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[Scheduler] Test browser implementation details (#16198)
The Scheduler implementation uses browser APIs like `MessageChannel`, `requestAnimationFrame`, and `setTimeout` to schedule work on the main thread. Most of our tests treat these as implementation details; however, the sequence and timing of these APIs are not precisely specified, and can vary wildly across browsers. To prevent regressions, we need the ability to simulate specific edge cases that we may encounter in various browsers. This adds a new test suite that mocks all browser methods used in our implementation. It assumes as little as possible about the order and timing of events. The only thing it assumes is that requestAnimationFrame is passed a frame time that is equal to or less than the time returned by performance.now. Everything else can be controlled at will. It also includes Scheduler-specific invariants, e.g. only one rAF callback can be scheduled at a time. It overlaps slightly with the existing SchedulerDOM-test suite, which also mocks the browser APIs, but exposes a higher-level set of testing primitives. I will consolidate the two suites in a follow-up.
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‎packages/scheduler/src/__tests__/SchedulerBrowser-test.js‎

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/**
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* Copyright (c) Facebook, Inc. and its affiliates.
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*
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* This source code is licensed under the MIT license found in the
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* LICENSE file in the root directory of this source tree.
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*
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* @emails react-core
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* @jest-environment node
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*/
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/* eslint-disable no-for-of-loops/no-for-of-loops */
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'use strict';
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let Scheduler;
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let runtime;
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let performance;
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let scheduleCallback;
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let NormalPriority;
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// The Scheduler implementation uses browser APIs like `MessageChannel`,
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// `requestAnimationFrame`, and `setTimeout` to schedule work on the main
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// thread. Most of our tests treat these as implementation details; however, the
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// sequence and timing of these APIs are not precisely specified, and can vary
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// wildly across browsers.
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//
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// To prevent regressions, we need the ability to simulate specific edge cases
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// that we may encounter in various browsers.
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//
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// This test suite mocks all browser methods used in our implementation. It
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// assumes as little as possible about the order and timing of events. The only
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// thing it assumes is that requestAnimationFrame is passed a frame time that is
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// equal to or less than the time returned by performance.now. Everything else
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// can be controlled at will.
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//
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// It also includes Scheduler-specific invariants, e.g. only one rAF callback
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// can be scheduled at a time.
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describe('SchedulerBrowser', () => {
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beforeEach(() => {
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jest.resetModules();
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// Un-mock scheduler
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jest.mock('scheduler', () => require.requireActual('scheduler'));
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jest.mock('scheduler/src/SchedulerHostConfig', () =>
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require.requireActual(
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'scheduler/src/forks/SchedulerHostConfig.default.js',
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),
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);
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runtime = installMockBrowserRuntime();
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performance = window.performance;
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Scheduler = require('scheduler');
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scheduleCallback = Scheduler.unstable_scheduleCallback;
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NormalPriority = Scheduler.unstable_NormalPriority;
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});
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afterEach(() => {
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if (!runtime.isLogEmpty()) {
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throw Error('Test exited without clearing log.');
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}
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});
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function installMockBrowserRuntime() {
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let VSYNC_INTERVAL = 33.33;
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let hasPendingMessageEvent = false;
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let rAFCallbackIDCounter = 0;
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let rAFCallback = null;
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let isRunningRAFCallback = false;
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let rICCallbackIDCounter = 0;
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let rICCallback = null;
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let timerIDCounter = 0;
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// let timerIDs = new Map();
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let eventLog = [];
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const window = {};
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global.window = window;
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let currentTime = 0;
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window.performance = {
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now() {
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return currentTime;
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},
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};
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window.requestAnimationFrame = cb => {
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if (rAFCallback !== null) {
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throw Error('rAF already scheduled');
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}
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if (isRunningRAFCallback) {
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log('Request Animation Frame [Reposted]');
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} else {
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log('Request Animation Frame');
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}
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rAFCallback = cb;
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return rAFCallbackIDCounter++;
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};
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window.cancelAnimationFrame = id => {
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rAFCallback = null;
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};
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window.requestIdleCallback = cb => {
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if (rICCallback !== null) {
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throw Error('rAF already scheduled');
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}
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log('Request Idle Callback');
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rICCallback = cb;
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return rICCallbackIDCounter++;
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};
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window.cancelIdleCallback = id => {
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rICCallback = null;
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};
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window.setTimeout = (cb, delay) => {
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const id = timerIDCounter++;
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log(`Set Timer`);
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// TODO
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return id;
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};
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window.clearTimeout = id => {
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// TODO
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};
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const port1 = {};
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const port2 = {
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postMessage() {
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if (hasPendingMessageEvent) {
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throw Error('Message event already scheduled');
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}
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log('Post Message');
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hasPendingMessageEvent = true;
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},
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};
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global.MessageChannel = function MessageChannel() {
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this.port1 = port1;
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this.port2 = port2;
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};
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function ensureLogIsEmpty() {
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if (eventLog.length !== 0) {
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throw Error('Log is not empty. Call assertLog before continuing.');
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}
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}
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function setHardwareFrameRate(fps) {
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VSYNC_INTERVAL = 1000 / fps;
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}
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function advanceTime(ms) {
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currentTime += ms;
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}
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function advanceTimeToNextFrame() {
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const targetTime =
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Math.ceil(currentTime / VSYNC_INTERVAL) * VSYNC_INTERVAL;
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if (targetTime === currentTime) {
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currentTime += VSYNC_INTERVAL;
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} else {
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currentTime = targetTime;
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}
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}
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function getMostRecentFrameNumber() {
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return Math.floor(currentTime / VSYNC_INTERVAL);
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}
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function fireMessageEvent() {
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ensureLogIsEmpty();
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if (!hasPendingMessageEvent) {
