Emit all Treehouse guest changes at the end of a frame

Prior to this changes were emitted when the applier's onEndChanges callback was invoked. Compose can do multiple rounds of changes within a single setContent call or sendFrame call resulting in multiple begin/end change callbacks on the applier.

redwood-compose/src/commonMain/kotlin/app/cash/redwood/compose/RedwoodComposition.kt

@@ -58,12 +58,15 @@ public interface RedwoodComposition {
 /**
  * @param scope A [CoroutineScope] whose [coroutineContext][kotlin.coroutines.CoroutineContext]
  * must have a [MonotonicFrameClock] key which is being ticked.
+ * @param onChanges Invoked when changes are being applied to the widget tree. Multiple rounds of
+ * changes can be applied in a single frame or setContent call. This should be relied on only for
+ * a signal that changes occurred, not for when they are complete.
  */
 public fun <W : Any> RedwoodComposition(
   scope: CoroutineScope,
   view: RedwoodView<W>,
   widgetSystem: WidgetSystem<W>,
-  onEndChanges: () -> Unit = {},
+  onChanges: () -> Unit = {},
 ): RedwoodComposition {
   view.children.remove(0, view.children.widgets.size)
 
@@ -100,13 +103,16 @@ public fun <W : Any> RedwoodComposition(
     saveableStateRegistry,
     view.uiConfiguration,
     widgetSystem,
-    onEndChanges,
+    onChanges,
   )
 }
 
 /**
  * @param scope A [CoroutineScope] whose [coroutineContext][kotlin.coroutines.CoroutineContext]
  * must have a [MonotonicFrameClock] key which is being ticked.
+ * @param onChanges Invoked when changes are being applied to the widget tree. Multiple rounds of
+ * changes can be applied in a single frame or setContent call. This should be relied on only for
+ * a signal that changes occurred, not for when they are complete.
  */
 public fun <W : Any> RedwoodComposition(
   scope: CoroutineScope,
@@ -115,14 +121,14 @@ public fun <W : Any> RedwoodComposition(
   saveableStateRegistry: SaveableStateRegistry?,
   uiConfigurations: StateFlow<UiConfiguration>,
   widgetSystem: WidgetSystem<W>,
-  onEndChanges: () -> Unit = {},
+  onChanges: () -> Unit = {},
 ): RedwoodComposition {
   return WidgetRedwoodComposition(
     scope,
     onBackPressedDispatcher,
     saveableStateRegistry,
     uiConfigurations,
-    NodeApplier(widgetSystem, container, onEndChanges),
+    NodeApplier(widgetSystem, container, onChanges),
   )
 }
 

redwood-compose/src/commonMain/kotlin/app/cash/redwood/compose/WidgetApplier.kt

@@ -54,7 +54,7 @@ import kotlin.math.min
 internal class NodeApplier<W : Any>(
   override val widgetSystem: WidgetSystem<W>,
   root: Widget.Children<W>,
-  private val onEndChanges: () -> Unit,
+  private val onChanges: () -> Unit,
 ) : AbstractApplier<Node<W>>(ChildrenNode(root)),
   RedwoodApplier<W> {
   private var closed = false
@@ -66,6 +66,13 @@ internal class NodeApplier<W : Any>(
     }
   }
 
+  override fun onBeginChanges() {
+    super.onBeginChanges()
+    // We invoke this here rather than in the end change callback to try and ensure
+    // no one relies on it to signal the end of changes.
+    onChanges.invoke()
+  }
+
   override fun onEndChanges() {
     check(!closed)
 
@@ -75,8 +82,6 @@ internal class NodeApplier<W : Any>(
       }
       changedWidgets.clear()
     }
-
-    onEndChanges.invoke()
   }
 
   override fun insertTopDown(index: Int, instance: Node<W>) {

redwood-protocol-guest/src/commonMain/kotlin/app/cash/redwood/protocol/guest/DefaultGuestProtocolAdapter.kt

@@ -170,7 +170,9 @@ public class DefaultGuestProtocolAdapter(
   }
 
   override fun emitChanges() {
-    changesSink.sendChanges(takeChanges())
+    if (changes.isNotEmpty()) {
+      changesSink.sendChanges(takeChanges())
+    }
   }
 
   public override fun removeWidget(id: Id) {

redwood-protocol-guest/src/commonMain/kotlin/app/cash/redwood/protocol/guest/GuestProtocolAdapter.kt

@@ -66,6 +66,10 @@ public abstract class GuestProtocolAdapter(
 
   public abstract fun initChangesSink(changesSink: ChangesSink)
 
+  /**
+   * Write changes to the underlying [ChangesSink].
+   * This function may no-op if there are no changes to send.
+   */
   public abstract fun emitChanges()
 
