Add a task-scheduler.

The current implementation is still pretty rudimentary.

packages/flutter/lib/src/scheduler/scheduler.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:async';
+import 'dart:ui' show VoidCallback;
+
+import 'package:collection/priority_queue.dart';
+import 'package:flutter/animation.dart' as animation show scheduler;
+
+/// An entry in the scheduler's priority queue.
+///
+/// Combines the task and its priority.
+class _SchedulerEntry {
+  final VoidCallback task;
+  final int priority;
+
+  const _SchedulerEntry(this.task, this.priority);
+}
+
+class Priority {
+  static const Priority idle = const Priority._(0);
+  static const Priority animation = const Priority._(100000);
+  static const Priority touch = const Priority._(200000);
+
+  /// Relative priorities are clamped by this offset.
+  ///
+  /// It is still possible to have priorities that are offset by more than this
+  /// amount by repeatedly taking relative offsets, but that's generally
+  /// discouraged.
+  static const int kMaxOffset = 10000;
+
+  const Priority._(this._value);
+
+  int get value => _value;
+  final int _value;
+
+  /// Returns a priority relative to this priority.
+  ///
+  /// A positive [offset] indicates a higher priority.
+  ///
+  /// The parameter [offset] is clamped to +/-[kMaxOffset].
+  Priority operator +(int offset) {
+    if (offset.abs() > kMaxOffset) {
+      // Clamp the input offset.
+      offset = kMaxOffset * offset.sign;
+    }
+    return new Priority._(_value + offset);
+  }
+
+  /// Returns a priority relative to this priority.
+  ///
+  /// A positive offset indicates a lower priority.
+  ///
+  /// The parameter [offset] is clamped to +/-[kMaxOffset].
+  Priority operator -(int offset) => this + (-offset);
+}
+
+/// Scheduler running tasks with specific priorities.
+///
+/// Combines the task's priority with remaining time in a frame to decide when
+/// the task should be run.
+///
+/// Tasks always run in the idle time after a frame has been committed.
+class TaskScheduler {
+  TaskScheduler._();
+
+  SchedulingStrategy schedulingStrategy = new DefaultSchedulingStrategy();
+
+  final PriorityQueue _queue = new HeapPriorityQueue<_SchedulerEntry>(
+    (_SchedulerEntry e1, _SchedulerEntry e2) {
+      // Note that we inverse the priority.
+      return -e1.priority.compareTo(e2.priority);
+    }
+  );
+
+  /// Wether this scheduler already requested to be woken up as soon as
+  /// possible.
+  bool _wakingNow = false;
+
+  /// Wether this scheduler already requested to be woken up in the next frame.
+  bool _wakingNextFrame = false;
+
+  /// Schedules the given [task] with the given [priority].
+  void schedule(VoidCallback task, Priority priority) {
+    bool isFirstTask = _queue.isEmpty;
+    _queue.add(new _SchedulerEntry(task, priority._value));
+    if (isFirstTask)
+      _wakeNow();
+  }
+
+  /// Invoked by the system when there is time to run tasks.
+  void tick() {
+    if (_queue.isEmpty)
+      return;
+    _SchedulerEntry entry = _queue.first;
+    if (schedulingStrategy.shouldRunTaskWithPriority(entry.priority)) {
+      try {
+        (_queue.removeFirst().task)();
+      } finally {
+        if (_queue.isNotEmpty)
+          _wakeNow();
+      }
+    } else {
+      _wakeNextFrame();
+    }
+  }
+
+  /// Tells the system that the scheduler is awake and should be called as
+  /// soon a there is time.
+  void _wakeNow() {
+    if (_wakingNow)
+      return;
+    _wakingNow = true;
+    Timer.run(() {
+      _wakingNow = false;
+      tick();
+    });
+  }
+
+  /// Tells the system that the scheduler needs to run again (ideally next
+  /// frame).
+  void _wakeNextFrame() {
+    if (_wakingNextFrame)
+      return;
+    _wakingNextFrame = true;
+    animation.scheduler.requestAnimationFrame((_) {
+      _wakingNextFrame = false;
+      // RequestAnimationFrame calls back at the beginning of a frame. We want
+      // to run in the idle-phase of an animation. We therefore request to be
+      // woken up as soon as possible.
+      _wakeNow();
+    });
+  }
+}
+
+final TaskScheduler tasks = new TaskScheduler._();
+
+abstract class SchedulingStrategy {
+  bool shouldRunTaskWithPriority(int priority);
+}
+
+class DefaultSchedulingStrategy implements SchedulingStrategy {
+  // TODO(floitsch): for now we only expose the priority. It might be
+  // interesting to provide more info (like, how long the task ran the last
+  // time).
+  bool shouldRunTaskWithPriority(int priority) {
+    if (animation.scheduler.transientCallbackCount > 0)
+      return priority >= Priority.animation._value;
+    return true;
+  }
+}

