WIP Reimplemented Workers using side effects.

Issues:
 - Fixes #82, which is the Kotlin half of square/workflow#1021.
 - Fixes #92.
 - Fixes square/workflow#1197.

samples/dungeon/app/src/main/java/com/squareup/sample/dungeon/DungeonAppWorkflow.kt

@@ -26,6 +26,7 @@ import com.squareup.workflow.Snapshot
 import com.squareup.workflow.StatefulWorkflow
 import com.squareup.workflow.action
 import com.squareup.workflow.renderChild
+import com.squareup.workflow.runningWorker
 import com.squareup.workflow.ui.WorkflowUiExperimentalApi
 import com.squareup.workflow.ui.modal.AlertContainerScreen
 

samples/dungeon/app/src/main/java/com/squareup/sample/dungeon/GameSessionWorkflow.kt

@@ -31,6 +31,7 @@ import com.squareup.workflow.WorkflowAction
 import com.squareup.workflow.WorkflowAction.Companion.noAction
 import com.squareup.workflow.WorkflowAction.Updater
 import com.squareup.workflow.action
+import com.squareup.workflow.runningWorker
 import com.squareup.workflow.ui.WorkflowUiExperimentalApi
 import com.squareup.workflow.ui.modal.AlertContainerScreen
 import com.squareup.workflow.ui.modal.AlertScreen

samples/dungeon/common/src/main/java/com/squareup/sample/dungeon/AiWorkflow.kt

@@ -28,6 +28,7 @@ import com.squareup.workflow.Snapshot
 import com.squareup.workflow.StatefulWorkflow
 import com.squareup.workflow.Worker
 import com.squareup.workflow.action
+import com.squareup.workflow.runningWorker
 import com.squareup.workflow.transform
 import kotlinx.coroutines.ExperimentalCoroutinesApi
 import kotlinx.coroutines.flow.transform

samples/dungeon/common/src/main/java/com/squareup/sample/dungeon/GameWorkflow.kt

@@ -36,6 +36,7 @@ import com.squareup.workflow.StatefulWorkflow
 import com.squareup.workflow.Worker
 import com.squareup.workflow.action
 import com.squareup.workflow.renderChild
+import com.squareup.workflow.runningWorker
 import kotlinx.coroutines.delay
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.flow

samples/dungeon/timemachine-shakeable/src/main/java/com/squareup/sample/timemachine/shakeable/ShakeableTimeMachineWorkflow.kt

@@ -28,6 +28,7 @@ import com.squareup.workflow.StatefulWorkflow
 import com.squareup.workflow.WorkflowAction
 import com.squareup.workflow.WorkflowAction.Updater
 import com.squareup.workflow.action
+import com.squareup.workflow.runningWorker
 import kotlin.time.Duration
 import kotlin.time.ExperimentalTime
 

samples/hello-terminal/hello-terminal-app/src/main/java/com/squareup/sample/helloterminal/BlinkingCursorWorkflow.kt

@@ -20,6 +20,7 @@ import com.squareup.workflow.Snapshot
 import com.squareup.workflow.StatefulWorkflow
 import com.squareup.workflow.Worker
 import com.squareup.workflow.action
+import com.squareup.workflow.runningWorker
 import kotlinx.coroutines.delay
 
 /**

samples/hello-terminal/hello-terminal-app/src/main/java/com/squareup/sample/helloterminal/HelloTerminalWorkflow.kt

@@ -29,6 +29,7 @@ import com.squareup.workflow.StatefulWorkflow
 import com.squareup.workflow.WorkflowAction
 import com.squareup.workflow.action
 import com.squareup.workflow.renderChild
+import com.squareup.workflow.runningWorker
 
 private typealias HelloTerminalAction = WorkflowAction<TerminalProps, State, ExitCode>
 

samples/tictactoe/common/src/main/java/com/squareup/sample/authworkflow/AuthWorkflow.kt

@@ -36,6 +36,7 @@ import com.squareup.workflow.StatefulWorkflow
 import com.squareup.workflow.Workflow
 import com.squareup.workflow.WorkflowAction
 import com.squareup.workflow.WorkflowAction.Updater
+import com.squareup.workflow.runningWorker
 import com.squareup.workflow.rx2.asWorker
 import com.squareup.workflow.ui.WorkflowUiExperimentalApi
 import com.squareup.workflow.ui.backstack.BackStackScreen

samples/tictactoe/common/src/main/java/com/squareup/sample/gameworkflow/RunGameWorkflow.kt