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throw Error('No message event was scheduled');
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}
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hasPendingMessageEvent = false;
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const onMessage = port1.onmessage;
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log('Message Event');
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onMessage();
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}
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function fireAnimationFrame() {
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ensureLogIsEmpty();
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if (isRunningRAFCallback) {
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throw Error('Cannot fire animation frame from inside another event.');
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}
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if (rAFCallback === null) {
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throw Error('No rAF scheduled.');
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}
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const mostRecentFrameNumber = getMostRecentFrameNumber();
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const rAFTime = mostRecentFrameNumber * VSYNC_INTERVAL;
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const cb = rAFCallback;
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rAFCallback = null;
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log(`Animation Frame [${mostRecentFrameNumber}]`);
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isRunningRAFCallback = true;
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try {
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cb(rAFTime);
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} finally {
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isRunningRAFCallback = false;
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}
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}
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function fireRIC() {
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ensureLogIsEmpty();
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if (rICCallback === null) {
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throw Error('No rIC scheduled.');
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}
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const cb = rICCallback;
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rICCallback = null;
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log('Idle Callback');
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cb();
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}
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function log(val) {
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eventLog.push(val);
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}
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function isLogEmpty() {
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return eventLog.length === 0;
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}
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function assertLog(expected) {
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const actual = eventLog;
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eventLog = [];
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expect(actual).toEqual(expected);
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}
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return {
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setHardwareFrameRate,
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advanceTime,
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advanceTimeToNextFrame,
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getMostRecentFrameNumber,
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fireMessageEvent,
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fireAnimationFrame,
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fireRIC,
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log,
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isLogEmpty,
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assertLog,
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};
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}
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it('callback with continuation', () => {
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scheduleCallback(NormalPriority, () => {
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runtime.log('Task');
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while (!Scheduler.unstable_shouldYield()) {
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runtime.advanceTime(1);
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}
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runtime.log(`Yield at ${performance.now()}ms`);
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return () => {
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runtime.log('Continuation');
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};
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});
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runtime.assertLog(['Request Animation Frame']);
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runtime.fireAnimationFrame();
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runtime.assertLog([
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'Animation Frame [0]',
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'Request Animation Frame [Reposted]',
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'Set Timer',
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'Post Message',
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]);
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runtime.fireMessageEvent();
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runtime.assertLog(['Message Event', 'Task', 'Yield at 34ms']);
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runtime.fireAnimationFrame();
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runtime.assertLog([
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'Animation Frame [1]',
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'Request Animation Frame [Reposted]',
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'Set Timer',
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'Post Message',
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]);
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runtime.fireMessageEvent();
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runtime.assertLog(['Message Event', 'Continuation']);
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runtime.advanceTimeToNextFrame();
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runtime.fireAnimationFrame();
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runtime.assertLog(['Animation Frame [2]']);
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});
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it('two rAF calls in the same frame', () => {
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scheduleCallback(NormalPriority, () => runtime.log('A'));
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runtime.assertLog(['Request Animation Frame']);
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runtime.fireAnimationFrame();
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runtime.assertLog([
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'Animation Frame [0]',
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'Request Animation Frame [Reposted]',
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'Set Timer',
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'Post Message',
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]);
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runtime.fireMessageEvent();
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runtime.assertLog(['Message Event', 'A']);
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// The Scheduler queue is now empty. We're still in frame 0.
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expect(runtime.getMostRecentFrameNumber()).toBe(0);
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// Post a task to Scheduler.
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scheduleCallback(NormalPriority, () => runtime.log('B'));
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// Did not request another animation frame, since one was already scheduled
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// during the previous rAF.
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runtime.assertLog([]);
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// Fire the animation frame.
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runtime.fireAnimationFrame();
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runtime.assertLog([
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'Animation Frame [0]',
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'Request Animation Frame [Reposted]',
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'Set Timer',
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'Post Message',
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]);
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runtime.fireMessageEvent();
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runtime.assertLog(['Message Event', 'B']);
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});
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it('adjusts frame rate by measuring inteval between rAF events', () => {
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runtime.setHardwareFrameRate(60);
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scheduleCallback(NormalPriority, () => runtime.log('Tick'));
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runtime.assertLog(['Request Animation Frame']);
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// Need to measure two consecutive intervals between frames.
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for (let i = 0; i < 2; i++) {
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runtime.fireAnimationFrame();
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runtime.assertLog([
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`Animation Frame [${runtime.getMostRecentFrameNumber()}]`,
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'Request Animation Frame [Reposted]',
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'Set Timer',
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'Post Message',
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]);
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runtime.fireMessageEvent();
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runtime.assertLog(['Message Event', 'Tick']);
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scheduleCallback(NormalPriority, () => runtime.log('Tick'));
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runtime.advanceTimeToNextFrame();
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}
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// Scheduler should observe that it's receiving rAFs every 16.6 ms and
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// adjust its frame rate accordingly. Test by blocking the thread until
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// Scheduler tells us to yield. Then measure how much time has elapsed.
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const start = performance.now();
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scheduleCallback(NormalPriority, () => {
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while (!Scheduler.unstable_shouldYield()) {
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runtime.advanceTime(1);
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}
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});
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runtime.fireAnimationFrame();
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runtime.assertLog([
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`Animation Frame [${runtime.getMostRecentFrameNumber()}]`,
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'Request Animation Frame [Reposted]',
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'Set Timer',
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'Post Message',
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]);
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runtime.fireMessageEvent();
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runtime.assertLog(['Message Event', 'Tick']);
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const end = performance.now();
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// Check how much time elapsed in the frame.
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expect(end - start).toEqual(17);
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});
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});

