diff --git a/CollectionKit.xcodeproj/project.pbxproj b/CollectionKit.xcodeproj/project.pbxproj
index edf99ae..55d1e2d 100644
--- a/CollectionKit.xcodeproj/project.pbxproj
+++ b/CollectionKit.xcodeproj/project.pbxproj
@@ -8,6 +8,8 @@
 
 /* Begin PBXBuildFile section */
 		0E3258451F5E23B000D455F8 /* FlowLayoutSpec.swift in Sources */ = {isa = PBXBuildFile; fileRef = 0E3258441F5E23B000D455F8 /* FlowLayoutSpec.swift */; };
+		A3066AAD2176997900467E3C /* BasicProvider+Convenience.swift in Sources */ = {isa = PBXBuildFile; fileRef = A3066AAC2176997800467E3C /* BasicProvider+Convenience.swift */; };
+		A3066AAF2176997F00467E3C /* ComposedHeaderProvider+Convenience.swift in Sources */ = {isa = PBXBuildFile; fileRef = A3066AAE2176997F00467E3C /* ComposedHeaderProvider+Convenience.swift */; };
 		A3299F342064ED690075A137 /* VisibleFrameInsetLayout.swift in Sources */ = {isa = PBXBuildFile; fileRef = A3299F332064ED690075A137 /* VisibleFrameInsetLayout.swift */; };
 		A363C56520C7EA44009A885F /* ComposedViewSource.swift in Sources */ = {isa = PBXBuildFile; fileRef = A363C56420C7EA44009A885F /* ComposedViewSource.swift */; };
 		A363C56620C7EA66009A885F /* AnyViewSource.swift in Sources */ = {isa = PBXBuildFile; fileRef = A363C56220C7EA2E009A885F /* AnyViewSource.swift */; };
@@ -74,6 +76,8 @@
 		0E3258441F5E23B000D455F8 /* FlowLayoutSpec.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = FlowLayoutSpec.swift; sourceTree = "<group>"; };
 		18407FD826F3703406A838D4 /* Pods_CollectionKitTests.framework */ = {isa = PBXFileReference; explicitFileType = wrapper.framework; includeInIndex = 0; path = Pods_CollectionKitTests.framework; sourceTree = BUILT_PRODUCTS_DIR; };
 		571F2E4FA7747B7E5CAC5A2D /* Pods-CollectionKitTests.debug.xcconfig */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = text.xcconfig; name = "Pods-CollectionKitTests.debug.xcconfig"; path = "Pods/Target Support Files/Pods-CollectionKitTests/Pods-CollectionKitTests.debug.xcconfig"; sourceTree = "<group>"; };
+		A3066AAC2176997800467E3C /* BasicProvider+Convenience.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = "BasicProvider+Convenience.swift"; sourceTree = "<group>"; };
+		A3066AAE2176997F00467E3C /* ComposedHeaderProvider+Convenience.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = "ComposedHeaderProvider+Convenience.swift"; sourceTree = "<group>"; };
 		A3299F332064ED690075A137 /* VisibleFrameInsetLayout.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = VisibleFrameInsetLayout.swift; sourceTree = "<group>"; };
 		A32B59BD213C063400DED8B4 /* CollectionKit.framework */ = {isa = PBXFileReference; explicitFileType = wrapper.framework; includeInIndex = 0; path = CollectionKit.framework; sourceTree = BUILT_PRODUCTS_DIR; };
 		A32B59BE213C063400DED8B4 /* CollectionKitTests.xctest */ = {isa = PBXFileReference; explicitFileType = wrapper.cfbundle; includeInIndex = 0; path = CollectionKitTests.xctest; sourceTree = BUILT_PRODUCTS_DIR; };
@@ -279,8 +283,10 @@
 			isa = PBXGroup;
 			children = (
 				B148237A1F1E87F60007A7E2 /* BasicProvider.swift */,
+				A3066AAC2176997800467E3C /* BasicProvider+Convenience.swift */,
 				B1E861F81F21520200621BA0 /* ComposedProvider.swift */,
 				A363C56D20CBD55B009A885F /* ComposedHeaderProvider.swift */,
+				A3066AAE2176997F00467E3C /* ComposedHeaderProvider+Convenience.swift */,
 				A363C56B20CACA89009A885F /* FlattenedProvider.swift */,
 				B144CE091F437A6B0073CF4E /* EmptyCollectionProvider.swift */,
 			);
@@ -526,6 +532,7 @@
 				B1688A2D1F25632900C229BD /* SpaceProvider.swift in Sources */,
 				A363C56520C7EA44009A885F /* ComposedViewSource.swift in Sources */,
 				B1B7B0611E7DAD2700F2483E /* Util.swift in Sources */,
+				A3066AAD2176997900467E3C /* BasicProvider+Convenience.swift in Sources */,
 				B144CE0A1F437A6B0073CF4E /* EmptyCollectionProvider.swift in Sources */,
 				B144CE061F437A1D0073CF4E /* RowLayout.swift in Sources */,
 				B19D64C91F23DCA800D02FDB /* CollectionReuseViewManager.swift in Sources */,
@@ -546,6 +553,7 @@
 				B1B7B05F1E7DAD2700F2483E /* CollectionView.swift in Sources */,
 				B157E51A1F6719F0008AEF83 /* SimpleLayout.swift in Sources */,
 				A3299F342064ED690075A137 /* VisibleFrameInsetLayout.swift in Sources */,
+				A3066AAF2176997F00467E3C /* ComposedHeaderProvider+Convenience.swift in Sources */,
 				B1E861F41F21517400621BA0 /* DataSource.swift in Sources */,
 				A3C76C1F20D10817003082BF /* LayoutableProvider.swift in Sources */,
 				B157E5181F66FEE5008AEF83 /* OverlayLayout.swift in Sources */,
diff --git a/CollectionKitTests/CollectionViewSpec.swift b/CollectionKitTests/CollectionViewSpec.swift
index cbdf81d..28cdfc8 100644
--- a/CollectionKitTests/CollectionViewSpec.swift
+++ b/CollectionKitTests/CollectionViewSpec.swift
@@ -21,11 +21,11 @@ class CollectionViewSpec: QuickSpec {
         dataSource = ArrayDataSource(data: [1, 2, 3, 4])
         provider = BasicProvider(
           dataSource: dataSource,
-          viewSource: ClosureViewSource(viewUpdater: { (label: UILabel, data: Int, index: Int) in
+          viewSource: { (label: UILabel, data: Int, index: Int) in
             label.backgroundColor = .red
             label.textAlignment = .center
             label.text = "\(data)"
-          }),
+          },
           sizeSource: { (index: Int, data: Int, collectionSize: CGSize) -> CGSize in
             return CGSize(width: 50, height: 50)
           }
@@ -228,10 +228,10 @@ class CollectionViewSpec: QuickSpec {
         expect((collectionView.cell(at: 1) as! UILabel).text) == "2"
 
