Makes ViewRegistry extra for experts instead of torture for the masses.

Introduces `ViewRendering` and `AndroidContainerView`.

 * `ViewRendering` is a marker interface in the UI common core, identifying UI views.

 * `AndroidContainerView` is able to tie particular `ViewRenderings` to particular `ViewFactories`,
   and is now built into `ViewRegistry` automatically -- just like `NamedViewFactory`.

To make this work, we teach `ViewRegistry` a new trick: if it can't find a
registered `ViewFactory` for a rendering, it checks to see if the rendering
itself implements `ViewFactory`, and if so calls it directly.

Given all this, we it's no longer crucial for developers to define and provide
a `ViewRegistry`, so we get rid of all the nannying that forced them to
provide one.

Author: rjrjr

samples/hello-workflow/src/main/java/com/squareup/sample/helloworkflow/HelloView.kt

@@ -0,0 +1,16 @@
+package com.squareup.sample.helloworkflow
+
+import com.squareup.sample.helloworkflow.HelloWorkflow.Rendering
+import com.squareup.sample.helloworkflow.databinding.HelloGoodbyeLayoutBinding
+import com.squareup.workflow1.ui.AndroidViewContainer
+import com.squareup.workflow1.ui.LayoutRunner
+import com.squareup.workflow1.ui.WorkflowUiExperimentalApi
+
+@Suppress("FunctionName")
+@OptIn(WorkflowUiExperimentalApi::class)
+fun HelloView(rendering: Rendering) = AndroidViewContainer(
+  rendering,
+  LayoutRunner.bind(HelloGoodbyeLayoutBinding::inflate) { r, _ ->
+    helloMessage.text = r.message
+    helloMessage.setOnClickListener { r.onClick() }
+  })

samples/hello-workflow/src/main/java/com/squareup/sample/helloworkflow/HelloViewFactory.kt

@@ -1,54 +1,44 @@
-/*
- * 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.
- */
 package com.squareup.sample.helloworkflow
 
-import com.squareup.sample.helloworkflow.HelloWorkflow.Rendering
 import com.squareup.sample.helloworkflow.HelloWorkflow.State
 import com.squareup.sample.helloworkflow.HelloWorkflow.State.Goodbye
 import com.squareup.sample.helloworkflow.HelloWorkflow.State.Hello
 import com.squareup.workflow1.Snapshot
 import com.squareup.workflow1.StatefulWorkflow
 import com.squareup.workflow1.action
 import com.squareup.workflow1.parse
+import com.squareup.workflow1.ui.AndroidViewContainer
+import com.squareup.workflow1.ui.ViewRendering
+import com.squareup.workflow1.ui.WorkflowUiExperimentalApi
 
-object HelloWorkflow : StatefulWorkflow<Unit, State, Nothing, Rendering>() {
+object HelloWorkflow : StatefulWorkflow<Unit, State, Nothing, AndroidViewContainer<*>>() {
   enum class State {
     Hello,
     Goodbye
   }
 
+  @OptIn(WorkflowUiExperimentalApi::class)
   data class Rendering(
     val message: String,
     val onClick: () -> Unit
-  )
+  ) : ViewRendering
 
   override fun initialState(
     props: Unit,
     snapshot: Snapshot?
   ): State = snapshot?.bytes?.parse { source -> if (source.readInt() == 1) Hello else Goodbye }
-      ?: Hello
+    ?: Hello
 
   override fun render(
     props: Unit,
     state: State,
     context: RenderContext
-  ): Rendering {
-    return Rendering(
+  ): AndroidViewContainer<*> {
+    return HelloView(
+      Rendering(
         message = state.name,
         onClick = { context.actionSink.send(helloAction) }
+      )
     )
   }
 

samples/hello-workflow/src/main/java/com/squareup/sample/helloworkflow/HelloWorkflow.kt

