Lazily initialize MessageLoop for faster thread startup

base/message_loop/incoming_task_queue.h

 // Copyright 2013 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 BASE_MESSAGE_LOOP_INCOMING_TASK_QUEUE_H_
 #define BASE_MESSAGE_LOOP_INCOMING_TASK_QUEUE_H_
 
 #include "base/base_export.h"
 #include "base/memory/ref_counted.h"
 #include "base/pending_task.h"
 #include "base/synchronization/lock.h"
 #include "base/time/time.h"
 
 namespace base {
 
 class MessageLoop;
 class WaitableEvent;
 
 namespace internal {
 
 // Implements a queue of tasks posted to the message loop running on the current
 // thread. This class takes care of synchronizing posting tasks from different
 // threads and together with MessageLoop ensures clean shutdown.
 class BASE_EXPORT IncomingTaskQueue
     : public RefCountedThreadSafe<IncomingTaskQueue> {
  public:
   explicit IncomingTaskQueue(MessageLoop* message_loop);
 
+  // Creates a task queue for the message loop that is to be initialized lazily
+  // (i.e. created by CreateForLazyInit()).

[rvargas (doing something else), 2015/03/26 02:19:26]

nit: try to keep cross-refs to a minimum (as in document what this function does
and not who call this). (and below)

[kinuko, 2015/04/13 02:02:59]

Done.

+  static IncomingTaskQueue* CreateForLazyInitMessageLoop(

[rvargas (doing something else), 2015/03/26 02:19:26]

return a scoped ptr (and same comment about this name suggesting that this is a
special case instead of the default)

[kinuko, 2015/04/13 02:02:59]

Done. (This is ref-counted so returning scoped_refptr)

+      MessageLoop* message_loop);
+
   // Appends a task to the incoming queue. Posting of all tasks is routed though
   // AddToIncomingQueue() or TryAddToIncomingQueue() to make sure that posting
   // task is properly synchronized between different threads.
   //
   // Returns true if the task was successfully added to the queue, otherwise
   // returns false. In all cases, the ownership of |task| is transferred to the
   // called method.
   bool AddToIncomingQueue(const tracked_objects::Location& from_here,
                           const Closure& task,
                           TimeDelta delay,
                           bool nestable);
 
   // Returns true if the queue contains tasks that require higher than default
   // timer resolution. Currently only needed for Windows.
   bool HasHighResolutionTasks();
 
   // Returns true if the message loop is "idle". Provided for testing.
   bool IsIdleForTesting();
 
   // Loads tasks from the |incoming_queue_| into |*work_queue|. Must be called
   // from the thread that is running the loop. Returns the number of tasks that
   // require high resolution timers.
   int ReloadWorkQueue(TaskQueue* work_queue);
 
   // Disconnects |this| from the parent message loop.
   void WillDestroyCurrentMessageLoop();
 
+  // Called when |message_loop_|'s LazyInit() is called, so that now it's ready
+  // for scheduling work. It is not valid to call this unless this task queue
+  // is created by CreateForLazyInitMessageLoop().
+  void DidInitializeMessageLoop();

[rvargas (doing something else), 2015/03/26 02:19:26]

Maybe StartScheduling() ? 

[kinuko, 2015/04/13 02:02:59]

Done.

+
+  bool empty();

[rvargas (doing something else), 2015/03/26 02:19:26]

nit: doc

[kinuko, 2015/04/13 02:02:59]

Done.

+
  private:
   friend class RefCountedThreadSafe<IncomingTaskQueue>;
   virtual ~IncomingTaskQueue();
 
   // Calculates the time at which a PendingTask should run.
   TimeTicks CalculateDelayedRuntime(TimeDelta delay);
 
   // Adds a task to |incoming_queue_|. The caller retains ownership of
   // |pending_task|, but this function will reset the value of
   // |pending_task->task|. This is needed to ensure that the posting call stack
@@ skipping 19 matching lines @@
   int next_sequence_num_;
 
   // True if our message loop has already been scheduled and does not need to be
   // scheduled again until an empty reload occurs.
   bool message_loop_scheduled_;
 
   // True if we always need to call ScheduleWork when receiving a new task, even
   // if the incoming queue was not empty.
   const bool always_schedule_work_;
 
+  // True if our message loop is created for lazy initialization and hasn't been
+  // initialized yet. Note that while this is true this task queue won't call
+  // ScheduleWork even if always_schedule_work_ is true.
+  bool message_loop_not_initialized_ = false;
+
   DISALLOW_COPY_AND_ASSIGN(IncomingTaskQueue);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_MESSAGE_LOOP_INCOMING_TASK_QUEUE_H_