         provider = BasicProvider(
-          dataSource: ArrayDataSource(data: [0, 0, 0, 0]),
-          viewSource: ClosureViewSource(viewUpdater: { (label: UILabel, data: Int, index: Int) in
+          dataSource: [0, 0, 0, 0],
+          viewSource: { (label: UILabel, data: Int, index: Int) in
             label.text = "\(data)"
-          }),
+          },
           sizeSource: { (index: Int, data: Int, collectionSize: CGSize) -> CGSize in
             return CGSize(width: 50, height: 50)
           }
@@ -264,10 +264,10 @@ class CollectionViewSpec: QuickSpec {
       it("handles tap") {
         var lastTappedIndex: Int = -1
         provider = BasicProvider(
-          dataSource: ArrayDataSource(data: [0, 1, 2, 3]),
-          viewSource: ClosureViewSource(viewUpdater: { (label: UILabel, data: Int, index: Int) in
+          dataSource: [0, 1, 2, 3],
+          viewSource: { (label: UILabel, data: Int, index: Int) in
             label.text = "\(data)"
-          }),
+          },
           sizeSource: { (index: Int, data: Int, collectionSize: CGSize) -> CGSize in
             return CGSize(width: 50, height: 50)
           },
diff --git a/CollectionKitTests/ComposedProviderSpec.swift b/CollectionKitTests/ComposedProviderSpec.swift
index 90e458e..75e02b6 100644
--- a/CollectionKitTests/ComposedProviderSpec.swift
+++ b/CollectionKitTests/ComposedProviderSpec.swift
@@ -102,9 +102,9 @@ class ComposedProviderSpec: QuickSpec {
 
         expect(collectionView.subviews[0].frame.origin) == CGPoint.zero
         expect(collectionView.subviews[1].frame.origin) == CGPoint(x: 50, y: 0)
-        provider1.sizeSource = { _, _, _ in
+        provider1.sizeSource = ClosureSizeSource(sizeSource: { _, _, _ in
           return CGSize(width: 30, height: 30)
-        }
+        })
         collectionView.layoutIfNeeded()
         expect(collectionView.reloadCount) == 1
         expect(collectionView.subviews[0].frame.origin) == CGPoint.zero
@@ -160,11 +160,11 @@ class ComposedProviderSpec: QuickSpec {
         let provider2 = SimpleTestProvider(data: ["a", "b"])
 
