override func screenDidChange(from previousScreen: MyScreen) {
+///     override func screenDidChange(from previousScreen: MyScreen, previousEnvironment: ViewEnvironment) {
 ///         // … update views as necessary
 ///     }
 /// }