@@ -1,36 +1,20 @@
-/*
- * 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.
- */
 package com.squareup.sample.helloworkflow
 
 import android.os.Bundle
 import androidx.appcompat.app.AppCompatActivity
 import com.squareup.workflow1.SimpleLoggingWorkflowInterceptor
+import com.squareup.workflow1.mapRendering
+import com.squareup.workflow1.ui.AndroidViewContainer
 import com.squareup.workflow1.ui.WorkflowUiExperimentalApi
 import com.squareup.workflow1.ui.ViewRegistry
 import com.squareup.workflow1.ui.WorkflowRunner
 import com.squareup.workflow1.ui.setContentWorkflow
 
-@OptIn(WorkflowUiExperimentalApi::class)
-private val viewRegistry = ViewRegistry(HelloViewFactory)
-
 @OptIn(WorkflowUiExperimentalApi::class)
 class HelloWorkflowActivity : AppCompatActivity() {
   override fun onCreate(savedInstanceState: Bundle?) {
     super.onCreate(savedInstanceState)
-    setContentWorkflow(viewRegistry) {
+    setContentWorkflow {
       WorkflowRunner.Config(
           HelloWorkflow,
           interceptors = listOf(SimpleLoggingWorkflowInterceptor())

samples/hello-workflow/src/main/java/com/squareup/sample/helloworkflow/HelloWorkflowActivity.kt

@@ -0,0 +1,29 @@
+package com.squareup.workflow1.ui
+
+import android.content.Context
+import android.view.View
+import android.view.ViewGroup
+
+@OptIn(WorkflowUiExperimentalApi::class)
+class AndroidViewContainer<V : ViewRendering>(
+  val rendering: V,
+  val viewFactory: ViewFactory<V>
+) : ViewFactory<AndroidViewContainer<V>> {
+  override val type = AndroidViewContainer::class
+
+  override fun buildView(
+    initialRendering: AndroidViewContainer<V>,
+    initialViewEnvironment: ViewEnvironment,
+    contextForNewView: Context,
+    container: ViewGroup?
+  ): View {
+    return viewFactory.buildView(
+      initialRendering.rendering, initialViewEnvironment, contextForNewView, container
+    ).also { view ->
+      val innerShowRendering = view.getShowRendering<V>()!!
+      view.bindShowRendering(initialRendering, initialViewEnvironment) { androidRendering, env ->
+        innerShowRendering(androidRendering.rendering, env)
+      }
+    }
+  }
+}

workflow-ui/core-android/src/main/java/com/squareup/workflow1/ui/AndroidViewContainer.kt

@@ -42,14 +42,7 @@ internal class CompositeViewRegistry private constructor(
 
   override fun <RenderingT : Any> getFactoryFor(
     renderingType: KClass<out RenderingT>
-  ): ViewFactory<RenderingT> = getRegistryFor(renderingType).getFactoryFor(renderingType)
-
-  private fun getRegistryFor(renderingType: KClass<out Any>): ViewRegistry {
-    return requireNotNull(registriesByKey[renderingType]) {
-      "A ${ViewFactory::class.java.name} should have been registered " +
-          "to display a $renderingType."
-    }
-  }
+  ): ViewFactory<RenderingT>? = registriesByKey[renderingType]?.getFactoryFor(renderingType)
 
   companion object {
     private fun mergeRegistries(vararg registries: ViewRegistry): Map<KClass<*>, ViewRegistry> {

workflow-ui/core-android/src/main/java/com/squareup/workflow1/ui/CompositeViewRegistry.kt

@@ -1,18 +1,3 @@
-/*
- * 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.
- */
 package com.squareup.workflow1.ui
 
 /**

workflow-ui/core-android/src/main/java/com/squareup/workflow1/ui/NamedViewFactory.kt

@@ -40,11 +40,8 @@ internal class TypedViewRegistry private constructor(
 
   override fun <RenderingT : Any> getFactoryFor(
     renderingType: KClass<out RenderingT>
-  ): ViewFactory<RenderingT> {
+  ): ViewFactory<RenderingT>? {
     @Suppress("UNCHECKED_CAST")
-    return requireNotNull(bindings[renderingType] as? ViewFactory<RenderingT>) {
-      "A ${ViewFactory::class.java.name} should have been registered " +
-          "to display a $renderingType."
-    }
+    return bindings[renderingType] as? ViewFactory<RenderingT>
   }
 }

workflow-ui/core-android/src/main/java/com/squareup/workflow1/ui/TypedViewRegistry.kt