         let tapProvider = BasicProvider(
-          dataSource: ArrayDataSource(data: [11, 12]),
-          viewSource: ClosureViewSource(viewUpdater: {
+          dataSource: [11, 12],
+          viewSource: {
             (label: UILabel, data: Int, index: Int) in
             label.text = "\(data)"
-          }),
+          },
           sizeSource: { (index: Int, data: Int, collectionSize: CGSize) -> CGSize in
             return CGSize(width: 50, height: 50)
           },
diff --git a/CollectionKitTests/TestUils.swift b/CollectionKitTests/TestUils.swift
index 3cc021d..df36119 100644
--- a/CollectionKitTests/TestUils.swift
+++ b/CollectionKitTests/TestUils.swift
@@ -54,9 +54,9 @@ class SimpleTestProvider<Data>: BasicProvider<Data, UILabel> {
         label.textAlignment = .center
         label.text = "\(data)"
       }),
-      sizeSource: { (index: Int, data: Data, collectionSize: CGSize) -> CGSize in
+      sizeSource: ClosureSizeSource(sizeSource: { (index: Int, data: Data, collectionSize: CGSize) -> CGSize in
         return CGSize(width: 50, height: 50)
-      }
+      })
     )
   }
 
diff --git a/Examples/CollectionKitExamples/AnimatorExample/AnimatorExampleViewController.swift b/Examples/CollectionKitExamples/AnimatorExample/AnimatorExampleViewController.swift
index b04ef1c..05b1d1f 100644
--- a/Examples/CollectionKitExamples/AnimatorExample/AnimatorExampleViewController.swift
+++ b/Examples/CollectionKitExamples/AnimatorExample/AnimatorExampleViewController.swift
@@ -24,7 +24,7 @@ class AnimatorExampleViewController: CollectionViewController {
     let visibleFrameInsets = UIEdgeInsets(top: 0, left: -100, bottom: 0, right: -100)
 
     let imageProvider = BasicProvider(
-      dataSource: ArrayDataSource(data: testImages),
+      dataSource: testImages,
       viewSource: ClosureViewSource(viewGenerator: { (data, index) -> UIImageView in
         let view = UIImageView()
         view.layer.cornerRadius = 5
@@ -33,7 +33,7 @@ class AnimatorExampleViewController: CollectionViewController {
       }, viewUpdater: { (view: UIImageView, data: UIImage, at: Int) in
         view.image = data
       }),
-      sizeSource: imageSizeProvider,
+      sizeSource: UIImageSizeSource(),
       layout: WaterfallLayout(columns: 2, spacing: 10).transposed().inset(by: bodyInset).insetVisibleFrame(by: visibleFrameInsets),
       animator: animators[0].1
     )
@@ -44,12 +44,12 @@ class AnimatorExampleViewController: CollectionViewController {
     buttonsCollectionView.showsHorizontalScrollIndicator = false
 
     let buttonsProvider = BasicProvider(
-      dataSource: ArrayDataSource(data: animators),
-      viewSource: ClosureViewSource(viewUpdater: { (view: SelectionButton, data: (String, Animator), at: Int) in
+      dataSource: animators,
+      viewSource: { (view: SelectionButton, data: (String, Animator), at: Int) in
         view.label.text = data.0
         view.label.textColor = imageProvider.animator === data.1 ? .white : .black
         view.backgroundColor = imageProvider.animator === data.1 ? .lightGray : .white
-      }),
+      },
       sizeSource: { _, data, maxSize in
         return CGSize(width: data.0.width(withConstraintedHeight: maxSize.height, font: UIFont.systemFont(ofSize:18)) + 20, height: maxSize.height)
       },
diff --git a/Examples/CollectionKitExamples/ArticleExample/ArticleExampleViewController.swift b/Examples/CollectionKitExamples/ArticleExample/ArticleExampleViewController.swift
index 5c3ccc4..3aa6623 100644
--- a/Examples/CollectionKitExamples/ArticleExample/ArticleExampleViewController.swift
+++ b/Examples/CollectionKitExamples/ArticleExample/ArticleExampleViewController.swift
@@ -25,11 +25,10 @@ class ArticleExampleViewController: CollectionViewController {
 
     collectionView.contentInset = UIEdgeInsets(top: 20, left: 16, bottom: 20, right: 16)
     provider = BasicProvider(
-      dataSource: ArrayDataSource(data: articles),
-      viewSource: ClosureViewSource(viewUpdater: {
-        (view: ArticleView, data: ArticleData, at: Int) in
+      dataSource: articles,
+      viewSource: { (view: ArticleView, data: ArticleData, at: Int) in
         view.populate(article: data)
-      }),
+      },
       sizeSource: { (_, view, size) -> CGSize in
         return CGSize(width: size.width, height: 200)
       },
diff --git a/Examples/CollectionKitExamples/GridExample/GridViewController.swift b/Examples/CollectionKitExamples/GridExample/GridViewController.swift
index 5807954..970a94f 100644
--- a/Examples/CollectionKitExamples/GridExample/GridViewController.swift
+++ b/Examples/CollectionKitExamples/GridExample/GridViewController.swift
@@ -31,11 +31,11 @@ class GridViewController: CollectionViewController {
     collectionView.contentInset = UIEdgeInsets(top: 10, left: 10, bottom: 10, right: 10)
     provider = BasicProvider(
       dataSource: dataSource,
-      viewSource: ClosureViewSource(viewUpdater: { (view: SquareView, data: Int, index: Int) in
+      viewSource: { (view: SquareView, data: Int, index: Int) in
         view.backgroundColor = UIColor(hue: CGFloat(index) / CGFloat(kGridSize.width * kGridSize.height),
                                        saturation: 0.68, brightness: 0.98, alpha: 1)
         view.text = "\(data)"
-      }),
+      },
       layout: layout,
       animator: WobbleAnimator()
     )
diff --git a/Examples/CollectionKitExamples/HeaderExample/HeaderExampleViewController.swift b/Examples/CollectionKitExamples/HeaderExample/HeaderExampleViewController.swift
index c3107dd..f373dc2 100644
--- a/Examples/CollectionKitExamples/HeaderExample/HeaderExampleViewController.swift
+++ b/Examples/CollectionKitExamples/HeaderExample/HeaderExampleViewController.swift
@@ -50,13 +50,12 @@ class HeaderExampleViewController: CollectionViewController {
     }
 
