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 "base/bind.h"
#include "base/location.h"
#include "base/memory/scoped_vector.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
// when |b| executes the callback, it does so on |a|'s current MessageLoop.
//
// Typical usage: request to be called back on the current thread:
// other->StartAsyncProcessAndCallMeBack(
//    media::BindToCurrentLoop(base::Bind(&MyClass::MyMethod, this)));
//
// Note that like base::Bind(), BindToCurrentLoop() can't bind non-constant
// references, and that *unlike* base::Bind(), BindToCurrentLoop() makes copies
// of its arguments, and thus can't be used with arrays.

namespace media {

// Mimic base::internal::CallbackForward, replacing std::move(p) with
// base::Passed(&p) to account for the extra layer of indirection.
namespace internal {
template <typename T>
T& TrampolineForward(T& t) { return t; }

template <typename T, typename R>
base::internal::PassedWrapper<std::unique_ptr<T, R>> TrampolineForward(
    std::unique_ptr<T, R>& p) {
  return base::Passed(&p);
}

template <typename T>
base::internal::PassedWrapper<ScopedVector<T> > TrampolineForward(
    ScopedVector<T>& p) { return base::Passed(&p); }

// First, tell the compiler TrampolineHelper 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 Sig> struct TrampolineHelper;

template <typename... Args>
struct TrampolineHelper<void(Args...)> {
  static void Run(
      const scoped_refptr<base::SingleThreadTaskRunner>& task_runner,
      const base::Callback<void(Args...)>& cb,
      Args... args) {
    task_runner->PostTask(FROM_HERE,
                          base::Bind(cb, TrampolineForward(args)...));
  }
};

}  // namespace internal

template<typename T>
static base::Callback<T> BindToCurrentLoop(
    const base::Callback<T>& cb) {
  return base::Bind(&internal::TrampolineHelper<T>::Run,
                    base::ThreadTaskRunnerHandle::Get(), cb);
}

}  // namespace media

#endif  // MEDIA_BASE_BIND_TO_CURRENT_LOOP_H_