@@ -46,6 +46,7 @@ import com.squareup.workflow.StatefulWorkflow
 import com.squareup.workflow.Workflow
 import com.squareup.workflow.WorkflowAction
 import com.squareup.workflow.WorkflowAction.Updater
+import com.squareup.workflow.runningWorker
 import com.squareup.workflow.rx2.asWorker
 import com.squareup.workflow.ui.WorkflowUiExperimentalApi
 import com.squareup.workflow.ui.modal.AlertContainerScreen

workflow-core/api/workflow-core.api

@@ -22,6 +22,7 @@ public final class com/squareup/workflow/ImpostorWorkflow$DefaultImpls {
 public abstract class com/squareup/workflow/LifecycleWorker : com/squareup/workflow/Worker {
 	public fun <init> ()V
 	public fun doesSameWorkAs (Lcom/squareup/workflow/Worker;)Z
+	public final fun getOutputType ()Lkotlin/reflect/KType;
 	public fun onStarted ()V
 	public fun onStopped ()V
 	public final fun run ()Lkotlinx/coroutines/flow/Flow;
@@ -33,14 +34,12 @@ public abstract interface class com/squareup/workflow/RenderContext {
 	public abstract fun onEvent (Lkotlin/jvm/functions/Function1;)Lkotlin/jvm/functions/Function1;
 	public abstract fun renderChild (Lcom/squareup/workflow/Workflow;Ljava/lang/Object;Ljava/lang/String;Lkotlin/jvm/functions/Function1;)Ljava/lang/Object;
 	public abstract fun runningSideEffect (Ljava/lang/String;Lkotlin/jvm/functions/Function1;)V
-	public abstract fun runningWorker (Lcom/squareup/workflow/Worker;Ljava/lang/String;Lkotlin/jvm/functions/Function1;)V
 }
 
 public final class com/squareup/workflow/RenderContext$DefaultImpls {
 	public static fun makeActionSink (Lcom/squareup/workflow/RenderContext;)Lcom/squareup/workflow/Sink;
 	public static fun onEvent (Lcom/squareup/workflow/RenderContext;Lkotlin/jvm/functions/Function1;)Lkotlin/jvm/functions/Function1;
 	public static synthetic fun renderChild$default (Lcom/squareup/workflow/RenderContext;Lcom/squareup/workflow/Workflow;Ljava/lang/Object;Ljava/lang/String;Lkotlin/jvm/functions/Function1;ILjava/lang/Object;)Ljava/lang/Object;
-	public static synthetic fun runningWorker$default (Lcom/squareup/workflow/RenderContext;Lcom/squareup/workflow/Worker;Ljava/lang/String;Lkotlin/jvm/functions/Function1;ILjava/lang/Object;)V
 }
 
 public abstract interface class com/squareup/workflow/Sink {
@@ -108,13 +107,15 @@ public abstract class com/squareup/workflow/StatelessWorkflow : com/squareup/wor
 public final class com/squareup/workflow/TypedWorker : com/squareup/workflow/Worker {
 	public fun <init> (Lkotlin/reflect/KType;Lkotlinx/coroutines/flow/Flow;)V
 	public fun doesSameWorkAs (Lcom/squareup/workflow/Worker;)Z
+	public fun getOutputType ()Lkotlin/reflect/KType;
 	public fun run ()Lkotlinx/coroutines/flow/Flow;
 	public fun toString ()Ljava/lang/String;
 }
 
 public abstract interface class com/squareup/workflow/Worker {
 	public static final field Companion Lcom/squareup/workflow/Worker$Companion;
 	public abstract fun doesSameWorkAs (Lcom/squareup/workflow/Worker;)Z
+	public abstract fun getOutputType ()Lkotlin/reflect/KType;
 	public abstract fun run ()Lkotlinx/coroutines/flow/Flow;
 }
 
