Throttle with wait

Summary: Throttle events whilst keeping track of latest

Reviewed By: LukeDefeo

Differential Revision: D39652348

fbshipit-source-id: 9c8fb5a1bb92872985f46a62d79c6594a37e8340

android/src/main/java/com/facebook/flipper/plugins/uidebugger/observers/DecorViewTreeObserver.kt

@@ -12,45 +12,47 @@ import android.view.View
 import android.view.ViewTreeObserver
 import com.facebook.flipper.plugins.uidebugger.LogTag
 import com.facebook.flipper.plugins.uidebugger.core.Context
+import com.facebook.flipper.plugins.uidebugger.scheduler.throttleLatest
 import java.lang.ref.WeakReference
+import kotlinx.coroutines.*
 
 typealias DecorView = View
 
 /** Responsible for subscribing to updates to the content view of an activity */
 class DecorViewObserver(val context: Context) : TreeObserver<DecorView>() {
 
-  val throttleTimeMs = 500
+  private val throttleTimeMs = 500L
 
   private var nodeRef: WeakReference<View>? = null
   private var listener: ViewTreeObserver.OnPreDrawListener? = null
 
   override val type = "DecorView"
 
+  private val waitScope = CoroutineScope(Dispatchers.IO)
+  private val mainScope = CoroutineScope(Dispatchers.Main)
+
   override fun subscribe(node: Any) {
     node as View
     nodeRef = WeakReference(node)
 
     Log.i(LogTag, "Subscribing to decor view changes")
 
-    // TODO: there's a problem with this. Some future changes may have been
-    // ignored and not sent. Need to keep track of the last one, always and react
-    // accordingly.
-    listener =
-        object : ViewTreeObserver.OnPreDrawListener {
-          var lastSend = 0L
-          override fun onPreDraw(): Boolean {
-            if (System.currentTimeMillis() - lastSend > throttleTimeMs) {
-              traverseAndSend(context, node)
+    val throttleSend =
+        throttleLatest<WeakReference<View>?>(throttleTimeMs, waitScope, mainScope) { weakView ->
+          weakView?.get()?.let { view -> traverseAndSend(context, view) }
+        }
 
-              lastSend = System.currentTimeMillis()
-            }
-            return true
-          }
+    listener =
+        ViewTreeObserver.OnPreDrawListener {
+          throttleSend(nodeRef)
+          true
         }
 
     node.viewTreeObserver.addOnPreDrawListener(listener)
-    // sometimes we are too late to the party and we miss the first draw
-    listener?.onPreDraw()
+
+    // It can be the case that the DecorView the current observer owns has already
+    // drawn. In this case, manually trigger an update.
+    throttleSend(nodeRef)
   }
 
   override fun unsubscribe() {

android/src/main/java/com/facebook/flipper/plugins/uidebugger/scheduler/CoroutineThrottle.kt

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+package com.facebook.flipper.plugins.uidebugger.scheduler
+
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Job
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.launch
+
+/**
+ * Throttle the execution of an executable for the specified interval.
+ *
+ * How does it work?
+ *
+ * The function `throttleLatest` returns a proxy for the given executable. This proxy captures the
+ * latest argument/param that was used on the last invocation. If the throttle job does not exist or
+ * has already completed, then create a new one.
+ *
+ * The job will wait on the waiting scope for the given amount of specified ms. Once it finishes
+ * waiting, then it will execute the given executable on the main scope with the latest captured
+ * param.
+ */
+fun <T> throttleLatest(
+    intervalMs: Long,
+    waitScope: CoroutineScope,
+    mainScope: CoroutineScope,
+    executable: (T) -> Unit
+): (T) -> Unit {
+  var throttleJob: Job? = null
+  var latestParam: T
+  return { param: T ->
+    latestParam = param
+    if (throttleJob == null || throttleJob?.isCompleted == true) {
+      throttleJob =
+          waitScope.launch {
+            delay(intervalMs)
+            mainScope.launch { executable(latestParam) }
+          }
+    }
+  }
+}