@@ -30,17 +30,11 @@ typealias ContainerHints = ViewEnvironment
  * its children via [View.showRendering][android.view.View.showRendering] et al.
  * Allows container views to give descendants information about the context in which
  * they're drawing.
- *
- * Every [ViewEnvironment] includes a [ViewRegistry]. This allows container views to
- * make recursive [ViewRegistry.buildView] calls to build child views to show nested renderings.
  */
 @WorkflowUiExperimentalApi
-class ViewEnvironment private constructor(
-  private val map: Map<ViewEnvironmentKey<*>, Any>
+class ViewEnvironment(
+  val map: Map<ViewEnvironmentKey<*>, Any> = emptyMap()
 ) {
-  constructor(registry: ViewRegistry) :
-      this(mapOf<ViewEnvironmentKey<*>, Any>(ViewRegistry to registry))
-
   @Suppress("UNCHECKED_CAST")
   operator fun <T : Any> get(key: ViewEnvironmentKey<T>): T = map[key] as? T ?: key.default
 

workflow-ui/core-android/src/main/java/com/squareup/workflow1/ui/ViewEnvironment.kt

@@ -70,17 +70,15 @@ interface ViewRegistry {
   /**
    * This method is not for general use, use [WorkflowViewStub] instead.
    *
-   * Returns the [ViewFactory] that was registered for the given [renderingType].
-   *
-   * @throws IllegalArgumentException if no factory can be found for type [RenderingT]
+   * Returns the [ViewFactory] that was registered for the given [renderingType], or null
+   * if none was found.
    */
   fun <RenderingT : Any> getFactoryFor(
     renderingType: KClass<out RenderingT>
-  ): ViewFactory<RenderingT>
+  ): ViewFactory<RenderingT>?
 
   companion object : ViewEnvironmentKey<ViewRegistry>(ViewRegistry::class) {
-    override val default: ViewRegistry
-      get() = error("There should always be a ViewRegistry hint, this is bug in Workflow.")
+    override val default: ViewRegistry get() = ViewRegistry()
   }
 }
 
@@ -105,7 +103,7 @@ fun ViewRegistry(): ViewRegistry = TypedViewRegistry()
  * It is usually more convenient to use [WorkflowViewStub] than to call this method directly.
  *
  * Creates a [View] to display [initialRendering], which can be updated via calls
- * to [View.showRendering].
+ * to [View.showRendering]. Uses
  *
  * @throws IllegalArgumentException if no factory can be find for type [RenderingT]
  *
@@ -119,19 +117,27 @@ fun <RenderingT : Any> ViewRegistry.buildView(
   contextForNewView: Context,
   container: ViewGroup? = null
 ): View {
-  return getFactoryFor(initialRendering::class)
-      .buildView(
-          initialRendering,
-          initialViewEnvironment,
-          contextForNewView,
-          container
-      )
-      .apply {
-        check(this.getRendering<Any>() != null) {
-          "View.bindShowRendering should have been called for $this, typically by the " +
-              "${ViewFactory::class.java.name} that created it."
-        }
+  @Suppress("UNCHECKED_CAST")
+  val factory = getFactoryFor(initialRendering::class)
+    ?: (initialRendering as? ViewFactory<RenderingT>)
+    ?: throw IllegalArgumentException(
+      "A ${ViewFactory::class.qualifiedName} should have been registered " +
+        "to display ${initialRendering::class.qualifiedName} instances, or that class " +
+        "should implement ${ViewFactory::class.simpleName}<${initialRendering::class.simpleName}>."
+    )
+
+  return factory.buildView(
+    initialRendering,
+    initialViewEnvironment,
+    contextForNewView,
+    container
+  )
+    .apply {
+      check(this.getRendering<Any>() != null) {
+        "View.bindShowRendering should have been called for $this, typically by the " +
+          "${ViewFactory::class.java.name} that created it."
       }
+    }
 }
 
 /**