1247: Partial Tree Rerendering

Track whether or not the state (or state of a child) has changed in the WorkflowNode. Pass lastRendering if its not.

Author: steve-the-edwards

workflow-runtime/src/commonMain/kotlin/com/squareup/workflow1/internal/WorkflowNode.kt

@@ -86,9 +86,11 @@ internal class WorkflowNode<PropsT, StateT, OutputT, RenderingT>(
   )
   private val sideEffects = ActiveStagingList<SideEffectNode>()
   private var lastProps: PropsT = initialProps
+  var lastRendering: RenderingT? = null
   private val eventActionsChannel =
     Channel<WorkflowAction<PropsT, StateT, OutputT>>(capacity = UNLIMITED)
   private var state: StateT
+  private var subtreeStateDidChange: Boolean = true
 
   private val baseRenderContext = RealRenderContext(
     renderer = subtreeManager,
@@ -205,16 +207,21 @@ internal class WorkflowNode<PropsT, StateT, OutputT, RenderingT>(
     coroutineContext.cancel(cause)
   }
 
-  // Call this after we have been passed any workflow instance, in [render] or [snapshot]. It may
-  // have changed and we should check to see if we need to update our cached instances.
+  /** Call this after we have been passed any workflow instance, in [render] or [snapshot]. It may
+   *  have changed and we should check to see if we need to update our cached instances.
+   *
+   *  @return true if the instance has changed, otherwise false.
+   */
   private fun updateCachedWorkflowInstance(
     workflow: StatefulWorkflow<PropsT, StateT, OutputT, RenderingT>
-  ) {
+  ): Boolean {
     if (workflow !== cachedWorkflowInstance) {
       // instance has changed.
       interceptedWorkflowInstance = interceptor.intercept(workflow, this)
       cachedWorkflowInstance = workflow
+      return true
     }
+    return false
   }
 
   /**
@@ -228,20 +235,22 @@ internal class WorkflowNode<PropsT, StateT, OutputT, RenderingT>(
     updateCachedWorkflowInstance(workflow)
     updatePropsAndState(props)
 
-    baseRenderContext.unfreeze()
-    val rendering = interceptedWorkflowInstance.render(props, state, context)
-    baseRenderContext.freeze()
+    if (subtreeStateDidChange) {
+      baseRenderContext.unfreeze()
+      lastRendering = interceptedWorkflowInstance.render(props, state, context)
+      baseRenderContext.freeze()
 
-    workflowTracer.trace("UpdateRuntimeTree") {
-      // Tear down workflows and workers that are obsolete.
-      subtreeManager.commitRenderedChildren()
-      // Side effect jobs are launched lazily, since they can send actions to the sink, and can only
-      // be started after context is frozen.
-      sideEffects.forEachStaging { it.job.start() }
-      sideEffects.commitStaging { it.job.cancel() }
+      workflowTracer.trace("UpdateRuntimeTree") {
+        // Tear down workflows and workers that are obsolete.
+        subtreeManager.commitRenderedChildren()
+        // Side effect jobs are launched lazily, since they can send actions to the sink, and can only
+        // be started after context is frozen.
+        sideEffects.forEachStaging { it.job.start() }
+        sideEffects.commitStaging { it.job.cancel() }
+      }
     }
 
-    return rendering
+    return lastRendering!!
   }
 
   private fun updatePropsAndState(
@@ -250,6 +259,7 @@ internal class WorkflowNode<PropsT, StateT, OutputT, RenderingT>(
     if (newProps != lastProps) {
       val newState = interceptedWorkflowInstance.onPropsChanged(lastProps, newProps, state)
       state = newState
+      subtreeStateDidChange = true
     }
     lastProps = newProps
   }
@@ -270,6 +280,8 @@ internal class WorkflowNode<PropsT, StateT, OutputT, RenderingT>(
       // Changing state is sticky, we pass it up if it ever changed.
       stateChanged = actionApplied.stateChanged || (childResult?.stateChanged ?: false)
     )
+    // Our state changed or one of our children's state changed.
+    subtreeStateDidChange = aggregateActionApplied.stateChanged
     return if (actionApplied.output != null) {
       emitAppliedActionToParent(aggregateActionApplied)
     } else {

workflow-runtime/src/commonTest/kotlin/com/squareup/workflow1/WorkflowOperatorsTest.kt