@@ -127,6 +128,7 @@ public final class com/squareup/workflow/Worker$Companion {
 
 public final class com/squareup/workflow/Worker$DefaultImpls {
 	public static fun doesSameWorkAs (Lcom/squareup/workflow/Worker;Lcom/squareup/workflow/Worker;)Z
+	public static fun getOutputType (Lcom/squareup/workflow/Worker;)Lkotlin/reflect/KType;
 }
 
 public abstract interface class com/squareup/workflow/Workflow {
@@ -217,8 +219,6 @@ public final class com/squareup/workflow/Workflows {
 	public static final fun invoke (Lcom/squareup/workflow/EventHandler;)V
 	public static final fun makeEventSink (Lcom/squareup/workflow/RenderContext;Lkotlin/jvm/functions/Function2;)Lcom/squareup/workflow/Sink;
 	public static final fun mapRendering (Lcom/squareup/workflow/Workflow;Lkotlin/jvm/functions/Function1;)Lcom/squareup/workflow/Workflow;
-	public static final fun onWorkerOutput (Lcom/squareup/workflow/RenderContext;Lcom/squareup/workflow/Worker;Ljava/lang/String;Lkotlin/jvm/functions/Function1;)V
-	public static synthetic fun onWorkerOutput$default (Lcom/squareup/workflow/RenderContext;Lcom/squareup/workflow/Worker;Ljava/lang/String;Lkotlin/jvm/functions/Function1;ILjava/lang/Object;)V
 	public static final fun renderChild (Lcom/squareup/workflow/RenderContext;Lcom/squareup/workflow/Workflow;Ljava/lang/Object;Ljava/lang/String;)Ljava/lang/Object;
 	public static final fun renderChild (Lcom/squareup/workflow/RenderContext;Lcom/squareup/workflow/Workflow;Ljava/lang/String;)Ljava/lang/Object;
 	public static final fun renderChild (Lcom/squareup/workflow/RenderContext;Lcom/squareup/workflow/Workflow;Ljava/lang/String;Lkotlin/jvm/functions/Function1;)Ljava/lang/Object;
@@ -227,6 +227,7 @@ public final class com/squareup/workflow/Workflows {
 	public static synthetic fun renderChild$default (Lcom/squareup/workflow/RenderContext;Lcom/squareup/workflow/Workflow;Ljava/lang/String;Lkotlin/jvm/functions/Function1;ILjava/lang/Object;)Ljava/lang/Object;
 	public static final fun rendering (Lcom/squareup/workflow/Workflow$Companion;Ljava/lang/Object;)Lcom/squareup/workflow/Workflow;
 	public static final fun runningWorker (Lcom/squareup/workflow/RenderContext;Lcom/squareup/workflow/Worker;Ljava/lang/String;)V
+	public static final fun runningWorker (Lcom/squareup/workflow/RenderContext;Lcom/squareup/workflow/Worker;Lkotlin/reflect/KType;Ljava/lang/String;Lkotlin/jvm/functions/Function1;)V
 	public static synthetic fun runningWorker$default (Lcom/squareup/workflow/RenderContext;Lcom/squareup/workflow/Worker;Ljava/lang/String;ILjava/lang/Object;)V
 	public static final fun sendAndAwaitApplication (Lcom/squareup/workflow/Sink;Lcom/squareup/workflow/WorkflowAction;Lkotlin/coroutines/Continuation;)Ljava/lang/Object;
 	public static final fun stateful (Lcom/squareup/workflow/Workflow$Companion;Ljava/lang/Object;Lkotlin/jvm/functions/Function2;)Lcom/squareup/workflow/StatefulWorkflow;

workflow-core/src/main/java/com/squareup/workflow/LifecycleWorker.kt

@@ -21,6 +21,7 @@ package com.squareup.workflow
 import kotlinx.coroutines.flow.Flow
 import kotlinx.coroutines.flow.flow
 import kotlinx.coroutines.suspendCancellableCoroutine
+import kotlin.reflect.KType
 
 /**
  * [Worker] that performs some action when the worker is started and/or stopped.
@@ -30,6 +31,8 @@ import kotlinx.coroutines.suspendCancellableCoroutine
  */
 abstract class LifecycleWorker : Worker<Nothing> {
 