‎packages/scheduler/src/forks/SchedulerHostConfig.default.js‎

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Original file line numberDiff line numberDiff line change
@@ -229,22 +229,11 @@ if (
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channel.port1.onmessage = performWorkUntilDeadline;
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const onAnimationFrame = rAFTime => {
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isRAFLoopRunning = false;
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if (scheduledHostCallback === null) {
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// No scheduled work. Exit.
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prevRAFTime = -1;
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prevRAFInterval = -1;
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return;
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}
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if (rAFTime - prevRAFTime < 0.1) {
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// Defensive coding. Received two rAFs in the same frame. Exit and wait
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// for the next frame.
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// TODO: This could be an indication that the frame rate is too high. We
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// don't currently handle the case where the browser dynamically lowers
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// the framerate, e.g. in low power situation (other than the rAF timeout,
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// but that's designed for when the tab is backgrounded and doesn't
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// optimize for maxiumum CPU utilization).
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isRAFLoopRunning = false;
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return;
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}
250239

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clearTimeout(rAFTimeoutID);
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onAnimationFrame(nextRAFTime);
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});
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// requestAnimationFrame is throttled when the tab is backgrounded. We
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// don't want to stop working entirely. So we'll fallback to a timeout loop.
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// TODO: Need a better heuristic for backgrounded work.
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};
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rAFTimeoutID = setTimeout(onTimeout, frameLength * 3);
273263

274-
if (prevRAFTime !== -1) {
264+
if (
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prevRAFTime !== -1 &&
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// Make sure this rAF time is different from the previous one. This check
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// could fail if two rAFs fire in the same frame.
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rAFTime - prevRAFTime > 0.1
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) {
275270
const rAFInterval = rAFTime - prevRAFTime;
276271
if (!fpsLocked && prevRAFInterval !== -1) {
277272
// We've observed two consecutive frame intervals. We'll use this to

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