@@ -207,23 +207,37 @@ class WorkflowOperatorsTest {
   private abstract class StateFlowWorkflow<T>(
     val name: String,
     val flow: StateFlow<T>
-  ) : StatelessWorkflow<Unit, Nothing, T>() {
+  ) : StatefulWorkflow<Unit, T, Nothing, T>() {
     var starts: Int = 0
       private set
 
+    override fun initialState(
+      props: Unit,
+      snapshot: Snapshot?
+    ): T {
+      return flow.value
+    }
+
     private val rerenderWorker = object : Worker<T> {
       override fun run(): Flow<T> = flow.onStart { starts++ }
     }
 
     override fun render(
       renderProps: Unit,
+      renderState: T,
       context: RenderContext
     ): T {
       // Listen to the flow to trigger a re-render when it updates.
-      context.runningWorker(rerenderWorker as Worker<Any?>) { WorkflowAction.noAction() }
-      return flow.value
+      context.runningWorker(rerenderWorker) { output: T ->
+        action("rerenderUpdate") {
+          state = output
+        }
+      }
+      return renderState
     }
 
+    override fun snapshotState(state: T): Snapshot? = null
+
     override fun toString(): String = "StateFlowWorkflow($name)"
   }
 }

workflow-tracing/src/test/resources/com/squareup/workflow1/diagnostic/tracing/expected_trace_file.txt

@@ -63,17 +63,6 @@
 {"name":"Sink received: Worker<String> (2)","cat":"update","ph":"i","ts":0,"pid":0,"tid":0,"s":"t","args":{"action":"action(EmitWorkerOutputAction(worker=TypedWorker(java.lang.String (Kotlin reflection is not available)), key=))"}},
 {"name":"WorkflowAction: Worker<String> (2)","cat":"update","ph":"i","ts":0,"pid":0,"tid":0,"s":"p","args":{"action":"action(EmitWorkerOutputAction(worker=TypedWorker(java.lang.String (Kotlin reflection is not available)), key=))","oldState":"0","newState":"{no change}","output":"fired!"}},
 {"name":"Worker<String> (2)","ph":"O","ts":0,"pid":0,"tid":0,"id":"2","args":{"snapshot":"0"}},
-{"name":"Render Pass","cat":"rendering","ph":"B","ts":0,"pid":0,"tid":0,"args":{"props":"3"}},
-{"name":"used/free memory","ph":"C","ts":0,"pid":0,"tid":0,"args":{"usedMemory":1,"freeMemory":42}},
-{"name":"TestWorkflow (0)","cat":"rendering","ph":"B","ts":0,"pid":0,"tid":0,"args":{"workflowId":"0","props":"3","state":"changed state"}},
-{"name":"TestWorkflow (1)","cat":"rendering","ph":"B","ts":0,"pid":0,"tid":0,"args":{"workflowId":"1","props":"0","state":"initial"}},
-{"name":"TestWorkflow (1)","cat":"rendering","ph":"E","ts":0,"pid":0,"tid":0,"args":{"rendering":"initial"}},
-{"name":"Props changed: Worker<String> (2)","ph":"i","ts":0,"pid":0,"tid":0,"s":"t","args":{"oldProps":"TypedWorker(java.lang.String (Kotlin reflection is not available))","newProps":"TypedWorker(java.lang.String (Kotlin reflection is not available))","oldState":"0","newState":"{no change}"}},
-{"name":"Worker<String> (2)","cat":"rendering","ph":"B","ts":0,"pid":0,"tid":0,"args":{"workflowId":"2","props":"TypedWorker(java.lang.String (Kotlin reflection is not available))","state":"0"}},
-{"name":"Worker<String> (2)","cat":"rendering","ph":"E","ts":0,"pid":0,"tid":0,"args":{"rendering":"kotlin.Unit"}},
-{"name":"TestWorkflow (0)","cat":"rendering","ph":"E","ts":0,"pid":0,"tid":0,"args":{"rendering":"rendering"}},
-{"name":"Render Pass","cat":"rendering","ph":"E","ts":0,"pid":0,"tid":0,"args":{"rendering":"rendering"}},
-{"name":"used/free memory","ph":"C","ts":0,"pid":0,"tid":0,"args":{"usedMemory":1,"freeMemory":42}},
 {"name":"Snapshot","ph":"B","ts":0,"pid":0,"tid":0,"args":{}},
 {"name":"Snapshot","ph":"E","ts":0,"pid":0,"tid":0,"args":{}},
 {"name":"Props changed: {root}","ph":"i","ts":0,"pid":0,"tid":0,"s":"t","args":{"oldProps":"3","newProps":"4","oldState":"changed state","newState":"{no change}"}},