     let provider = ComposedHeaderProvider(
-      headerViewSource: ClosureViewSource(
-        viewUpdater: { (view: UILabel, data, index) in
+      headerViewSource: { (view: UILabel, data, index) in
           view.backgroundColor = UIColor.darkGray
           view.textColor = .white
           view.textAlignment = .center
           view.text = "Header \(data.index)"
-      }),
+      },
       headerSizeSource: { (index, data, maxSize) -> CGSize in
         return CGSize(width: maxSize.width, height: 40)
       },
diff --git a/Examples/CollectionKitExamples/HorizontalGalleryExample/HorizontalGalleryViewController.swift b/Examples/CollectionKitExamples/HorizontalGalleryExample/HorizontalGalleryViewController.swift
index a5a0cfc..7f9e7b9 100644
--- a/Examples/CollectionKitExamples/HorizontalGalleryExample/HorizontalGalleryViewController.swift
+++ b/Examples/CollectionKitExamples/HorizontalGalleryExample/HorizontalGalleryViewController.swift
@@ -9,19 +9,6 @@
 import UIKit
 import CollectionKit
 
-func imageSizeProvider(at: Int, data: UIImage, collectionSize: CGSize) -> CGSize {
-  var imageSize = data.size
-  if imageSize.width > collectionSize.width {
-    imageSize.height /= imageSize.width/collectionSize.width
-    imageSize.width = collectionSize.width
-  }
-  if imageSize.height > collectionSize.height {
-    imageSize.width /= imageSize.height/collectionSize.height
-    imageSize.height = collectionSize.height
-  }
-  return imageSize
-}
-
 class HorizontalGalleryViewController: CollectionViewController {
   override func viewDidLoad() {
     super.viewDidLoad()
@@ -30,7 +17,7 @@ class HorizontalGalleryViewController: CollectionViewController {
 
     let visibleFrameInsets = UIEdgeInsets(top: 0, left: -100, bottom: 0, right: -100)
     provider = BasicProvider(
-      dataSource: ArrayDataSource(data: testImages),
+      dataSource: testImages,
       viewSource: ClosureViewSource(viewGenerator: { (data, index) -> UIImageView in
         let view = UIImageView()
         view.layer.cornerRadius = 5
@@ -39,7 +26,7 @@ class HorizontalGalleryViewController: CollectionViewController {
       }, viewUpdater: { (view: UIImageView, data: UIImage, at: Int) in
         view.image = data
       }),
-      sizeSource: imageSizeProvider,
+      sizeSource: UIImageSizeSource(),
       layout: WaterfallLayout(columns: 2, spacing: 10).transposed().insetVisibleFrame(by: visibleFrameInsets),
       animator: WobbleAnimator()
     )
diff --git a/Examples/CollectionKitExamples/ReloadAnimationExample/ReloadAnimationViewController.swift b/Examples/CollectionKitExamples/ReloadAnimationExample/ReloadAnimationViewController.swift
index 96dbd63..be042e9 100644
--- a/Examples/CollectionKitExamples/ReloadAnimationExample/ReloadAnimationViewController.swift
+++ b/Examples/CollectionKitExamples/ReloadAnimationExample/ReloadAnimationViewController.swift
@@ -111,13 +111,13 @@ class ReloadAnimationViewController: CollectionViewController {
 
     provider = BasicProvider(
       dataSource: dataSource,
-      viewSource: ClosureViewSource(viewUpdater: { (view: SquareView, data: Int, index: Int) in
+      viewSource: { (view: SquareView, data: Int, index: Int) in
         view.backgroundColor = UIColor(hue: CGFloat(data) / 30,
                                        saturation: 0.68,
                                        brightness: 0.98,
                                        alpha: 1)
         view.text = "\(data)"
-      }),
+      },
       sizeSource: { (index, data, _) in
         return CGSize(width: 80, height: 80)
       },
diff --git a/Examples/CollectionKitExamples/ReloadDataExample/ReloadDataViewController.swift b/Examples/CollectionKitExamples/ReloadDataExample/ReloadDataViewController.swift
index cb39a28..1dc51b8 100644
--- a/Examples/CollectionKitExamples/ReloadDataExample/ReloadDataViewController.swift
+++ b/Examples/CollectionKitExamples/ReloadDataExample/ReloadDataViewController.swift
@@ -38,13 +38,13 @@ class ReloadDataViewController: CollectionViewController {
 