+  final override val outputType: KType? get() = null
+
   /**
    * Called when this worker is started. It is executed concurrently with the parent workflow –
    * the first render pass that starts this worker *will not* wait for this method to return, and
@@ -73,6 +76,5 @@ abstract class LifecycleWorker : Worker<Nothing> {
   /**
    * Equates [LifecycleWorker]s that have the same concrete class.
    */
-  override fun doesSameWorkAs(otherWorker: Worker<*>): Boolean =
-    this::class == otherWorker::class
+  override fun doesSameWorkAs(otherWorker: Worker<*>): Boolean = true
 }

workflow-core/src/main/java/com/squareup/workflow/RenderContext.kt

@@ -21,6 +21,8 @@ package com.squareup.workflow
 
 import com.squareup.workflow.WorkflowAction.Companion.noAction
 import com.squareup.workflow.WorkflowAction.Updater
+import kotlin.reflect.KType
+import kotlin.reflect.typeOf
 
 /**
  * Facilities for a [Workflow] to interact with other [Workflow]s and the outside world from inside
@@ -109,19 +111,6 @@ interface RenderContext<out PropsT, StateT, in OutputT> {
     handler: (ChildOutputT) -> WorkflowAction<PropsT, StateT, OutputT>
   ): ChildRenderingT
 
-  /**
-   * Ensures [worker] is running. When the [Worker] emits an output, [handler] is called
-   * to determine the [WorkflowAction] to take. When the worker finishes, nothing happens (although
-   * another render pass may be triggered).
-   *
-   * @param key An optional string key that is used to distinguish between identical [Worker]s.
-   */
-  fun <T> runningWorker(
-    worker: Worker<T>,
-    key: String = "",
-    handler: (T) -> WorkflowAction<PropsT, StateT, OutputT>
-  )
-
   /**
    * Ensures [sideEffect] is running with the given [key].
    *
@@ -202,6 +191,48 @@ fun <PropsT, StateT, OutputT> RenderContext<PropsT, StateT, OutputT>.runningWork
   }
 }
 
+/**
+ * Ensures [worker] is running. When the [Worker] emits an output, [handler] is called
+ * to determine the [WorkflowAction] to take. When the worker finishes, nothing happens (although
+ * another render pass may be triggered).
+ *
+ * @param key An optional string key that is used to distinguish between identical [Worker]s.
+ */
+@OptIn(ExperimentalStdlibApi::class)
+/* ktlint-disable parameter-list-wrapping */
+inline fun <reified T, reified W : Worker<T>, PropsT, StateT, OutputT>
+    RenderContext<PropsT, StateT, OutputT>.runningWorker(
+  worker: W,
+  key: String = "",
+  noinline handler: (T) -> WorkflowAction<PropsT, StateT, OutputT>
+) {
+/* ktlint-enable parameter-list-wrapping */
+  runningWorker(worker, typeOf<W>(), key, handler)
+}
+
+/**
+ * Ensures [worker] is running. When the [Worker] emits an output, [handler] is called
+ * to determine the [WorkflowAction] to take. When the worker finishes, nothing happens (although
+ * another render pass may be triggered).
+ *
+ * @param workerType `typeOf<W>()`
+ * @param key An optional string key that is used to distinguish between identical [Worker]s.
+ */
+@OptIn(ExperimentalStdlibApi::class)
+@PublishedApi
+/* ktlint-disable parameter-list-wrapping */
+internal fun <T, PropsT, StateT, OutputT>
+    RenderContext<PropsT, StateT, OutputT>.runningWorker(
+  worker: Worker<T>,
+  workerType: KType,
+  key: String = "",
+  handler: (T) -> WorkflowAction<PropsT, StateT, OutputT>
+) {
+/* ktlint-enable parameter-list-wrapping */
+  val workerWorkflow = WorkerWorkflow<T>(workerType, key)
+  renderChild(workerWorkflow, props = worker, key = key, handler = handler)
+}
+
 /**
  * Alternative to [RenderContext.actionSink] that allows externally defined
  * event types to be mapped to anonymous [WorkflowAction]s.
@@ -223,8 +254,8 @@ fun <EventT, PropsT, StateT, OutputT> RenderContext<PropsT, StateT, OutputT>.mak
     "Use runningWorker",
     ReplaceWith("runningWorker(worker, key, handler)", "com.squareup.workflow.runningWorker")
 )
-fun <PropsT, StateT, OutputT, T> RenderContext<PropsT, StateT, OutputT>.onWorkerOutput(
+inline fun <PropsT, StateT, OutputT, reified T> RenderContext<PropsT, StateT, OutputT>.onWorkerOutput(
   worker: Worker<T>,
   key: String = "",
-  handler: (T) -> WorkflowAction<PropsT, StateT, OutputT>
+  noinline handler: (T) -> WorkflowAction<PropsT, StateT, OutputT>
 ) = runningWorker(worker, key, handler)

