Introduce OnceClosure and BindOnce

base/callback_forward.h

Index: base/callback_forward.h
diff --git a/base/callback_forward.h b/base/callback_forward.h
index 8b9b89cdc231bcf1020111e91bcbda14fd4e678c..58b3f29e2e942f5c9f7e6035ead5a07a497e23c6 100644
--- a/base/callback_forward.h
+++ b/base/callback_forward.h
@@ -15,16 +15,36 @@ enum class CopyMode {
   MoveOnly, Copyable,
 };
 
+enum class RepeatMode {
+  Once,

[dcheng, 2016/09/08 04:09:01]

Super minor optional nit: here, the order is Once, Repeating, but everywhere
else is Repeating, Once.

I think it would be nice to be uniform (my preferred order is Once, Repeating
since we want to guide people to use OnceCallback) but that requires moving more
things around.

Again, this is totally optional and up to you if you want to.

[tzik, 2016/09/12 11:00:34]

Done. I moved all .*Once.* before Repeating.

+  Repeating,
+};
+
 }  // namespace internal
 
 template <typename Signature,
-          internal::CopyMode copy_mode = internal::CopyMode::Copyable>
+          internal::CopyMode copy_mode = internal::CopyMode::Copyable,
+          internal::RepeatMode repeat_mode = internal::RepeatMode::Repeating>
 class Callback;
 
 // Syntactic sugar to make Callback<void()> easier to declare since it
 // will be used in a lot of APIs with delayed execution.
 using Closure = Callback<void()>;
 
+namespace internal {
+
+template <typename Signature>
+using RepeatingCallback = Callback<Signature,
+                                   internal::CopyMode::Copyable,
+                                   internal::RepeatMode::Repeating>;
+template <typename Signature>
+using OnceCallback = Callback<Signature,
+                              internal::CopyMode::MoveOnly,
+                              internal::RepeatMode::Once>;
+using RepeatingClosure = RepeatingCallback<void()>;
+using OnceClosure = OnceCallback<void()>;
+
+}  // namespace internal
 }  // namespace base
 
 #endif  // BASE_CALLBACK_FORWARD_H_