     provider = BasicProvider(
       dataSource: dataSource,
-      viewSource: ClosureViewSource(viewUpdater: { (view: SquareView, data: Int, index: Int) in
+      viewSource: { (view: SquareView, data: Int, index: Int) in
         view.backgroundColor = UIColor(hue: CGFloat(data) / 30,
                                        saturation: 0.68,
                                        brightness: 0.98,
                                        alpha: 1)
         view.text = "\(data)"
-      }),
+      },
       sizeSource: { (index, data, _) in
         return CGSize(width: 80, height: data % 3 == 0 ? 120 : 80)
       },
diff --git a/Examples/CollectionKitExamples/ViewController.swift b/Examples/CollectionKitExamples/ViewController.swift
index 947aa80..8a20ea9 100644
--- a/Examples/CollectionKitExamples/ViewController.swift
+++ b/Examples/CollectionKitExamples/ViewController.swift
@@ -33,11 +33,10 @@ class ViewController: CollectionViewController {
     super.viewDidLoad()
 
     let examplesSection = BasicProvider(
-      dataSource: ArrayDataSource(data: examples),
-      viewSource: ClosureViewSource(viewUpdater: {
-        (view: ExampleView, data: (String, UIViewController.Type), at: Int) in
+      dataSource: examples,
+      viewSource: { (view: ExampleView, data: (String, UIViewController.Type), at: Int) in
         view.populate(title: data.0, contentViewControllerType: data.1)
-      }),
+      },
       sizeSource: { (_, _, size) -> CGSize in
         return CGSize(width: size.width, height: max(360, UIScreen.main.bounds.height * 0.7))
       },
diff --git a/Sources/Layout/LayoutContext.swift b/Sources/Layout/LayoutContext.swift
index ad1b553..35fbc42 100644
--- a/Sources/Layout/LayoutContext.swift
+++ b/Sources/Layout/LayoutContext.swift
@@ -13,5 +13,5 @@ public protocol LayoutContext {
   var numberOfItems: Int { get }
   func data(at: Int) -> Any
   func identifier(at: Int) -> String
-  func size(at: Int, collectionSize: CGSize) -> CGSize
+  func size(at index: Int, collectionSize: CGSize) -> CGSize
 }
diff --git a/Sources/Other/Deprecated.swift b/Sources/Other/Deprecated.swift
index aa2705e..1ac7666 100644
--- a/Sources/Other/Deprecated.swift
+++ b/Sources/Other/Deprecated.swift
@@ -78,7 +78,7 @@ extension BasicProvider {
                           dataProvider: DataSource<Data>,
                           viewProvider: ViewSource<Data, View>,
                           layout: Layout = FlowLayout(),
-                          sizeProvider: @escaping SizeSource<Data> = defaultSizeSource,
+                          sizeProvider: SizeSource<Data> = SizeSource<Data>(),
                           presenter: Animator? = nil,
                           willReloadHandler: (() -> Void)? = nil,
                           didReloadHandler: (() -> Void)? = nil,
@@ -98,7 +98,7 @@ extension BasicProvider {
                           viewGenerator: ((Data, Int) -> View)? = nil,
                           viewUpdater: @escaping (View, Data, Int) -> Void,
                           layout: Layout = FlowLayout(),
-                          sizeProvider: @escaping SizeSource<Data> = defaultSizeSource,
+                          sizeProvider: SizeSource<Data> = SizeSource<Data>(),
                           presenter: Animator? = nil,
                           willReloadHandler: (() -> Void)? = nil,
                           didReloadHandler: (() -> Void)? = nil,
@@ -118,7 +118,7 @@ extension BasicProvider {
                           viewGenerator: ((Data, Int) -> View)? = nil,
                           viewUpdater: @escaping (View, Data, Int) -> Void,
                           layout: Layout = FlowLayout(),
-                          sizeProvider: @escaping SizeSource<Data> = defaultSizeSource,
+                          sizeProvider: SizeSource<Data> = SizeSource<Data>(),
                           presenter: Animator? = nil,
                           willReloadHandler: (() -> Void)? = nil,
                           didReloadHandler: (() -> Void)? = nil,
diff --git a/Sources/Other/SizeSource.swift b/Sources/Other/SizeSource.swift
index be1fb22..60e00d5 100644
--- a/Sources/Other/SizeSource.swift
+++ b/Sources/Other/SizeSource.swift
@@ -8,8 +8,42 @@
 
 import UIKit
 
-public typealias SizeSource<Data> = (Int, Data, CGSize) -> CGSize
+public typealias ClosureSizeSourceFn<Data> = (Int, Data, CGSize) -> CGSize
 