   @RedwoodCodegenApi

redwood-protocol-guest/src/commonMain/kotlin/app/cash/redwood/protocol/guest/ProtocolRedwoodComposition.kt

@@ -38,7 +38,6 @@ public fun ProtocolRedwoodComposition(
   onBackPressedDispatcher: OnBackPressedDispatcher,
   saveableStateRegistry: SaveableStateRegistry?,
   uiConfigurations: StateFlow<UiConfiguration>,
-  onEndChanges: () -> Unit = {},
 ): RedwoodComposition {
   val composition = RedwoodComposition(
     scope = scope,
@@ -47,7 +46,6 @@ public fun ProtocolRedwoodComposition(
     saveableStateRegistry = saveableStateRegistry,
     uiConfigurations = uiConfigurations,
     widgetSystem = guestAdapter.widgetSystem,
-    onEndChanges = onEndChanges,
   )
   return ProtocolRedwoodComposition(composition, widgetVersion)
 }

redwood-protocol-guest/src/commonTest/kotlin/app/cash/redwood/protocol/guest/ProtocolTest.kt

@@ -46,7 +46,6 @@ import app.cash.redwood.ui.UiConfiguration
 import assertk.assertFailure
 import assertk.assertThat
 import assertk.assertions.containsExactly
-import assertk.assertions.isEmpty
 import assertk.assertions.isEqualTo
 import assertk.assertions.isInstanceOf
 import assertk.assertions.message
@@ -86,9 +85,6 @@ class ProtocolTest {
       },
       saveableStateRegistry = null,
       uiConfigurations = MutableStateFlow(UiConfiguration()),
-      onEndChanges = {
-        assertThat(guestAdapter.takeChanges()).isEmpty()
-      },
     )
 
     var actualDisplayVersion = 0U

redwood-testing/src/commonMain/kotlin/app/cash/redwood/testing/TestRedwoodComposition.kt

@@ -120,7 +120,7 @@ private class RealTestRedwoodComposition<W : Any, S>(
     saveableStateRegistry = savedStateRegistry,
     uiConfigurations = uiConfigurations,
     widgetSystem = widgetSystem,
-    onEndChanges = { hasChanges = true },
+    onChanges = { hasChanges = true },
   )
 
   override fun setContent(content: @Composable () -> Unit) {

redwood-testing/src/commonTest/kotlin/app/cash/redwood/testing/ViewTreesTest.kt

@@ -22,7 +22,6 @@ import app.cash.redwood.compose.current
 import app.cash.redwood.layout.testing.RedwoodLayoutTestingWidgetFactory
 import app.cash.redwood.lazylayout.testing.RedwoodLazyLayoutTestingWidgetFactory
 import app.cash.redwood.leaks.LeakDetector
-import app.cash.redwood.protocol.Change
 import app.cash.redwood.protocol.ChildrenChange.Add
 import app.cash.redwood.protocol.ChildrenTag
 import app.cash.redwood.protocol.Create
@@ -115,15 +114,13 @@ class ViewTreesTest {
       .isEqualTo(expected)
 
     // Validate that the normal Compose protocol backend produces the same list of changes.
-    val protocolChanges = mutableListOf<Change>()
     val guestAdapter = DefaultGuestProtocolAdapter(
       hostVersion = hostRedwoodVersion,
       widgetSystemFactory = TestSchemaProtocolWidgetSystemFactory,
     )
     val composition = ProtocolRedwoodComposition(
       scope = this + BroadcastFrameClock(),
       guestAdapter = guestAdapter,
-      onEndChanges = { protocolChanges += guestAdapter.takeChanges() },
       widgetVersion = UInt.MAX_VALUE,
       onBackPressedDispatcher = object : OnBackPressedDispatcher {
         override fun addCallback(onBackPressedCallback: OnBackPressedCallback): Cancellable {
@@ -138,7 +135,7 @@ class ViewTreesTest {
     composition.setContent(content)
     composition.cancel()
 
-    assertThat(protocolChanges).isEqualTo(expected)
+    assertThat(guestAdapter.takeChanges()).isEqualTo(expected)
 
     // Ensure when the changes are applied with the widget protocol we get equivalent values.
     val widgetSystem = TestSchemaWidgetSystem(

redwood-treehouse-guest/src/commonMain/kotlin/app/cash/redwood/treehouse/StandardAppLifecycle.kt