workflow-core/src/main/java/com/squareup/workflow/Worker.kt

@@ -52,6 +52,11 @@ import kotlin.reflect.typeOf
  * See the documentation on [doesSameWorkAs] for more details on how and when workers are compared
  * and the worker lifecycle.
  *
+ * Implementations of this interface that are themselves parameterized on a type should override
+ * [outputType] to return the [KType] of their type parameter. This allows the runtime to compare
+ * workers by the output type as well as the concrete type. If [outputType] returns null, all
+ * workers of the same concrete type will be considered equivalent.
+ *
  * ## Example: Network request
  *
  * Let's say you have a network service with an API that returns a number, and you want to
@@ -117,6 +122,14 @@ import kotlin.reflect.typeOf
  */
 interface Worker<out OutputT> {
 
+  /**
+   * Should be overridden in subclasses that have their own type parameters, to allow the runtime to
+   * make use of the value of those type parameters to compare workers. Two workers of the same
+   * concrete [Worker] class will be considered equivalent if and only if their [outputType]s are
+   * also equivalent. If this property returns null, the worker will be treated as a `Worker<*>`.
+   */
+  val outputType: KType? get() = null
+
   /**
    * Returns a [Flow] to execute the work represented by this worker.
    *
@@ -165,8 +178,7 @@ interface Worker<out OutputT> {
    * that performs a network request might check that two workers are requests to the same endpoint
    * and have the same request data.
    *
-   * Most implementations of this method will check for concrete type equality, and then match
-   * on constructor parameters.
+   * Most implementations of this method should compare constructor parameters.
    *
    * E.g:
    *
@@ -179,7 +191,7 @@ interface Worker<out OutputT> {
    * }
    * ```
    */
-  fun doesSameWorkAs(otherWorker: Worker<*>): Boolean = otherWorker::class == this::class
+  fun doesSameWorkAs(otherWorker: Worker<*>): Boolean = true
 
   companion object {
 
@@ -384,23 +396,18 @@ fun <T, R> Worker<T>.transform(
 
 /**
  * A generic [Worker] implementation that defines equivalent workers as those having equivalent
- * [type]s. This is used by all the [Worker] builder functions.
+ * [outputType]s. This is used by all the [Worker] builder functions.
  */
 @PublishedApi
 internal class TypedWorker<OutputT>(
-  private val type: KType,
+  override val outputType: KType,
   private val work: Flow<OutputT>
 ) : Worker<OutputT> {
-
   override fun run(): Flow<OutputT> = work
-
-  override fun doesSameWorkAs(otherWorker: Worker<*>): Boolean =
-    otherWorker is TypedWorker && otherWorker.type == type
-
-  override fun toString(): String = "TypedWorker($type)"
+  override fun toString(): String = "TypedWorker($outputType)"
 }
 
-private class TimerWorker(
+private data class TimerWorker(
   private val delayMs: Long,
   private val key: String
 ) : Worker<Unit> {
@@ -412,17 +419,14 @@ private class TimerWorker(
 
   override fun doesSameWorkAs(otherWorker: Worker<*>): Boolean =
     otherWorker is TimerWorker && otherWorker.key == key
-
-  override fun toString(): String = "TimerWorker(delayMs=$delayMs)"
 }
 
 private object FinishedWorker : Worker<Nothing> {
   override fun run(): Flow<Nothing> = emptyFlow()
-  override fun doesSameWorkAs(otherWorker: Worker<*>): Boolean = otherWorker === FinishedWorker
   override fun toString(): String = "FinishedWorker"
 }
 
-private class WorkerWrapper<T, R>(
+private data class WorkerWrapper<T, R>(
   private val wrapped: Worker<T>,
   private val flow: Flow<R>
 ) : Worker<R> {