-public func defaultSizeSource<Data>(at: Int, data: Data, collectionSize: CGSize) -> CGSize {
-  return collectionSize
+open class SizeSource<Data> {
+
+  public init() {}
+
+  // override point for subclass
+  open func size(at index: Int, data: Data, collectionSize: CGSize) -> CGSize {
+    return collectionSize
+  }
+}
+
+open class UIImageSizeSource: SizeSource<UIImage> {
+  open override func size(at index: Int, data: UIImage, collectionSize: CGSize) -> CGSize {
+    var imageSize = data.size
+    if imageSize.width > collectionSize.width {
+      imageSize.height /= imageSize.width / collectionSize.width
+      imageSize.width = collectionSize.width
+    }
+    if imageSize.height > collectionSize.height {
+      imageSize.width /= imageSize.height / collectionSize.height
+      imageSize.height = collectionSize.height
+    }
+    return imageSize
+  }
+}
+
+open class ClosureSizeSource<Data>: SizeSource<Data> {
+  open var sizeSource: ClosureSizeSourceFn<Data>
+
+  public init(sizeSource: @escaping ClosureSizeSourceFn<Data>) {
+    self.sizeSource = sizeSource
+    super.init()
+  }
+
+  open override func size(at index: Int, data: Data, collectionSize: CGSize) -> CGSize {
+    return sizeSource(index, data, collectionSize)
+  }
 }
diff --git a/Sources/Provider/BasicProvider+Convenience.swift b/Sources/Provider/BasicProvider+Convenience.swift
new file mode 100644
index 0000000..dfa2600
--- /dev/null
+++ b/Sources/Provider/BasicProvider+Convenience.swift
@@ -0,0 +1,162 @@
+//
+//  BasicProvider+Convenience.swift
+//  CollectionKit
+//
+//  Created by Luke Zhao on 2018-10-15.
+//
+
+extension BasicProvider {
+  public convenience init(identifier: String? = nil,
+                          dataSource: DataSource<Data>,
+                          viewSource: @escaping ViewUpdaterFn<Data, View>,
+                          sizeSource: SizeSource<Data>,
+                          layout: Layout = FlowLayout(),
+                          animator: Animator? = nil,
+                          tapHandler: TapHandler? = nil) {
+    self.init(identifier: identifier,
+              dataSource: dataSource,
+              viewSource: ClosureViewSource(viewUpdater: viewSource),
+              sizeSource: sizeSource,
+              layout: layout,
+              animator: animator,
+              tapHandler: tapHandler)
+  }
+
+  public convenience init(identifier: String? = nil,
+                          dataSource: DataSource<Data>,
+                          viewSource: ViewSource<Data, View>,
+                          sizeSource: @escaping ClosureSizeSourceFn<Data>,
+                          layout: Layout = FlowLayout(),
+                          animator: Animator? = nil,
+                          tapHandler: TapHandler? = nil) {
+    self.init(identifier: identifier,
+              dataSource: dataSource,
+              viewSource: viewSource,
+              sizeSource: ClosureSizeSource(sizeSource: sizeSource),
+              layout: layout,
+              animator: animator,
+              tapHandler: tapHandler)
+  }
+
+  public convenience init(identifier: String? = nil,
+                          dataSource: DataSource<Data>,
+                          viewSource: @escaping ViewUpdaterFn<Data, View>,
+                          sizeSource: @escaping ClosureSizeSourceFn<Data>,
+                          layout: Layout = FlowLayout(),
+                          animator: Animator? = nil,
+                          tapHandler: TapHandler? = nil) {
+    self.init(identifier: identifier,
+              dataSource: dataSource,
+              viewSource: ClosureViewSource(viewUpdater: viewSource),
+              sizeSource: ClosureSizeSource(sizeSource: sizeSource),
+              layout: layout,
+              animator: animator,
+              tapHandler: tapHandler)
+  }
+
+  public convenience init(identifier: String? = nil,
+                          dataSource: [Data],
+                          viewSource: ViewSource<Data, View>,
+                          sizeSource: SizeSource<Data>,
+                          layout: Layout = FlowLayout(),
+                          animator: Animator? = nil,
+                          tapHandler: TapHandler? = nil) {
+    self.init(identifier: identifier,
+              dataSource: ArrayDataSource(data: dataSource),
+              viewSource: viewSource,
+              sizeSource: sizeSource,
+              layout: layout,
+              animator: animator,
+              tapHandler: tapHandler)
+  }
+
+  public convenience init(identifier: String? = nil,
+                          dataSource: [Data],
+                          viewSource: @escaping ViewUpdaterFn<Data, View>,
+                          sizeSource: SizeSource<Data>,
+                          layout: Layout = FlowLayout(),
+                          animator: Animator? = nil,
+                          tapHandler: TapHandler? = nil) {
+    self.init(identifier: identifier,
+              dataSource: ArrayDataSource(data: dataSource),
