Scheduler should be able to cancel a queued callback

Previously, once a callback was in the execution queue, it couldn't be
canceled. Now we check whether the callback was canceled before executing it.

packages/flutter/lib/base/scheduler.dart

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+import 'dart:collection';
 import 'dart:sky' as sky;
 
 import 'package:sky/base/debug.dart';
@@ -12,9 +13,10 @@ bool _haveScheduledVisualUpdate = false;
 int _nextCallbackId = 1;
 
 final List<Callback> _persistentCallbacks = new List<Callback>();
-Map<int, Callback> _transientCallbacks = new Map<int, Callback>();
+Map<int, Callback> _transientCallbacks = new LinkedHashMap<int, Callback>();
+final Set<int> _removedIds = new Set<int>();
 
-void _beginFrame(double timeStamp) {
+void beginFrame(double timeStamp) {
   timeStamp /= timeDilation;
 
   _haveScheduledVisualUpdate = false;
@@ -23,15 +25,17 @@ void _beginFrame(double timeStamp) {
   _transientCallbacks = new Map<int, Callback>();
 
   callbacks.forEach((id, callback) {
-    callback(timeStamp);
+    if (!_removedIds.contains(id))
+      callback(timeStamp);
   });
+  _removedIds.clear();
 
   for (Callback callback in _persistentCallbacks)
     callback(timeStamp);
 }
 
 void init() {
-  sky.view.setFrameCallback(_beginFrame);
+  sky.view.setFrameCallback(beginFrame);
 }
 
 void addPersistentFrameCallback(Callback callback) {
@@ -47,6 +51,7 @@ int requestAnimationFrame(Callback callback) {
 
 void cancelAnimationFrame(int id) {
   _transientCallbacks.remove(id);
+  _removedIds.add(id);
 }
 
 void ensureVisualUpdate() {

packages/unit/test/base/scheduler_test.dart

+import 'package:sky/base/scheduler.dart' as scheduler;
+import 'package:test/test.dart';
+
+void main() {
+  test("Can cancel queued callback", () {
+    int secondId;
+
+    bool firstCallbackRan = false;
+    bool secondCallbackRan = false;
+
+    void firstCallback(double timeStamp) {
+      expect(firstCallbackRan, isFalse);
+      expect(secondCallbackRan, isFalse);
+      expect(timeStamp, equals(16.0));
+      firstCallbackRan = true;
+      scheduler.cancelAnimationFrame(secondId);
+    }
+
+    void secondCallback(double timeStamp) {
+      expect(firstCallbackRan, isTrue);
+      expect(secondCallbackRan, isFalse);
+      expect(timeStamp, equals(16.0));
+      secondCallbackRan = true;
+    }
+
+    scheduler.requestAnimationFrame(firstCallback);
+    secondId = scheduler.requestAnimationFrame(secondCallback);
+
+    scheduler.beginFrame(16.0);
+
+    expect(firstCallbackRan, isTrue);
+    expect(secondCallbackRan, isFalse);
+
+    firstCallbackRan = false;
+    secondCallbackRan = false;
+
+    scheduler.beginFrame(32.0);
+
+    expect(firstCallbackRan, isFalse);
+    expect(secondCallbackRan, isFalse);
+  });
+}