Add version of scheduler that only swaps MessageChannel for postTask (#20206)

* Fork SchedulerDOM to SchedulerPostTaskOnly

* Swap in postTask for MessageChannel

* Add SchedulerPostTaskOnly-test.js

* Update getCurrentTime

* Gate tests to source

* Prettier

Author: rickhanlonii

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

@@ -0,0 +1,275 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @emails react-core
+ * @jest-environment node
+ */
+
+/* eslint-disable no-for-of-loops/no-for-of-loops */
+
+'use strict';
+
+let Scheduler;
+let runtime;
+let performance;
+let cancelCallback;
+let scheduleCallback;
+let NormalPriority;
+
+describe('SchedulerPostTaskOnly', () => {
+  beforeEach(() => {
+    if (!process.env.IS_BUILD) {
+      jest.resetModules();
+
+      // Un-mock scheduler
+      jest.mock('scheduler', () =>
+        require.requireActual('scheduler/unstable_post_task_only'),
+      );
+
+      runtime = installMockBrowserRuntime();
+      performance = window.performance;
+      Scheduler = require('scheduler');
+      cancelCallback = Scheduler.unstable_cancelCallback;
+      scheduleCallback = Scheduler.unstable_scheduleCallback;
+      NormalPriority = Scheduler.unstable_NormalPriority;
+    }
+  });
+
+  afterEach(() => {
+    if (!process.env.IS_BUILD) {
+      if (!runtime.isLogEmpty()) {
+        throw Error('Test exited without clearing log.');
+      }
+    }
+  });
+
+  function installMockBrowserRuntime() {
+    let hasPendingTask = false;
+    let timerIDCounter = 0;
+    let eventLog = [];
+
+    // Mock window functions
+    const window = {};
+    global.window = window;
+
+    // TODO: Scheduler no longer requires these methods to be polyfilled. But
+    // maybe we want to continue warning if they don't exist, to preserve the
+    // option to rely on it in the future?
+    window.requestAnimationFrame = window.cancelAnimationFrame = () => {};
+
+    let currentTime = 0;
+    window.performance = {
+      now() {
+        return currentTime;
+      },
+    };
+
+    window.setTimeout = (cb, delay) => {
+      const id = timerIDCounter++;
+      log(`Set Timer`);
+      // TODO
+      return id;
+    };
+    window.clearTimeout = id => {
+      // TODO
+    };
+
+    // Mock browser scheduler.
+    const scheduler = {};
+    global.scheduler = scheduler;
+
+    let nextTask;
+    scheduler.postTask = function(callback) {
+      if (hasPendingTask) {
+        throw Error('Task already scheduled');
+      }
+      log('Post Task');
+      hasPendingTask = true;
+      nextTask = callback;
+    };
+
+    function ensureLogIsEmpty() {
+      if (eventLog.length !== 0) {
+        throw Error('Log is not empty. Call assertLog before continuing.');
+      }
+    }
+
+    function advanceTime(ms) {
+      currentTime += ms;
+    }
+
+    function fireNextTask() {
+      ensureLogIsEmpty();
+      if (!hasPendingTask) {
+        throw Error('No task was scheduled');
+      }
+      hasPendingTask = false;
+
+      log('Task Event');
+
+      // If there's a continuation, it will call postTask again
+      // which will set nextTask. That means we need to clear
+      // nextTask before the invocation, otherwise we would
+      // delete the continuation task.
+      const task = nextTask;
+      nextTask = null;
+      task();
+    }
+
+    function log(val) {
+      eventLog.push(val);
+    }
+
+    function isLogEmpty() {
+      return eventLog.length === 0;
+    }
+
+    function assertLog(expected) {
+      const actual = eventLog;
+      eventLog = [];
+      expect(actual).toEqual(expected);
+    }
+
+    return {
+      advanceTime,
