Convert some audio code to OnceCallback.

media/base/bind_to_current_loop.h

 // Copyright (c) 2015 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 MEDIA_BASE_BIND_TO_CURRENT_LOOP_H_
 #define MEDIA_BASE_BIND_TO_CURRENT_LOOP_H_
 
 #include <memory>
+#include <utility>
 
 #include "base/bind.h"
 #include "base/location.h"
 #include "base/memory/ptr_util.h"
 #include "base/single_thread_task_runner.h"
 #include "base/threading/thread_task_runner_handle.h"
 
 // This is a helper utility for base::Bind()ing callbacks to the current
 // MessageLoop. The typical use is when |a| (of class |A|) wants to hand a
 // callback such as base::Bind(&A::AMethod, a) to |b|, but needs to ensure that
@@ skipping 31 matching lines @@
 
   ~TrampolineHelper() {
     task_runner_->PostTask(
         posted_from_,
         base::Bind(&TrampolineHelper::ClearCallbackOnTargetTaskRunner,
                    base::Passed(&callback_)));
   }
 
  private:
   static void ClearCallbackOnTargetTaskRunner(CallbackType) {}
-  static void RunOnceClosure(base::OnceClosure cb) { std::move(cb).Run(); }
 
   tracked_objects::Location posted_from_;
   scoped_refptr<base::SequencedTaskRunner> task_runner_;
   CallbackType callback_;
 };
 
 template <>
 inline void TrampolineHelper<void()>::Run() {
   task_runner_->PostTask(posted_from_, callback_);
 }
 
 template <typename... Args>
 inline void TrampolineHelper<void(Args...)>::Run(Args... args) {
-  // TODO(tzik): Use OnceCallback directly without RunOnceClosure, once
-  // TaskRunner::PostTask migrates to OnceClosure.
   base::OnceClosure cb = base::BindOnce(callback_, std::forward<Args>(args)...);
-  task_runner_->PostTask(
-      posted_from_,
-      base::Bind(&TrampolineHelper::RunOnceClosure, base::Passed(&cb)));
+  task_runner_->PostTask(posted_from_, std::move(cb));
+}
+
+// First, tell the compiler OnceTrampolineHelper is a struct template with one
+// type parameter.  Then define specializations where the type is a function
+// returning void and taking zero or more arguments.
+template <typename Signature>
+class OnceTrampolineHelper;
+
+template <typename... Args>
+class OnceTrampolineHelper<void(Args...)> {
+ public:
+  using CallbackType = base::OnceCallback<void(Args...)>;
+
+  OnceTrampolineHelper(const tracked_objects::Location& posted_from,
+                       scoped_refptr<base::SequencedTaskRunner> task_runner,
+                       CallbackType callback)
+      : posted_from_(posted_from),
+        task_runner_(std::move(task_runner)),
+        callback_(std::move(callback)) {
+    DCHECK(task_runner_);
+    DCHECK(callback_);
+  }
+
+  inline void Run(Args... args);
+
+  ~OnceTrampolineHelper() {
+    if (callback_) {
+      task_runner_->PostTask(
+          posted_from_,
+          base::BindOnce(&OnceTrampolineHelper::ClearCallbackOnTargetTaskRunner,
+                         std::move(callback_)));
+    }
+  }
+
+ private:
+  static void ClearCallbackOnTargetTaskRunner(CallbackType) {}
+
+  tracked_objects::Location posted_from_;
+  scoped_refptr<base::SequencedTaskRunner> task_runner_;
+  CallbackType callback_;
+};
+
+template <>
+inline void OnceTrampolineHelper<void()>::Run() {
+  task_runner_->PostTask(posted_from_, std::move(callback_));
+}
+
+template <typename... Args>
+inline void OnceTrampolineHelper<void(Args...)>::Run(Args... args) {
+  base::OnceClosure cb =
+      base::BindOnce(std::move(callback_), std::forward<Args>(args)...);
+  task_runner_->PostTask(posted_from_, std::move(cb));
 }
 
 }  // namespace internal
 
 template <typename T>
+inline base::OnceCallback<T> BindToCurrentLoop(base::OnceCallback<T> cb) {
+  return base::BindOnce(
+      &internal::OnceTrampolineHelper<T>::Run,
+      base::MakeUnique<internal::OnceTrampolineHelper<T>>(
+          FROM_HERE,  // TODO(tzik): Propagate FROM_HERE from the caller.
+          base::ThreadTaskRunnerHandle::Get(), std::move(cb)));
+}
+
+template <typename T>
 inline base::Callback<T> BindToCurrentLoop(base::Callback<T> cb) {
   return base::Bind(
       &internal::TrampolineHelper<T>::Run,
       base::MakeUnique<internal::TrampolineHelper<T>>(
           FROM_HERE,  // TODO(tzik): Propagate FROM_HERE from the caller.
           base::ThreadTaskRunnerHandle::Get(), std::move(cb)));
 }
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_BIND_TO_CURRENT_LOOP_H_