Lazily initialize MessageLoop for faster thread startup

base/message_loop/message_loop.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_MESSAGE_LOOP_H_
 #define BASE_MESSAGE_LOOP_MESSAGE_LOOP_H_
 
 #include <queue>
 #include <string>
 
@@ skipping 91 matching lines @@
   enum Type {
     TYPE_DEFAULT,
     TYPE_UI,
     TYPE_CUSTOM,
     TYPE_IO,
 #if defined(OS_ANDROID)
     TYPE_JAVA,
 #endif  // defined(OS_ANDROID)
   };
 
+  using MessagePumpFactoryCallback = Callback<scoped_ptr<MessagePump>()>;
+
+  // Creates a MessageLoop of |type|.  If |type| is TYPE_CUSTOM
+  // non-null |pump_factory| must be also given to create a message pump
+  // for this message loop.  Otherwise a default message pump for
+  // the |type| is created.
+  //
+  // It is valid to create a new message loop on one thread, and then pass
+  // it to the thread  where the message loop actually runs. The message
+  // loop's BindToCurrentThread() method must be called on the thread
+  // the message loop runs on, before calling Run().

[Nico, 2015/04/27 22:49:34]

I'm not sure how to best communicate what can be done with a message loop in
this state. Maybe there should be a paragraph like "Only the Post*Task()
functions can be called on the MessageLoop while it's in this state, all other
functions may only be called after the loop has been bound to a thread"?

Or should we make the BindToCurrentThread() method private, make Thread a
friend, and assume that MessageLoops are only ever created by Thread? (This CL
couples them somewhat tightly.)

Then people reading this header would know that they're always talking  to
fully-constructed MessageLoop objects.

[kinuko, 2015/04/28 15:43:25]

Yeah that might be less confusing.  In the new CL I also made this constructor a
private static with a specific name (i.e. CreateUnbound) to make it clear that
only friend'ed classes like Thread instantiates MessageLoop that way. How does
this look to you now?

+  //
   // Normally, it is not necessary to instantiate a MessageLoop.  Instead, it
   // is typical to make use of the current thread's MessageLoop instance.
+  MessageLoop(Type type, MessagePumpFactoryCallback pump_factory);
+
+  // TODO(kinuko): Cleanup these non-default constructors in a follow-up CL
+  // for crbug.com/465458.
+  // Creates a MessageLoop of |type| for the current thread. Usually this
+  // constructor is used only for testing. No need to call Init if the
+  // message loop is constructed this way.
   explicit MessageLoop(Type type = TYPE_DEFAULT);
-  // Creates a TYPE_CUSTOM MessageLoop with the supplied MessagePump, which must
-  // be non-NULL.
-  explicit MessageLoop(scoped_ptr<base::MessagePump> pump);
+  // Creates a TYPE_CUSTOM MessageLoop for the current thread with the
+  // supplied MessagePump, which must be non-NULL. Usually used only for
+  // testing. No need to call Init if the message loop is constructed this way.

[Nico, 2015/04/27 22:49:34]

s/Init/BindToCurrentThread/

(I still think it's weird that this is different for different constructors, but
ok if you're going to do a cleanup pass afterwards)

[kinuko, 2015/04/28 15:43:25]

Done.

+  explicit MessageLoop(scoped_ptr<MessagePump> pump);
+
   ~MessageLoop() override;
 
   // Returns the MessageLoop object for the current thread, or null if none.
   static MessageLoop* current();
 
   static void EnableHistogrammer(bool enable_histogrammer);
 
   typedef scoped_ptr<MessagePump> (MessagePumpFactory)();
   // Uses the given base::MessagePumpForUIFactory to override the default
   // MessagePump implementation for 'TYPE_UI'. Returns true if the factory
@@ skipping 12 matching lines @@
   // not be run.  Instead, they will be deleted.
   //
   class BASE_EXPORT DestructionObserver {
    public:
     virtual void WillDestroyCurrentMessageLoop() = 0;
 
    protected:
     virtual ~DestructionObserver();
   };
 
+  // Configure various members and bind this message loop to the current thread.
+  void BindToCurrentThread();
+
   // Add a DestructionObserver, which will start receiving notifications
   // immediately.
   void AddDestructionObserver(DestructionObserver* destruction_observer);
 
   // Remove a DestructionObserver.  It is safe to call this method while a
   // DestructionObserver is receiving a notification callback.
   void RemoveDestructionObserver(DestructionObserver* destruction_observer);
 
   // NOTE: Deprecated; prefer task_runner() and the TaskRunner interfaces.
   // TODO(skyostil): Remove these functions (crbug.com/465354).
@@ skipping 109 matching lines @@
   void QuitNow();
 
   // TODO(jbates) remove this. crbug.com/131220. See QuitWhenIdleClosure().
   static Closure QuitClosure() { return QuitWhenIdleClosure(); }
 
   // Deprecated: use RunLoop instead.
   // Construct a Closure that will call QuitWhenIdle(). Useful to schedule an
   // arbitrary MessageLoop to QuitWhenIdle.
   static Closure QuitWhenIdleClosure();
 
   // Set the timer slack for this message loop.