+      fireNextTask,
+      log,
+      isLogEmpty,
+      assertLog,
+    };
+  }
+
+  // @gate source
+  it('task that finishes before deadline', () => {
+    scheduleCallback(NormalPriority, () => {
+      runtime.log('Task');
+    });
+    runtime.assertLog(['Post Task']);
+    runtime.fireNextTask();
+    runtime.assertLog(['Task Event', 'Task']);
+  });
+
+  // @gate source
+  it('task with continuation', () => {
+    scheduleCallback(NormalPriority, () => {
+      runtime.log('Task');
+      while (!Scheduler.unstable_shouldYield()) {
+        runtime.advanceTime(1);
+      }
+      runtime.log(`Yield at ${performance.now()}ms`);
+      return () => {
+        runtime.log('Continuation');
+      };
+    });
+    runtime.assertLog(['Post Task']);
+
+    runtime.fireNextTask();
+    runtime.assertLog(['Task Event', 'Task', 'Yield at 5ms', 'Post Task']);
+
+    runtime.fireNextTask();
+    runtime.assertLog(['Task Event', 'Continuation']);
+  });
+
+  // @gate source
+  it('multiple tasks', () => {
+    scheduleCallback(NormalPriority, () => {
+      runtime.log('A');
+    });
+    scheduleCallback(NormalPriority, () => {
+      runtime.log('B');
+    });
+    runtime.assertLog(['Post Task']);
+    runtime.fireNextTask();
+    runtime.assertLog(['Task Event', 'A', 'B']);
+  });
+
+  // @gate source
+  it('multiple tasks with a yield in between', () => {
+    scheduleCallback(NormalPriority, () => {
+      runtime.log('A');
+      runtime.advanceTime(4999);
+    });
+    scheduleCallback(NormalPriority, () => {
+      runtime.log('B');
+    });
+    runtime.assertLog(['Post Task']);
+    runtime.fireNextTask();
+    runtime.assertLog([
+      'Task Event',
+      'A',
+      // Ran out of time. Post a continuation event.
+      'Post Task',
+    ]);
+    runtime.fireNextTask();
+    runtime.assertLog(['Task Event', 'B']);
+  });
+
+  // @gate source
+  it('cancels tasks', () => {
+    const task = scheduleCallback(NormalPriority, () => {
+      runtime.log('Task');
+    });
+    runtime.assertLog(['Post Task']);
+    cancelCallback(task);
+    runtime.assertLog([]);
+  });
+
+  // @gate source
+  it('throws when a task errors then continues in a new event', () => {
+    scheduleCallback(NormalPriority, () => {
+      runtime.log('Oops!');
+      throw Error('Oops!');
+    });
+    scheduleCallback(NormalPriority, () => {
+      runtime.log('Yay');
+    });
+    runtime.assertLog(['Post Task']);
+
+    expect(() => runtime.fireNextTask()).toThrow('Oops!');
+    runtime.assertLog(['Task Event', 'Oops!', 'Post Task']);
+
+    runtime.fireNextTask();
+    runtime.assertLog(['Task Event', 'Yay']);
+  });
+
+  // @gate source
+  it('schedule new task after queue has emptied', () => {
+    scheduleCallback(NormalPriority, () => {
+      runtime.log('A');
+    });
+
+    runtime.assertLog(['Post Task']);
+    runtime.fireNextTask();
+    runtime.assertLog(['Task Event', 'A']);
+
+    scheduleCallback(NormalPriority, () => {
+      runtime.log('B');
+    });
+    runtime.assertLog(['Post Task']);
+    runtime.fireNextTask();
+    runtime.assertLog(['Task Event', 'B']);
+  });
+
+  // @gate source
+  it('schedule new task after a cancellation', () => {
+    const handle = scheduleCallback(NormalPriority, () => {
+      runtime.log('A');
+    });
+
+    runtime.assertLog(['Post Task']);
+    cancelCallback(handle);
+
+    runtime.fireNextTask();
+    runtime.assertLog(['Task Event']);
+
+    scheduleCallback(NormalPriority, () => {
+      runtime.log('B');
+    });
+    runtime.assertLog(['Post Task']);
+    runtime.fireNextTask();
+    runtime.assertLog(['Task Event', 'B']);
+  });
+});