base: Make it possible to replace the MessageLoop's task runner

base/message_loop/message_loop.cc

 // 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.
 
 #include "base/message_loop/message_loop.h"
 
 #include <algorithm>
 
 #include "base/bind.h"
 #include "base/compiler_specific.h"
@@ skipping 153 matching lines @@
 
   // Let interested parties have one last shot at accessing this.
   FOR_EACH_OBSERVER(DestructionObserver, destruction_observers_,
                     WillDestroyCurrentMessageLoop());
 
   thread_task_runner_handle_.reset();
 
   // Tell the incoming queue that we are dying.
   incoming_task_queue_->WillDestroyCurrentMessageLoop();
   incoming_task_queue_ = NULL;
+  unbound_task_runner_ = NULL;
   task_runner_ = NULL;
 
   // OK, now make it so that no one can find us.
   lazy_tls_ptr.Pointer()->Set(NULL);
 }
 
 // static
 MessageLoop* MessageLoop::current() {
   // TODO(darin): sadly, we cannot enable this yet since people call us even
   // when they have no intention of using us.
@@ skipping 177 matching lines @@
 }
 
 bool MessageLoop::IsIdleForTesting() {
   // We only check the imcoming queue|, since we don't want to lock the work
   // queue.
   return incoming_task_queue_->IsIdleForTesting();
 }
 
 //------------------------------------------------------------------------------
 
+// static
 scoped_ptr<MessageLoop> MessageLoop::CreateUnbound(
     Type type, MessagePumpFactoryCallback pump_factory) {
   return make_scoped_ptr(new MessageLoop(type, pump_factory));
 }
 
 MessageLoop::MessageLoop(Type type, MessagePumpFactoryCallback pump_factory)
     : type_(type),
 #if defined(OS_WIN)
       pending_high_res_tasks_(0),
       in_high_res_mode_(false),
 #endif
       nestable_tasks_allowed_(true),
 #if defined(OS_WIN)
       os_modal_loop_(false),
 #endif  // OS_WIN
       pump_factory_(pump_factory),
       message_histogram_(NULL),
       run_loop_(NULL),
       incoming_task_queue_(new internal::IncomingTaskQueue(this)),
-      task_runner_(new internal::MessageLoopTaskRunner(incoming_task_queue_)) {
+      unbound_task_runner_(
+          new internal::MessageLoopTaskRunner(incoming_task_queue_)),
+      task_runner_(unbound_task_runner_) {
   // If type is TYPE_CUSTOM non-null pump_factory must be given.
   DCHECK_EQ(type_ == TYPE_CUSTOM, !pump_factory_.is_null());
 }
 
 void MessageLoop::BindToCurrentThread() {
   DCHECK(!pump_);
   if (!pump_factory_.is_null())
     pump_ = pump_factory_.Run();
   else
     pump_ = CreateMessagePumpForType(type_);
 
   DCHECK(!current()) << "should only have one message loop per thread";
   lazy_tls_ptr.Pointer()->Set(this);
 
   incoming_task_queue_->StartScheduling();
-  task_runner_->BindToCurrentThread();
+  unbound_task_runner_->BindToCurrentThread();
+  unbound_task_runner_ = nullptr;
+  SetThreadTaskRunnerHandle();
+}
+
+void MessageLoop::SetTaskRunner(
+    scoped_refptr<SingleThreadTaskRunner> task_runner) {
+  DCHECK_EQ(this, current());
+  DCHECK(task_runner->BelongsToCurrentThread());
+  DCHECK(!unbound_task_runner_);
+  task_runner_ = task_runner.Pass();
+  SetThreadTaskRunnerHandle();
+}
+
+void MessageLoop::SetThreadTaskRunnerHandle() {
+  DCHECK_EQ(this, current());
+  // Clear the previous thread task runner first because only one can exist at
+  // a time.
+  thread_task_runner_handle_.reset();
   thread_task_runner_handle_.reset(new ThreadTaskRunnerHandle(task_runner_));
 }
 
 void MessageLoop::RunHandler() {
   DCHECK_EQ(this, current());
 
   StartHistogrammer();
 
 #if defined(OS_WIN)
   if (run_loop_->dispatcher_ && type() == TYPE_UI) {
@@ skipping 308 matching lines @@
       persistent,
       mode,
       controller,
       delegate);
 }
 #endif
 
 #endif  // !defined(OS_NACL_SFI)
 
 }  // namespace base