media/base/bind_to_current_loop.h

   1 // Copyright (c) 2015 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #ifndef MEDIA_BASE_BIND_TO_CURRENT_LOOP_H_
   6 #define MEDIA_BASE_BIND_TO_CURRENT_LOOP_H_
   7
   8 #include "base/bind.h"
   9 #include "base/location.h"
  10 #include "base/memory/scoped_vector.h"
  11 #include "base/single_thread_task_runner.h"
  12 #include "base/thread_task_runner_handle.h"
  13
  14 // This is a helper utility for base::Bind()ing callbacks to the current
  15 // MessageLoop. The typical use is when |a| (of class |A|) wants to hand a
  16 // callback such as base::Bind(&A::AMethod, a) to |b|, but needs to ensure that
  17 // when |b| executes the callback, it does so on |a|'s current MessageLoop.
  18 //
  19 // Typical usage: request to be called back on the current thread:
  20 // other->StartAsyncProcessAndCallMeBack(
  21 //    media::BindToCurrentLoop(base::Bind(&MyClass::MyMethod, this)));
  22 //
  23 // Note that like base::Bind(), BindToCurrentLoop() can't bind non-constant
  24 // references, and that *unlike* base::Bind(), BindToCurrentLoop() makes copies
  25 // of its arguments, and thus can't be used with arrays.
  26
  27 namespace media {
  28
  29 // Mimic base::internal::CallbackForward, replacing p.Pass() with
  30 // base::Passed(&p) to account for the extra layer of indirection.
  31 namespace internal {
  32 template <typename T>
  33 T& TrampolineForward(T& t) { return t; }
  34
  35 template <typename T, typename R>
  36 base::internal::PassedWrapper<scoped_ptr<T, R> > TrampolineForward(
  37     scoped_ptr<T, R>& p) { return base::Passed(&p); }
  38
  39 template <typename T>
  40 base::internal::PassedWrapper<ScopedVector<T> > TrampolineForward(
  41     ScopedVector<T>& p) { return base::Passed(&p); }
  42
  43 // First, tell the compiler TrampolineHelper is a struct template with one
  44 // type parameter.  Then define specializations where the type is a function
  45 // returning void and taking zero or more arguments.
  46 template <typename Sig> struct TrampolineHelper;
  47
  48 template <typename... Args>
  49 struct TrampolineHelper<void(Args...)> {
  50   static void Run(
  51       const scoped_refptr<base::SingleThreadTaskRunner>& task_runner,
  52       const base::Callback<void(Args...)>& cb,
  53       Args... args) {
  54     task_runner->PostTask(FROM_HERE,
  55                           base::Bind(cb, TrampolineForward(args)...));
  56   }
  57 };
  58
  59 }  // namespace internal
  60
  61 template<typename T>
  62 static base::Callback<T> BindToCurrentLoop(
  63     const base::Callback<T>& cb) {
  64   return base::Bind(&internal::TrampolineHelper<T>::Run,
  65                     base::ThreadTaskRunnerHandle::Get(), cb);
  66 }
  67
  68 }  // namespace media
  69
  70 #endif  // MEDIA_BASE_BIND_TO_CURRENT_LOOP_H_