Adds the ability for MessageLoop to take a MessagePump

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 122 matching lines @@
     TYPE_UI,
 #if defined(TOOLKIT_GTK)
     TYPE_GPU,
 #endif
     TYPE_IO,
 #if defined(OS_ANDROID)
     TYPE_JAVA,
 #endif // defined(OS_ANDROID)
   };
 
+  // Creates a MessageLoop. If |message_pump| is NULL a MessagePump based on
+  // |type| is used, otherwise MessageLoop takes ownership of |message_pump| and
+  // uses it as the MessagePump.
   // Normally, it is not necessary to instantiate a MessageLoop.  Instead, it
   // is typical to make use of the current thread's MessageLoop instance.
-  explicit MessageLoop(Type type = TYPE_DEFAULT);
+  // TODO(sky): remove default args.
+  MessageLoop(Type type = TYPE_DEFAULT, MessagePump* message_pump = NULL);

[darin (slow to review), 2013/11/07 00:17:54]

Can we use scoped_ptr<MessagePump> here? Is it hard to express a null default
value in that case?

Another option would be to just override the constructor entirely.  I suspect
the Type field is not meaningful when initializing a MessageLoop with a custom
MessagePump. Internally, we should probably have a MessageLoop type value of
TYPE_CUSTOM.

   virtual ~MessageLoop();
 
   // Returns the MessageLoop object for the current thread, or null if none.
   static MessageLoop* current();
 
   static void EnableHistogrammer(bool enable_histogrammer);
 
   typedef MessagePump* (MessagePumpFactory)();
   // Uses the given base::MessagePumpForUIFactory to override the default
   // MessagePump implementation for 'TYPE_UI'. Returns true if the factory
@@ skipping 575 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_