Lazily initialize MessageLoop for faster thread startup

base/message_loop/message_loop.h

Index: base/message_loop/message_loop.h
diff --git a/base/message_loop/message_loop.h b/base/message_loop/message_loop.h
index fd7596a79204ed89291f32b2f9135108a73b2882..c08ef7455682e16706ebe981cddca0e647733f7b 100644
--- a/base/message_loop/message_loop.h
+++ b/base/message_loop/message_loop.h
@@ -109,12 +109,33 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
 #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.
@@ -147,6 +168,9 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
     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);
@@ -394,10 +418,6 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
   // 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_; }
@@ -411,9 +431,12 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
 
  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();
@@ -490,6 +513,10 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
   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_;