Update //docs/callback.md for OnceCallback and RepeatingCallback

docs/callback.md

-# base::Callback<> and base::Bind()
+# Callback<> and Bind()
 
 ## Introduction
 
 The templated `Callback<>` class is a generalized function object. Together with
 the `Bind()` function in base/bind.h, they provide a type-safe method for
 performing partial application of functions.
 
 Partial application (or "currying") is the process of binding a subset of a
 function's arguments to produce another function that takes fewer arguments.
 This can be used to pass around a unit of delayed execution, much like lexical
 closures are used in other languages. For example, it is used in Chromium code
 to schedule tasks on different MessageLoops.
 
 A callback with no unbound input parameters (`Callback<void()>`) is called a
 `Closure`. Note that this is NOT the same as what other languages refer to as a
 closure -- it does not retain a reference to its enclosing environment.
 
+### OnceCallback<> And RepeatingCallback<>
+
+`OnceCallback<>` and `RepeatingCallback<>` are next gen callback classes, which
+are under development.
+
+`OnceCallback<>` is created by `BindOnce()`. This is a callback variant that is
+a move-only type and can be run only once. This moves out bound parameters from
+its internal storage to the bound function by default, so it's easier to use
+with movable types. This should be the preferred callback type: since the
+lifetime of the callback is clear, it's simpler to reason about when a callback
+that is passed between threads is destroyed.
+
+`RepeatingCallback<>` is created by `BindRepeating()`. This is a callback
+variant that is copyable that can be run multiple times. It uses internal
+ref-counting to make copies cheap. However, since ownership is shared, it is
+harder to reason about when the callback and the bound state are destroyed,
+especially when the callback is passed between threads.
+
+The legacy `Callback<>` is currently aliased to `RepeatingCallback<>`. In new
+code, prefer to explicitly use `OnceCallback<>` where possible or

[Yuta Kitamura, 2016/10/05 06:15:53]

"to explicitly use" => remove

"or `RepeatingCallback<>` otherwise"
=> ", and use `RepeatingCallback<>` otherwise"

[tzik, 2016/10/05 08:04:21]

Done.

+`RepeatingCallback<>` otherwise. Once the migration is complete, the type alias
+will be removed and `OnceCallback<>` will be renamed to `Callback<>` to
+emphasize that it should be preferred.
+
+`RepeatingCallback<>` is convertible to `OnceCallback<>` by the implicit
+conversion.
+
 ### Memory Management And Passing
 
-The Callback objects themselves should be passed by const-reference, and stored
-by copy. They internally store their state via a refcounted class and thus do
-not need to be deleted.
+Pass `Callback` objects by value if ownership is transferred; otherwise, pass it
+by const-reference.
 
-The reason to pass via a const-reference is to avoid unnecessary AddRef/Release
-pairs to the internal state.
+```cpp
+// |Foo| just refers to |cb| but doesn't store it nor consume it.
+bool Foo(const OnceCallback<void(int)>& cb) {
+  return cb.is_null();
+}
+
+// |Bar| takes the ownership of |cb| and stores |cb| into |g_cb|.
+OnceCallback<void(int)> g_cb;
+void Bar(OnceCallback<void(int)> cb) {
+  g_cb = std::move(cb);
+}
+
+// |Baz| takes the ownership of |cb| and consumes |cb| by Run().
+void Baz(OnceCallback<void(int)> cb) {
+  std::move(cb).Run(42);
+}
+
+// |Qux| takes the ownership of |cb| and transfers ownership to PostTask(),
+// which also takes the ownership of |cb|.
+void Qux(OnceCallback<void(int)> cb) {
+  PostTask(FROM_HERE, std::move(cb));
+}
+```
+
+When you pass a `Callback` object to a function parameter, use `std::move()` if
+you don't need to keep a reference to it, otherwise, pass the object directly.
+You may see a compile error when the function requires the exclusive ownership,
+and you didn't pass the callback by move.
 