+              viewSource: ClosureViewSource(viewUpdater: viewSource),
+              sizeSource: sizeSource,
+              layout: layout,
+              animator: animator,
+              tapHandler: tapHandler)
+  }
+
+  public convenience init(identifier: String? = nil,
+                          dataSource: [Data],
+                          viewSource: ViewSource<Data, View>,
+                          sizeSource: @escaping ClosureSizeSourceFn<Data>,
+                          layout: Layout = FlowLayout(),
+                          animator: Animator? = nil,
+                          tapHandler: TapHandler? = nil) {
+    self.init(identifier: identifier,
+              dataSource: ArrayDataSource(data: dataSource),
+              viewSource: viewSource,
+              sizeSource: ClosureSizeSource(sizeSource: sizeSource),
+              layout: layout,
+              animator: animator,
+              tapHandler: tapHandler)
+  }
+
+  public convenience init(identifier: String? = nil,
+                          dataSource: [Data],
+                          viewSource: @escaping ViewUpdaterFn<Data, View>,
+                          sizeSource: @escaping ClosureSizeSourceFn<Data>,
+                          layout: Layout = FlowLayout(),
+                          animator: Animator? = nil,
+                          tapHandler: TapHandler? = nil) {
+    self.init(identifier: identifier,
+              dataSource: ArrayDataSource(data: dataSource),
+              viewSource: ClosureViewSource(viewUpdater: viewSource),
+              sizeSource: ClosureSizeSource(sizeSource: sizeSource),
+              layout: layout,
+              animator: animator,
+              tapHandler: tapHandler)
+  }
+
+  public convenience init(identifier: String? = nil,
+                          dataSource: [Data],
+                          viewSource: @escaping ViewUpdaterFn<Data, View>,
+                          layout: Layout = FlowLayout(),
+                          animator: Animator? = nil,
+                          tapHandler: TapHandler? = nil) {
+    self.init(identifier: identifier,
+              dataSource: ArrayDataSource(data: dataSource),
+              viewSource: ClosureViewSource(viewUpdater: viewSource),
+              layout: layout,
+              animator: animator,
+              tapHandler: tapHandler)
+  }
+
+  public convenience init(identifier: String? = nil,
+                          dataSource: [Data],
+                          viewSource: ViewSource<Data, View>,
+                          layout: Layout = FlowLayout(),
+                          animator: Animator? = nil,
+                          tapHandler: TapHandler? = nil) {
+    self.init(identifier: identifier,
+              dataSource: ArrayDataSource(data: dataSource),
+              viewSource: viewSource,
+              layout: layout,
+              animator: animator,
+              tapHandler: tapHandler)
+  }
+
+  public convenience init(identifier: String? = nil,
+                          dataSource: DataSource<Data>,
+                          viewSource: @escaping ViewUpdaterFn<Data, View>,
+                          layout: Layout = FlowLayout(),
+                          animator: Animator? = nil,
+                          tapHandler: TapHandler? = nil) {
+    self.init(identifier: identifier,
+              dataSource: dataSource,
+              viewSource: ClosureViewSource(viewUpdater: viewSource),
+              layout: layout,
+              animator: animator,
+              tapHandler: tapHandler)
+  }
+}
diff --git a/Sources/Provider/BasicProvider.swift b/Sources/Provider/BasicProvider.swift
index a984d03..027334d 100644
--- a/Sources/Provider/BasicProvider.swift
+++ b/Sources/Provider/BasicProvider.swift
@@ -37,7 +37,7 @@ open class BasicProvider<Data, View: UIView>: ItemProvider, LayoutableProvider,
   public init(identifier: String? = nil,
               dataSource: DataSource<Data>,
               viewSource: ViewSource<Data, View>,
-              sizeSource: @escaping SizeSource<Data> = defaultSizeSource,
+              sizeSource: SizeSource<Data> = SizeSource<Data>(),
               layout: Layout = FlowLayout(),
               animator: Animator? = nil,
               tapHandler: TapHandler? = nil) {
@@ -95,7 +95,7 @@ struct BasicProviderLayoutContext<Data>: LayoutContext {
   func identifier(at: Int) -> String {
     return dataSource.identifier(at: at)
   }
-  func size(at: Int, collectionSize: CGSize) -> CGSize {
-    return sizeSource(at, dataSource.data(at: at), collectionSize)
+  func size(at index: Int, collectionSize: CGSize) -> CGSize {
+    return sizeSource.size(at: index, data: dataSource.data(at: index), collectionSize: collectionSize)
   }
 }
diff --git a/Sources/Provider/ComposedHeaderProvider+Convenience.swift b/Sources/Provider/ComposedHeaderProvider+Convenience.swift
new file mode 100644
index 0000000..1ef2f6f
--- /dev/null
+++ b/Sources/Provider/ComposedHeaderProvider+Convenience.swift
@@ -0,0 +1,56 @@
+//
