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