[SyncFS] Refine callback helpers

chrome/browser/sync_file_system/drive_backend/callback_helper.h

 // Copyright 2014 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 CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_CALLBACK_HELPER_H_
 #define CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_CALLBACK_HELPER_H_
 
 #include "base/bind.h"
 #include "base/location.h"
-#include "base/message_loop/message_loop_proxy.h"
+#include "base/logging.h"
+#include "base/thread_task_runner_handle.h"
+
+// TODO(tzik): Merge this file to media/base/bind_to_current_loop.h.
 
 namespace sync_file_system {
 namespace drive_backend {
 
 namespace internal {
 
 template <typename T>
 typename base::enable_if<
     base::internal::IsMoveOnlyType<T>::value,
     base::internal::PassedWrapper<T> >::type
 RebindForward(T& t) {
   return base::Passed(&t);
 }
 
 template <typename T>
 typename base::enable_if<
     !base::internal::IsMoveOnlyType<T>::value,
     T&>::type
 RebindForward(T& t) {
   return t;
 }
 
-template <typename T>
-void RelayToTaskRunner1(base::SequencedTaskRunner* task_runner,
-                        const tracked_objects::Location& from_here,
-                        const base::Callback<void(T)>& callback,
-                        T arg) {
-  task_runner->PostTask(
-      from_here, base::Bind(callback, RebindForward(arg)));
-}
+template <typename>
+struct RelayToTaskRunner;
+
+template <>
+struct RelayToTaskRunner<void()> {
+  static void Run(base::TaskRunner* task_runner,
+                  const tracked_objects::Location& from_here,
+                  const base::Callback<void()>& callback) {
+    task_runner->PostTask(from_here, base::Bind(
+        callback));
+  }
+};
+
+template <typename T1>
+struct RelayToTaskRunner<void(T1)> {
+  static void Run(base::TaskRunner* task_runner,
+                  const tracked_objects::Location& from_here,
+                  const base::Callback<void(T1)>& callback,
+                  T1 t1) {
+    task_runner->PostTask(from_here, base::Bind(
+        callback,
+        RebindForward(t1)));
+  }
+};
 
 template <typename T1, typename T2>
-void RelayToTaskRunner2(base::SequencedTaskRunner* task_runner,
-                        const tracked_objects::Location& from_here,
-                        const base::Callback<void(T1, T2)>& callback,
-                        T1 arg1,
-                        T2 arg2) {
-  task_runner->PostTask(
-      from_here, base::Bind(callback,
-                            RebindForward(arg1),
-                            RebindForward(arg2)));
-}
+struct RelayToTaskRunner<void(T1, T2)> {
+  static void Run(base::TaskRunner* task_runner,
+                  const tracked_objects::Location& from_here,
+                  const base::Callback<void(T1, T2)>& callback,
+                  T1 t1,
+                  T2 t2) {
+    task_runner->PostTask(from_here, base::Bind(
+        callback,
+        RebindForward(t1),
+        RebindForward(t2)));
+  }
+};
 
 }  // namespace internal
 
 template <typename T>
-base::Callback<void(T)> CreateRelayedCallback(
-    const base::Callback<void(T)>& callback) {
-  return base::Bind(&internal::RelayToTaskRunner1<T>,
-                    base::MessageLoopProxy::current(),
-                    FROM_HERE,
+base::Callback<T> RelayCallbackToTaskRunner(
+    base::TaskRunner* task_runner,
+    const tracked_objects::Location& from_here,
+    const base::Callback<T>& callback) {
+  DCHECK(task_runner->RunsTasksOnCurrentThread());
+  return base::Bind(&internal::RelayToTaskRunner<T>::Run,
+                    make_scoped_refptr(task_runner), from_here,
                     callback);
 }
 
-template <typename T1, typename T2>
-base::Callback<void(T1, T2)> CreateRelayedCallback(
-    const base::Callback<void(T1, T2)>& callback) {
-  return base::Bind(&internal::RelayToTaskRunner2<T1, T2>,
-                    base::MessageLoopProxy::current(),
-                    FROM_HERE,
-                    callback);
+template <typename T>
+base::Callback<T> RelayCallbackToCurrentThread(
+    const tracked_objects::Location& from_here,
+    const base::Callback<T>& callback) {
+  return RelayCallbackToTaskRunner(
+      base::ThreadTaskRunnerHandle::Get(),
+      from_here, callback);
 }
 
 }  // namespace drive_backend
 }  // namespace sync_file_system
 
 #endif  // CHROME_BROWSER_SYNC_FILE_SYSTEM_DRIVE_BACKEND_CALLBACK_HELPER_H_