+//  ComposedHeaderProvider+Convenience.swift
+//  CollectionKit
+//
+//  Created by Luke Zhao on 2018-10-15.
+//
+
+extension ComposedHeaderProvider {
+  public convenience init(identifier: String? = nil,
+                          layout: Layout = FlowLayout(),
+                          animator: Animator? = nil,
+                          headerViewSource: @escaping ViewUpdaterFn<HeaderData, HeaderView>,
+                          headerSizeSource: @escaping ClosureSizeSourceFn<HeaderData>,
+                          sections: [Provider] = [],
+                          tapHandler: TapHandler? = nil) {
+    self.init(identifier: identifier,
+              layout: layout,
+              animator: animator,
+              headerViewSource: ClosureViewSource(viewUpdater: headerViewSource),
+              headerSizeSource: ClosureSizeSource(sizeSource: headerSizeSource),
+              sections: sections,
+              tapHandler: tapHandler)
+  }
+
+  public convenience init(identifier: String? = nil,
+                          layout: Layout = FlowLayout(),
+                          animator: Animator? = nil,
+                          headerViewSource: HeaderViewSource,
+                          headerSizeSource: @escaping ClosureSizeSourceFn<HeaderData>,
+                          sections: [Provider] = [],
+                          tapHandler: TapHandler? = nil) {
+    self.init(identifier: identifier,
+              layout: layout,
+              animator: animator,
+              headerViewSource: headerViewSource,
+              headerSizeSource: ClosureSizeSource(sizeSource: headerSizeSource),
+              sections: sections,
+              tapHandler: tapHandler)
+  }
+
+  public convenience init(identifier: String? = nil,
+                          layout: Layout = FlowLayout(),
+                          animator: Animator? = nil,
+                          headerViewSource: @escaping ViewUpdaterFn<HeaderData, HeaderView>,
+                          headerSizeSource: HeaderSizeSource,
+                          sections: [Provider] = [],
+                          tapHandler: TapHandler? = nil) {
+    self.init(identifier: identifier,
+              layout: layout,
+              animator: animator,
+              headerViewSource: ClosureViewSource(viewUpdater: headerViewSource),
+              headerSizeSource: headerSizeSource,
+              sections: sections,
+              tapHandler: tapHandler)
+  }
+}
diff --git a/Sources/Provider/ComposedHeaderProvider.swift b/Sources/Provider/ComposedHeaderProvider.swift
index 43c3740..71bc906 100644
--- a/Sources/Provider/ComposedHeaderProvider.swift
+++ b/Sources/Provider/ComposedHeaderProvider.swift
@@ -73,7 +73,7 @@ open class ComposedHeaderProvider<HeaderView: UIView>:
               layout: Layout = FlowLayout(),
               animator: Animator? = nil,
               headerViewSource: HeaderViewSource,
-              headerSizeSource: @escaping HeaderSizeSource,
+              headerSizeSource: HeaderSizeSource,
               sections: [Provider] = [],
               tapHandler: TapHandler? = nil) {
     self.animator = animator
@@ -182,12 +182,14 @@ open class ComposedHeaderProvider<HeaderView: UIView>:
         return sectionIdentifier
       }
     }
-    func size(at: Int, collectionSize: CGSize) -> CGSize {
-      if at % 2 == 0 {
-        return headerSizeSource(at / 2, data(at: at) as! HeaderData, collectionSize)
+    func size(at index: Int, collectionSize: CGSize) -> CGSize {
+      if index % 2 == 0 {
+        return headerSizeSource.size(at: index / 2,
+                                     data: data(at: index) as! HeaderData,
+                                     collectionSize: collectionSize)
       } else {
-        sections[at / 2].layout(collectionSize: collectionSize)
-        return sections[at / 2].contentSize
+        sections[index / 2].layout(collectionSize: collectionSize)
+        return sections[index / 2].contentSize
       }
     }
   }
diff --git a/Sources/ViewSource/ClosureViewSource.swift b/Sources/ViewSource/ClosureViewSource.swift
index c7f5758..5222894 100644
--- a/Sources/ViewSource/ClosureViewSource.swift
+++ b/Sources/ViewSource/ClosureViewSource.swift
@@ -8,11 +8,15 @@
 
 import UIKit
 
+public typealias ViewUpdaterFn<Data, View> = (View, Data, Int) -> Void
+public typealias ViewGeneratorFn<Data, View> = (Data, Int) -> View
+
 public class ClosureViewSource<Data, View>: ViewSource<Data, View> where View: UIView {
-  public var viewUpdater: (View, Data, Int) -> Void
-  public var viewGenerator: ((Data, Int) -> View)?
+  public var viewGenerator: ViewGeneratorFn<Data, View>?
+  public var viewUpdater: ViewUpdaterFn<Data, View>
 
-  public init(viewGenerator: ((Data, Int) -> View)? = nil, viewUpdater: @escaping (View, Data, Int) -> Void) {
+  public init(viewGenerator: ViewGeneratorFn<Data, View>? = nil,
+              viewUpdater: @escaping ViewUpdaterFn<Data, View>) {
     self.viewGenerator = viewGenerator
     self.viewUpdater = viewUpdater
     super.init()