 ## Quick reference for basic stuff
 
 ### Binding A Bare Function
 
 ```cpp
 int Return5() { return 5; }
-base::Callback<int()> func_cb = base::Bind(&Return5);
+OnceCallback<int()> func_cb = BindOnce(&Return5);
+LOG(INFO) << std::move(func_cb).Run();  // Prints 5.
+```
+
+```cpp
+int Return5() { return 5; }
+RepeatingCallback<int()> func_cb = BindRepeating(&Return5);
 LOG(INFO) << func_cb.Run();  // Prints 5.
 ```
 
+### Binding A Captureless Lambda
+
+```cpp
+Callback<int()> lambda_cb = Bind([] { return 4; });
+LOG(INFO) << lambda_cb.Run();  // Print 4.
+
+OnceCallback<int()> lambda_cb2 = BindOnce([] { return 3; });
+LOG(INFO) << std::move(lambda_cb2).Run();  // Print 3.
+```
+
 ### Binding A Class Method
 
 The first argument to bind is the member function to call, the second is the
 object on which to call it.
 
 ```cpp
-class Ref : public base::RefCountedThreadSafe<Ref> {
+class Ref : public RefCountedThreadSafe<Ref> {
  public:
   int Foo() { return 3; }
-  void PrintBye() { LOG(INFO) << "bye."; }
 };
 scoped_refptr<Ref> ref = new Ref();
-base::Callback<void()> ref_cb = base::Bind(&Ref::Foo, ref);
+Callback<void()> ref_cb = Bind(&Ref::Foo, ref);
 LOG(INFO) << ref_cb.Run();  // Prints out 3.
 ```
 
 By default the object must support RefCounted or you will get a compiler
-error. If you're passing between threads, be sure it's
-RefCountedThreadSafe! See "Advanced binding of member functions" below if
-you don't want to use reference counting.
+error. If you're passing between threads, be sure it's RefCountedThreadSafe! See
+"Advanced binding of member functions" below if you don't want to use reference
+counting.
 
 ### Running A Callback
 
-Callbacks can be run with their `Run` method, which has the same
-signature as the template argument to the callback.
+Callbacks can be run with their `Run` method, which has the same signature as
+the template argument to the callback. Note that `OnceCallback::Run` consumes
+the callback object and can only be invoked on a callback rvalue.
 
 ```cpp
-void DoSomething(const base::Callback<void(int, std::string)>& callback) {
+void DoSomething(const Callback<void(int, std::string)>& callback) {
   callback.Run(5, "hello");
 }
+
+void DoSomethingOther(OnceCallback<void(int, std::string)> callback) {
+  std::move(callback).Run(5, "hello");
+}
 ```
 
-Callbacks can be run more than once (they don't get deleted or marked when
-run). However, this precludes using base::Passed (see below).
+RepeatingCallbacks can be run more than once (they don't get deleted or marked
+when run). However, this precludes using Passed (see below).
 
 ```cpp
-void DoSomething(const base::Callback<double(double)>& callback) {
+void DoSomething(const RepeatingCallback<double(double)>& callback) {
   double myresult = callback.Run(3.14159);
   myresult += callback.Run(2.71828);
 }
 ```
 
 ### Passing Unbound Input Parameters
 
 Unbound parameters are specified at the time a callback is `Run()`. They are
 specified in the `Callback` template type:
 
 ```cpp
 void MyFunc(int i, const std::string& str) {}
-base::Callback<void(int, const std::string&)> cb = base::Bind(&MyFunc);
+Callback<void(int, const std::string&)> cb = Bind(&MyFunc);
 cb.Run(23, "hello, world");
 ```
 
 ### Passing Bound Input Parameters
 
 Bound parameters are specified when you create the callback as arguments to
 `Bind()`. They will be passed to the function and the `Run()`ner of the callback
 doesn't see those values or even know that the function it's calling.
 
 ```cpp
 void MyFunc(int i, const std::string& str) {}
-base::Callback<void()> cb = base::Bind(&MyFunc, 23, "hello world");
+Callback<void()> cb = Bind(&MyFunc, 23, "hello world");
 cb.Run();
 ```
 
-A callback with no unbound input parameters (`base::Callback<void()>`) is called
-a `base::Closure`. So we could have also written:
+A callback with no unbound input parameters (`Callback<void()>`) is called a
+`Closure`. So we could have also written:
 
 ```cpp
-base::Closure cb = base::Bind(&MyFunc, 23, "hello world");
+Closure cb = Bind(&MyFunc, 23, "hello world");
 ```
 
 When calling member functions, bound parameters just go after the object
 pointer.
 
 ```cpp
-base::Closure cb = base::Bind(&MyClass::MyFunc, this, 23, "hello world");
+Closure cb = Bind(&MyClass::MyFunc, this, 23, "hello world");
 ```
 
 ### Partial Binding Of Parameters
 
 You can specify some parameters when you create the callback, and specify the
 rest when you execute the callback.
 
 ```cpp
 void MyFunc(int i, const std::string& str) {}
-base::Callback<void(const std::string&)> cb = base::Bind(&MyFunc, 23);
+Callback<void(const std::string&)> cb = Bind(&MyFunc, 23);
 cb.Run("hello world");
 ```
 
 When calling a function bound parameters are first, followed by unbound
 parameters.
 
