Add support for priorities to TaskQueue.

webrtc/base/task_queue.h

 /*
  *  Copyright 2016 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 143 matching lines @@
 //
 // A note on destruction:
 //
 // When a TaskQueue is deleted, pending tasks will not be executed but they will
 // be deleted.  The deletion of tasks may happen asynchronously after the
 // TaskQueue itself has been deleted or it may happen synchronously while the
 // TaskQueue instance is being deleted.  This may vary from one OS to the next
 // so assumptions about lifetimes of pending tasks should not be made.
 class LOCKABLE TaskQueue {
  public:
-  explicit TaskQueue(const char* queue_name);
-  // TODO(tommi): Implement move semantics?
+  // TaskQueue priority levels. On some platforms these will map to thread
+  // priorities, on others such as Mac and iOS, GCD queue priorities.
+  enum class Priority {
+    NORMAL = 0,
+    HIGH,
+    LOW,
+  };
+
+  explicit TaskQueue(const char* queue_name,
+                     Priority priority = Priority::NORMAL);
   ~TaskQueue();
 
   static TaskQueue* Current();
 
   // Used for DCHECKing the current queue.
   static bool IsCurrent(const char* queue_name);
   bool IsCurrent() const;
 
   // TODO(tommi): For better debuggability, implement RTC_FROM_HERE.
 
@@ skipping 92 matching lines @@
 #elif defined(WEBRTC_WIN)
   class MultimediaTimer;
   typedef std::unordered_map<UINT_PTR, std::unique_ptr<QueuedTask>>
       DelayedTasks;
   static void ThreadMain(void* context);
   static bool ProcessQueuedMessages(DelayedTasks* delayed_tasks,
                                     std::vector<MultimediaTimer>* timers);
 
   class WorkerThread : public PlatformThread {
    public:
-    WorkerThread(ThreadRunFunction func, void* obj, const char* thread_name)
-        : PlatformThread(func, obj, thread_name) {}
+    WorkerThread(ThreadRunFunction func,
+                 void* obj,
+                 const char* thread_name,
+                 ThreadPriority priority)
+        : PlatformThread(func, obj, thread_name, priority) {}
 
     bool QueueAPC(PAPCFUNC apc_function, ULONG_PTR data) {
       return PlatformThread::QueueAPC(apc_function, data);
     }
   };
   WorkerThread thread_;
 #else
 #error not supported.
 #endif
 
   RTC_DISALLOW_COPY_AND_ASSIGN(TaskQueue);
 };
 
 }  // namespace rtc
 
 #endif  // WEBRTC_BASE_TASK_QUEUE_H_