Introduce OnceClosure and BindOnce

base/bind.h

 // Copyright (c) 2011 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 BASE_BIND_H_
 #define BASE_BIND_H_
 
 #include "base/bind_internal.h"
 
 // -----------------------------------------------------------------------------
 // Usage documentation
 // -----------------------------------------------------------------------------
 //
 // See base/callback.h for documentation.
 //
 //
 // -----------------------------------------------------------------------------
 // Implementation notes
 // -----------------------------------------------------------------------------
 //
 // If you're reading the implementation, before proceeding further, you should
 // read the top comment of base/bind_internal.h for a definition of common
 // terms and concepts.
 
 namespace base {
 
+// Default Bind.
 template <typename Functor, typename... Args>
-inline base::Callback<MakeUnboundRunType<Functor, Args...>> Bind(
-    Functor&& functor,
-    Args&&... args) {
+inline base::Callback<MakeUnboundRunType<Functor, Args...>>

[hiroshige, 2016/07/04 10:04:22]

It might be unclear for developers which Bind()/Callback to use, especially
because Callback and RepeatingCallback are the same class.

IIUC Bind()/Callback are legacy/unannotated, and new CLs should use
BindRepeating() or BindOneShot().
We have two names of Bind()/BindRepeating() (and Callback/RepeatingCallback) of
the same type, just for annotating purposes.
I think we need comments (and/or PSA?).

[tzik, 2016/07/06 07:36:13]

Hmm, I think it's still not ready to use for devs, since other threading
primitives uses old Callbacks. Let me hide new variants into `internal`
namespace.
I'd propose to unveil them with another wip CL that updates TaskRunners:
https://codereview.chromium.org/2122543002/

[hiroshige, 2016/08/12 06:00:47]

Placing new variants in internal namespace looks good to me.

+Bind(Functor&& functor, Args&&... args) {
   using BindState = internal::MakeBindStateType<Functor, Args...>;
   using UnboundRunType = MakeUnboundRunType<Functor, Args...>;
   using Invoker = internal::Invoker<BindState, UnboundRunType>;
 
   using CallbackType = Callback<UnboundRunType>;
   return CallbackType(new BindState(std::forward<Functor>(functor),
                                     std::forward<Args>(args)...),
                       &Invoker::Run);
 }
 
+// Bind as RepeatingCallback.
+template <typename Functor, typename... Args>
+inline base::RepeatingCallback<MakeUnboundRunType<Functor, Args...>>
+BindRepeating(Functor&& functor, Args&&... args) {
+  using BindState = internal::MakeBindStateType<Functor, Args...>;
+  using UnboundRunType = MakeUnboundRunType<Functor, Args...>;
+  using Invoker = internal::Invoker<BindState, UnboundRunType>;
+
+  using CallbackType = RepeatingCallback<UnboundRunType>;
+  return CallbackType(new BindState(std::forward<Functor>(functor),
+                                    std::forward<Args>(args)...),
+                      &Invoker::Run);
+}
+
+// Bind as OneShotCallback.
+template <typename Functor, typename... Args>
+inline base::OneShotCallback<MakeUnboundRunType<Functor, Args...>>
+BindOneShot(Functor&& functor, Args&&... args) {
+  using BindState = internal::MakeBindStateType<Functor, Args...>;
+  using UnboundRunType = MakeUnboundRunType<Functor, Args...>;
+  using Invoker = internal::Invoker<BindState, UnboundRunType>;
+
+  using CallbackType = OneShotCallback<UnboundRunType>;
+  return CallbackType(new BindState(std::forward<Functor>(functor),
+                                    std::forward<Args>(args)...),
+                      &Invoker::RunOneShot);
+}
+
 }  // namespace base
 
 #endif  // BASE_BIND_H_