added idle screen support for empty stack

Author: zach-klippenstein

samples/containers/thingy/src/main/java/com/squareup/sample/thingy/BackStackWorkflow.kt

@@ -11,10 +11,11 @@ import kotlinx.coroutines.flow.flowOf
 
 /**
  * Creates a [BackStackWorkflow]. See the docs on [BackStackWorkflow.runBackStack] for more
- * information about what [block] can do.
+ * information about what [runBackStack] can do.
  */
 public inline fun <PropsT, OutputT> backStackWorkflow(
-  crossinline block: suspend BackStackScope.(
+  crossinline createIdleScreen: () -> Screen,
+  crossinline runBackStack: suspend BackStackScope.(
     props: StateFlow<PropsT>,
     emitOutput: (OutputT) -> Unit
   ) -> Unit
@@ -24,8 +25,10 @@ public inline fun <PropsT, OutputT> backStackWorkflow(
       props: StateFlow<PropsT>,
       emitOutput: (OutputT) -> Unit
     ) {
-      block(props, emitOutput)
+      runBackStack(props, emitOutput)
     }
+
+    override fun createIdleScreen(): Screen = createIdleScreen()
   }
 
 /**
@@ -96,6 +99,12 @@ public abstract class BackStackWorkflow<PropsT, OutputT> :
     emitOutput: (OutputT) -> Unit
   )
 
+  /**
+   * Called to provide a screen to display when [runBackStack] has not shown anything yet, or when
+   * a workflow's output handler is idle (not showing an active screen).
+   */
+  abstract fun createIdleScreen(): Screen
+
   final override fun asStatefulWorkflow():
     StatefulWorkflow<PropsT, *, OutputT, BackStackScreen<Screen>> =
     BackStackWorkflowImpl(this)

samples/containers/thingy/src/main/java/com/squareup/sample/thingy/BackStackWorkflowImpl.kt

@@ -93,8 +93,11 @@ internal class BackStackWorkflowImpl<PropsT, OutputT>(
       }
     }
 
-    // TODO show a loading screen if renderings is empty.
-    return renderings.toBackStackScreen()
+    return if (renderings.isEmpty()) {
+      BackStackScreen(workflow.createIdleScreen())
+    } else {
+      renderings.toBackStackScreen()
+    }
   }
 
   override fun snapshotState(state: BackStackState): Snapshot? = null

samples/containers/thingy/src/main/java/com/squareup/sample/thingy/MyWorkflow.kt

@@ -19,6 +19,8 @@ data class RetryScreen(
   val onCancelClicked: () -> Unit,
 ) : Screen
 
+data object LoadingScreen : Screen
+
 class MyWorkflow(
   private val child1: Workflow<Unit, String, Screen>,
   private val child2: Workflow<Unit, String, Screen>,
@@ -60,6 +62,8 @@ class MyWorkflow(
     }
   }
 
+  override fun createIdleScreen(): Screen = LoadingScreen
+
   private suspend fun BackStackParentScope.networkCallWithRetry(
     request: String
   ): String {