Introduce the concept of media task runner.

chromecast/media/cma/base/balanced_media_task_runner_factory.h

+// Copyright 2014 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.
+
+#ifndef CHROMECAST_MEDIA_CMA_BASE_BALANCED_TASK_RUNNER_FACTORY_H_
+#define CHROMECAST_MEDIA_CMA_BASE_BALANCED_TASK_RUNNER_FACTORY_H_
+
+#include <set>
+
+#include "base/macros.h"
+#include "base/memory/ref_counted.h"
+#include "base/synchronization/lock.h"
+#include "base/time/time.h"
+
+namespace base {
+class SingleThreadTaskRunner;
+}
+
+namespace chromecast {
+namespace media {
+class BalancedMediaTaskRunner;

[lcwu1, 2014/09/04 00:32:54]

nit: Add an empty line between namespace and class lines.

+class MediaTaskRunner;
+
+// BalancedMediaTaskRunnerFactory -
+// Create media tasks runners that are loosely synchronized between each other.
+// For two tasks T1 and T2 with timestamps ts1 and ts2, the scheduler ensures
+// T2 is not scheduled before T1 if ts2 > ts1 + |max_delta|.
+class BalancedMediaTaskRunnerFactory
+    : public base::RefCountedThreadSafe<BalancedMediaTaskRunnerFactory> {
+ public:
+  explicit BalancedMediaTaskRunnerFactory(base::TimeDelta max_delta);
+
+  // Creates a media task runner using |task_runner| as the underlying
+  // regular task runner.
+  // Restriction on the returned media task runner:
+  // - can only schedule only one media task at a time.
+  // - timestamps of tasks posted on that task runner must be increasing.
+  scoped_refptr<MediaTaskRunner> CreateMediaTaskRunner(
+      const scoped_refptr<base::SingleThreadTaskRunner>& task_runner);
+
+ private:
+  typedef std::set<scoped_refptr<BalancedMediaTaskRunner> > MediaTaskRunnerSet;
+
+  friend class base::RefCountedThreadSafe<BalancedMediaTaskRunnerFactory>;
+  virtual ~BalancedMediaTaskRunnerFactory();
+
+  // Invoked when one of the registered media task runners received a new media
+  // task.
+  void OnNewTask();
+
+  // Unregister a media task runner.
+  void UnregisterMediaTaskRunner(
+      const scoped_refptr<BalancedMediaTaskRunner>& media_task_runner);
+
+  // Maximum timestamp deviation between tasks from the registered task runners.
+  const base::TimeDelta max_delta_;
+
+  // Task runners created by the factory that have not been unregistered yet.
+  base::Lock lock_;
+  MediaTaskRunnerSet task_runners_;
+
+  DISALLOW_COPY_AND_ASSIGN(BalancedMediaTaskRunnerFactory);
+};
+
+}  // namespace media
+}  // namespace chromecast
+
+#endif  // CHROMECAST_MEDIA_CMA_BASE_BALANCED_TASK_RUNNER_FACTORY_H_