@@ -15,8 +15,6 @@
  */
 package app.cash.redwood.treehouse
 
-import androidx.compose.runtime.BroadcastFrameClock
-import androidx.compose.runtime.MonotonicFrameClock
 import app.cash.redwood.protocol.EventTag
 import app.cash.redwood.protocol.Id
 import app.cash.redwood.protocol.RedwoodVersion
@@ -38,6 +36,7 @@ public class StandardAppLifecycle(
 ) : AppLifecycle {
   private var started = false
   private lateinit var host: Host
+  private val frameListeners = mutableListOf<FrameListener>()
 
   override val guestProtocolVersion: RedwoodVersion
     get() = guestRedwoodVersion
@@ -50,12 +49,11 @@ public class StandardAppLifecycle(
     }
   }
 
-  private val broadcastFrameClock: BroadcastFrameClock = BroadcastFrameClock {
+  internal fun requestHostFrame() {
     if (started) {
       host.requestFrame()
     }
   }
-  public val frameClock: MonotonicFrameClock get() = broadcastFrameClock
 
   internal val mismatchHandler: ProtocolMismatchHandler = object : ProtocolMismatchHandler {
     override fun onUnknownEvent(widgetTag: WidgetTag, tag: EventTag) {
@@ -87,6 +85,20 @@ public class StandardAppLifecycle(
   }
 
   override fun sendFrame(timeNanos: Long) {
-    broadcastFrameClock.sendFrame(timeNanos)
+    for (frameListener in frameListeners) {
+      frameListener.onFrame(timeNanos)
+    }
+  }
+
+  internal fun addFrameListener(listener: FrameListener) {
+    frameListeners += listener
+  }
+
+  internal fun removeFrameListener(listener: FrameListener) {
+    frameListeners.remove(listener)
+  }
+
+  internal fun interface FrameListener {
+    fun onFrame(timeNanos: Long)
   }
 }

redwood-treehouse-guest/src/commonMain/kotlin/app/cash/redwood/treehouse/treehouseCompose.kt

@@ -15,6 +15,7 @@
  */
 package app.cash.redwood.treehouse
 
+import androidx.compose.runtime.BroadcastFrameClock
 import androidx.compose.runtime.saveable.SaveableStateRegistry
 import app.cash.redwood.compose.RedwoodComposition
 import app.cash.redwood.protocol.Change
@@ -56,7 +57,18 @@ private class RedwoodZiplineTreehouseUi(
   private val guestAdapter: GuestProtocolAdapter,
 ) : ZiplineTreehouseUi,
   ZiplineScoped,
-  EventSink by guestAdapter {
+  EventSink by guestAdapter,
+  StandardAppLifecycle.FrameListener {
+
+  private val clock = BroadcastFrameClock {
+    appLifecycle.requestHostFrame()
+  }
+  private var hasChanges = false
+
+  override fun onFrame(timeNanos: Long) {
+    clock.sendFrame(timeNanos)
+    guestAdapter.emitChanges()
+  }
 
   /**
    * By overriding [ZiplineScoped.scope], all services passed into [start] are added to this scope,
@@ -125,18 +137,24 @@ private class RedwoodZiplineTreehouseUi(
 
     guestAdapter.initChangesSink(changesSink)
 
+    appLifecycle.addFrameListener(this)
+
     val composition = ProtocolRedwoodComposition(
-      scope = coroutineScope + appLifecycle.frameClock,
+      scope = coroutineScope + clock,
       guestAdapter = guestAdapter,
       widgetVersion = appLifecycle.widgetVersion,
-      onEndChanges = { guestAdapter.emitChanges() },
       onBackPressedDispatcher = host.asOnBackPressedDispatcher(),
       saveableStateRegistry = saveableStateRegistry,
       uiConfigurations = host.uiConfigurations,
     )
     this.composition = composition
 
     composition.bind(treehouseUi)
+
+    // Explicitly emit the initial changes produced by calling 'setContent' (within 'bind').
+    // All other changes are initiated and emitted by the [onFrame] callback.
+    hasChanges = false
+    guestAdapter.emitChanges()
   }
 
   override fun snapshotState(): StateSnapshot {
@@ -145,6 +163,7 @@ private class RedwoodZiplineTreehouseUi(
   }
 
   override fun close() {
+    appLifecycle.removeFrameListener(this)
     coroutineScope.coroutineContext.job.invokeOnCompletion {
       scope.close()
     }

redwood-treehouse-guest/src/jsMain/kotlin/app/cash/redwood/treehouse/ProtocolBridgeJs.kt

@@ -220,8 +220,10 @@ internal class FastGuestProtocolAdapter(
   }
 
   override fun emitChanges() {
-    sendChanges(changesSinkService, arrayOf(changes))
-    changes.clear()
+    if (changes.length > 0) {
+      sendChanges(changesSinkService, arrayOf(changes))
+      changes.clear()
+    }
   }
 
   override fun removeWidget(id: Id) {