packages/flutter/lib/task.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+/// The Flutter Task library.
+///
+/// This library provides means to schedule tasks at given priorities.
+/// The library will make sure that tasks are only run when appropriate.
+/// For example, an idle-task will not run when an animation is running.
+library task;
+
+export 'src/scheduler/scheduler.dart';

packages/flutter/pubspec.yaml

@@ -4,6 +4,7 @@ author: Flutter Authors <flutter-dev@googlegroups.com>
 description: A framework for writing Flutter applications
 homepage: http://flutter.io
 dependencies:
+  collection: '>=1.1.3 <2.0.0'
   intl: '>=0.12.4+2 <0.13.0'
   material_design_icons: '>=0.0.3 <0.1.0'
   sky_engine: 0.0.55

packages/unit/test/scheduler/scheduler_test.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/task.dart';
+import 'package:test/test.dart';
+
+class TestStrategy implements SchedulingStrategy {
+  int allowedPriority = 10000;
+
+  bool shouldRunTaskWithPriority(int priority) {
+    return priority >= allowedPriority;
+  }
+}
+
+void main() {
+  test("Tasks are executed in the right order", () {
+    var strategy = new TestStrategy();
+    tasks.schedulingStrategy = strategy;
+    List input = [2, 23, 23, 11, 0, 80, 3];
+    List executedTasks = [];
+
+    void scheduleAddingTask(int x) {
+      tasks.schedule(() { executedTasks.add(x); }, Priority.idle + x);
+    }
+
+    for (int x in input) {
+      scheduleAddingTask(x);
+    }
+    strategy.allowedPriority = 100;
+    for (int i = 0; i < 3; i++) tasks.tick();
+    expect(executedTasks.isEmpty, isTrue);
+
+    strategy.allowedPriority = 50;
+    for (int i = 0; i < 3; i++) tasks.tick();
+    expect(executedTasks.length, equals(1));
+    expect(executedTasks.single, equals(80));
+    executedTasks.clear();
+
+    strategy.allowedPriority = 20;
+    for (int i = 0; i < 3; i++) tasks.tick();
+    expect(executedTasks.length, equals(2));
+    expect(executedTasks[0], equals(23));
+    expect(executedTasks[1], equals(23));
+    executedTasks.clear();
+
+    scheduleAddingTask(99);
+    scheduleAddingTask(19);
+    scheduleAddingTask(5);
+    scheduleAddingTask(97);
+    for (int i = 0; i < 3; i++) tasks.tick();
+    expect(executedTasks.length, equals(2));
+    expect(executedTasks[0], equals(99));
+    expect(executedTasks[1], equals(97));
+    executedTasks.clear();
+
+    strategy.allowedPriority = 10;
+    for (int i = 0; i < 3; i++) tasks.tick();
+    expect(executedTasks.length, equals(2));
+    expect(executedTasks[0], equals(19));
+    expect(executedTasks[1], equals(11));
+    executedTasks.clear();
+
+    strategy.allowedPriority = 1;
+    for (int i = 0; i < 4; i++) tasks.tick();
+    expect(executedTasks.length, equals(3));
+    expect(executedTasks[0], equals(5));
+    expect(executedTasks[1], equals(3));
+    expect(executedTasks[2], equals(2));
+    executedTasks.clear();
+
+    strategy.allowedPriority = 0;
+    tasks.tick();
+    expect(executedTasks.length, equals(1));
+    expect(executedTasks[0], equals(0));
+  });
+}