[scheduler] Post to MessageChannel instead of window

Scheduler needs to schedule a task that fires after paint. To do this,
it currently posts a message event to `window`. This happens on every
frame until the queue is empty. An unfortunate consequence is that every
other message event handler also gets called on every frame; even if
they exit immediately, this adds up to significant per-frame overhead.

Instead, we'll create a MessageChannel and post to that, with a
fallback to the old behavior if MessageChannel does not exist.

Author: acdlite

packages/scheduler/src/Scheduler.js

@@ -533,13 +533,14 @@ if (typeof window !== 'undefined' && window._schedMock) {
   };
 
   // We use the postMessage trick to defer idle work until after the repaint.
+  var port = null;
   var messageKey =
     '__reactIdleCallback$' +
     Math.random()
       .toString(36)
       .slice(2);
   var idleTick = function(event) {
-    if (event.source !== window || event.data !== messageKey) {
+    if (event.source !== port || event.data !== messageKey) {
       return;
     }
 
@@ -583,9 +584,6 @@ if (typeof window !== 'undefined' && window._schedMock) {
       }
     }
   };
-  // Assumes that we have addEventListener in this environment. Might need
-  // something better for old IE.
-  window.addEventListener('message', idleTick, false);
 
   var animationTick = function(rafTime) {
     if (scheduledHostCallback !== null) {
@@ -629,7 +627,7 @@ if (typeof window !== 'undefined' && window._schedMock) {
     frameDeadline = rafTime + activeFrameTime;
     if (!isMessageEventScheduled) {
       isMessageEventScheduled = true;
-      window.postMessage(messageKey, '*');
+      port.postMessage(messageKey, '*');
     }
   };
 
@@ -638,7 +636,7 @@ if (typeof window !== 'undefined' && window._schedMock) {
     timeoutTime = absoluteTimeout;
     if (isFlushingHostCallback || absoluteTimeout < 0) {
       // Don't wait for the next frame. Continue working ASAP, in a new event.
-      window.postMessage(messageKey, '*');
+      port.postMessage(messageKey, '*');
     } else if (!isAnimationFrameScheduled) {
       // If rAF didn't already schedule one, we need to schedule a frame.
       // TODO: If this rAF doesn't materialize because the browser throttles, we
@@ -649,6 +647,19 @@ if (typeof window !== 'undefined' && window._schedMock) {
     }
   };
 
+  if (typeof MessageChannel === 'function') {
+    // Use a MessageChannel, if support exists
+    var channel = new MessageChannel();
+    channel.port1.onmessage = idleTick;
+    port = channel.port2;
+  } else {
+    // Otherwise post a message to the window. This isn't ideal because message
+    // handlers will fire on every frame until the queue is empty, including
+    // some browser extensions.
+    window.addEventListener('message', idleTick, false);
+    port = window;
+  }
+
   cancelHostCallback = function() {
     scheduledHostCallback = null;
     isMessageEventScheduled = false;

packages/scheduler/src/__tests__/SchedulerDOM-test.js

@@ -64,33 +64,26 @@ describe('SchedulerDOM', () => {
   let currentTime = 0;
 
   beforeEach(() => {
-    // TODO pull this into helper method, reduce repetition.
-    // mock the browser APIs which are used in schedule:
-    // - requestAnimationFrame should pass the DOMHighResTimeStamp argument
-    // - calling 'window.postMessage' should actually fire postmessage handlers
-    // - Date.now should return the correct thing
-    // - test with native performance.now()
     delete global.performance;
     global.requestAnimationFrame = function(cb) {
       return rAFCallbacks.push(() => {
         cb(startOfLatestFrame);
       });
     };
-    const originalAddEventListener = global.addEventListener;
-    postMessageCallback = null;
     postMessageEvents = [];
     postMessageErrors = [];
-    global.addEventListener = function(eventName, callback, useCapture) {
-      if (eventName === 'message') {
-        postMessageCallback = callback;
-      } else {
-        originalAddEventListener(eventName, callback, useCapture);
-      }
+    const port1 = {};
+    const port2 = {
+      postMessage(messageKey) {
+        const postMessageEvent = {source: port2, data: messageKey};
+        postMessageEvents.push(postMessageEvent);
+      },
     };
-    global.postMessage = function(messageKey, targetOrigin) {
-      const postMessageEvent = {source: window, data: messageKey};
-      postMessageEvents.push(postMessageEvent);
+    global.MessageChannel = function MessageChannel() {
+      this.port1 = port1;
+      this.port2 = port2;
     };
+    postMessageCallback = event => port1.onmessage(event);
     global.Date.now = function() {
       return currentTime;
     };