+### Move In and Move Out

[dcheng, 2016/10/04 22:26:25]

Random thought: maybe title this section "Avoiding Copies with Callback
Parameters" or "Moving instead of Copying Callback Parameters" or "Binding Types
without Copying", etc.

I don't particularly like any of my suggestions, but the idea is to make it
obvious that this section is for how to bind types without copying.

[tzik, 2016/10/05 08:04:21]

Done.

+
+A parameter of `Bind()` is moved into its internal storage if it is passed as a
+rvalue.
+
+```cpp
+std::vector<int> v = {1, 2, 3};
+// |v| is moved into the internal storage without copy.
+Bind(&Foo, std::move(v));
+```
+
+```cpp
+std::vector<int> v = {1, 2, 3};
+// The vector is moved into the internal storage without copy.
+Bind(&Foo, std::vector<int>({1, 2, 3}));
+```
+
+A bound object is moved out to the target function if you use `Passed()` for
+the parameter. If you use `BindOnce()`, the bound object is moved out even
+without `Passed()`.
+
+```cpp
+void Foo(std::unique_ptr<int>) {}
+std::unique_ptr<int> p(new int(42));
+
+// |p| is moved into the internal storage of Bind(), and moved out to |Foo|.
+BindOnce(&Foo, std::move(p));
+BindRepeating(&Foo, Passed(&p));
+```
+
 ## Quick reference for advanced binding
 
 ### Binding A Class Method With Weak Pointers
 
 ```cpp
-base::Bind(&MyClass::Foo, GetWeakPtr());
+Bind(&MyClass::Foo, GetWeakPtr());
 ```
 
 The callback will not be run if the object has already been destroyed.
 **DANGER**: weak pointers are not threadsafe, so don't use this when passing between
 threads!
 
 ### Binding A Class Method With Manual Lifetime Management
 
 ```cpp
-base::Bind(&MyClass::Foo, base::Unretained(this));
+Bind(&MyClass::Foo, Unretained(this));
 ```
 
 This disables all lifetime management on the object. You're responsible for
 making sure the object is alive at the time of the call. You break it, you own
 it!
 
 ### Binding A Class Method And Having The Callback Own The Class
 
 ```cpp
 MyClass* myclass = new MyClass;
-base::Bind(&MyClass::Foo, base::Owned(myclass));
+Bind(&MyClass::Foo, Owned(myclass));
 ```
 
 The object will be deleted when the callback is destroyed, even if it's not run
 (like if you post a task during shutdown). Potentially useful for "fire and
 forget" cases.
 
+Smart pointers (e.g. `std::unique_ptr<>`) are also supported as the receiver.
+
+```cpp
+std::unique_ptr<MyClass> myclass(new MyClass);
+Bind(&MyClass::Foo, std::move(myclass));
+```
+
 ### Ignoring Return Values
 
 Sometimes you want to call a function that returns a value in a callback that
 doesn't expect a return value.
 
 ```cpp
 int DoSomething(int arg) { cout << arg << endl; }
-base::Callback<void(int)> cb =
-    base::Bind(base::IgnoreResult(&DoSomething));
+Callback<void(int)> cb =
+    Bind(IgnoreResult(&DoSomething));
 ```
 
 ## Quick reference for binding parameters to Bind()
 
 Bound parameters are specified as arguments to `Bind()` and are passed to the
 function. A callback with no parameters or no unbound parameters is called a
-`Closure` (`base::Callback<void()>` and `base::Closure` are the same thing).
+`Closure` (`Callback<void()>` and `Closure` are the same thing).
 
 ### Passing Parameters Owned By The Callback
 
 ```cpp
 void Foo(int* arg) { cout << *arg << endl; }
 int* pn = new int(1);
-base::Closure foo_callback = base::Bind(&foo, base::Owned(pn));
+Closure foo_callback = Bind(&foo, Owned(pn));
 ```
 
 The parameter will be deleted when the callback is destroyed, even if it's not
 run (like if you post a task during shutdown).
 