[Nico, 2015/04/27 22:49:34]

// Must not be called before BindToCurrentThread() was called.

   void SetTimerSlack(TimerSlack timer_slack) {
     pump_->SetTimerSlack(timer_slack);
   }
 
   // Returns true if this loop is |type|. This allows subclasses (especially
   // those in tests) to specialize how they are identified.
   virtual bool IsType(Type type) const;
 
   // Returns the type passed to the constructor.
   Type type() const { return type_; }
@@ skipping 31 matching lines @@
   // The specific case where tasks get queued is:
   // - The thread is running a message loop.
   // - It receives a task #1 and execute it.
   // - The task #1 implicitly start a message loop, like a MessageBox in the
   //   unit test. This can also be StartDoc or GetSaveFileName.
   // - The thread receives a task #2 before or while in this second message
   //   loop.
   // - With NestableTasksAllowed set to true, the task #2 will run right away.
   //   Otherwise, it will get executed right after task #1 completes at "thread
   //   message loop level".
   void SetNestableTasksAllowed(bool allowed);

[Nico, 2015/04/27 22:49:34]

// Must not be called before BindToCurrentThread() was called.

   bool NestableTasksAllowed() const;
 
   // Enables nestable tasks on |loop| while in scope.
   class ScopedNestableTaskAllower {
    public:
     explicit ScopedNestableTaskAllower(MessageLoop* loop)
         : loop_(loop),
           old_state_(loop_->NestableTasksAllowed()) {
       loop_->SetNestableTasksAllowed(true);
     }
@@ skipping 45 matching lines @@
   // Can only be called from the thread that owns the MessageLoop.
   bool is_running() const;
 
   // Returns true if the message loop has high resolution timers enabled.
   // Provided for testing.
   bool HasHighResolutionTasks();
 
   // Returns true if the message loop is "idle". Provided for testing.
   bool IsIdleForTesting();
 
-  // Wakes up the message pump. Can be called on any thread. The caller is
-  // responsible for synchronizing ScheduleWork() calls.
-  void ScheduleWork();
-
   // Returns the TaskAnnotator which is used to add debug information to posted
   // tasks.
   debug::TaskAnnotator* task_annotator() { return &task_annotator_; }
 
   // Runs the specified PendingTask.
   void RunTask(const PendingTask& pending_task);
 
   //----------------------------------------------------------------------------
  protected:
   scoped_ptr<MessagePump> pump_;
 
  private:
   friend class RunLoop;
+  friend class internal::IncomingTaskQueue;
+  friend class ScheduleWorkTest;
 
-  // Configures various members for the two constructors.
-  void Init();
+  // Wakes up the message pump. Can be called on any thread. The caller is
+  // responsible for synchronizing ScheduleWork() calls.
+  void ScheduleWork();
 
   // Invokes the actual run loop using the message pump.
   void RunHandler();
 
   // Called to process any delayed non-nestable tasks.
   bool ProcessNextDelayedNonNestableTask();
 
   // Calls RunTask or queues the pending_task on the deferred task list if it
   // cannot be run right now.  Returns true if the task was run.
   bool DeferOrRunPendingTask(const PendingTask& pending_task);
@@ skipping 56 matching lines @@
   // A recursion block that prevents accidentally running additional tasks when
   // insider a (accidentally induced?) nested message pump.
   bool nestable_tasks_allowed_;
 
 #if defined(OS_WIN)
   // Should be set to true before calling Windows APIs like TrackPopupMenu, etc
   // which enter a modal message loop.
   bool os_modal_loop_;
 #endif
 
+  // pump_factory_.Run() is called to create a message pump for this loop
+  // if type_ is TYPE_CUSTOM and pump_ is null.
+  MessagePumpFactoryCallback pump_factory_;
+
   std::string thread_name_;
   // A profiling histogram showing the counts of various messages and events.
   HistogramBase* message_histogram_;
 
   RunLoop* run_loop_;
 
   ObserverList<TaskObserver> task_observers_;
 
   debug::TaskAnnotator task_annotator_;
 
@@ skipping 151 matching lines @@
 
 // Do not add any member variables to MessageLoopForIO!  This is important b/c
 // MessageLoopForIO is often allocated via MessageLoop(TYPE_IO).  Any extra
 // data that you need should be stored on the MessageLoop's pump_ instance.
 COMPILE_ASSERT(sizeof(MessageLoop) == sizeof(MessageLoopForIO),
                MessageLoopForIO_should_not_have_extra_member_variables);
 
 }  // namespace base
 
 #endif  // BASE_MESSAGE_LOOP_MESSAGE_LOOP_H_