 ### Passing Parameters As A unique_ptr
 
 ```cpp
 void TakesOwnership(std::unique_ptr<Foo> arg) {}
 std::unique_ptr<Foo> f(new Foo);
 // f becomes null during the following call.
-base::Closure cb = base::Bind(&TakesOwnership, base::Passed(&f));
+RepeatingClosure cb = BindRepeating(&TakesOwnership, Passed(&f));
 ```
 
 Ownership of the parameter will be with the callback until the callback is run,
 and then ownership is passed to the callback function. This means the callback
 can only be run once. If the callback is never run, it will delete the object
 when it's destroyed.
 
 ### Passing Parameters As A scoped_refptr
 
 ```cpp
 void TakesOneRef(scoped_refptr<Foo> arg) {}
-scoped_refptr<Foo> f(new Foo)
-base::Closure cb = base::Bind(&TakesOneRef, f);
+scoped_refptr<Foo> f(new Foo);
+Closure cb = Bind(&TakesOneRef, f);
 ```
 
 This should "just work." The closure will take a reference as long as it is
 alive, and another reference will be taken for the called function.
 
+```cpp
+void DontTakeRef(Foo* arg) {}
+scoped_refptr<Foo> f(new Foo);
+Closure cb = Bind(&DontTakeRef, RetainedRef(f));
+```
+
+`RetainedRef` holds a reference to the object and passes a raw pointer to
+the object when the Callback is run.
+
 ### Passing Parameters By Reference
 
 Const references are *copied* unless `ConstRef` is used. Example:
 
 ```cpp
 void foo(const int& arg) { printf("%d %p\n", arg, &arg); }
 int n = 1;
-base::Closure has_copy = base::Bind(&foo, n);
-base::Closure has_ref = base::Bind(&foo, base::ConstRef(n));
+Closure has_copy = Bind(&foo, n);
+Closure has_ref = Bind(&foo, ConstRef(n));
 n = 2;
 foo(n);                        // Prints "2 0xaaaaaaaaaaaa"
 has_copy.Run();                // Prints "1 0xbbbbbbbbbbbb"
 has_ref.Run();                 // Prints "2 0xaaaaaaaaaaaa"
 ```
 
 Normally parameters are copied in the closure.
-**DANGER**: ConstRef stores a const reference instead, referencing the original
-parameter. This means that you must ensure the object outlives the callback!
+**DANGER**: `ConstRef` stores a const reference instead, referencing the
+original parameter. This means that you must ensure the object outlives the
+callback!
 
 ## Implementation notes
 
 ### Where Is This Design From:
 
 The design `Callback` and Bind is heavily influenced by C++'s `tr1::function` /
 `tr1::bind`, and by the "Google Callback" system used inside Google.
 
+### Customizing the behavior
+
+There are several injection points that controls `Bind` behavior from outside of
+its implementation.
+
+```cpp
+template <typename Receiver>
+struct IsWeakReceiver {
+  static constexpr bool value = false;
+};
+
+template <typename Obj>
+struct UnwrapTraits {
+  template <typename T>
+  T&& Unwrap(T&& obj) {
+    return std::forward<T>(obj);
+  }
+};
+```
+
+If `IsWeakReceiver<Receiver>::value` is true on a receiver of a method, `Bind`
+checks if the receiver is evaluated to true and cancels the invocation if it's
+evaluated to false. You can specialize `IsWeakReceiver` to make an external
+smart pointer as a weak pointer.
+
+`UnwrapTraits<BoundObject>::Unwrap()` is called for each bound arguments right
+before `Callback` calls the target function. You can specialize this to define
+an argument wrapper such as `Unretained`, `ConstRef`, `Owned`, `RetainedRef` and
+`Passed`.
+
 ### How The Implementation Works:
 
 There are three main components to the system:
-  1) The Callback classes.
+  1) The `Callback<>` classes.
   2) The `Bind()` functions.
   3) The arguments wrappers (e.g., `Unretained()` and `ConstRef()`).
 
 The Callback classes represent a generic function pointer. Internally, it stores
 a refcounted piece of state that represents the target function and all its
-bound parameters.  Each `Callback` specialization has a templated constructor
-that takes an `BindState<>*`.  In the context of the constructor, the static
+bound parameters. The `Callback` constructor takes a `BindStateBase*`, which is
+upcasted from a `BindState<>`. In the context of the constructor, the static
 type of this `BindState<>` pointer uniquely identifies the function it is
 representing, all its bound parameters, and a `Run()` method that is capable of
 invoking the target.
 
-`Callback`'s constructor takes the `BindState<>*` that has the full static type
-and erases the target function type as well as the types of the bound
-parameters.  It does this by storing a pointer to the specific `Run()` function,
-and upcasting the state of `BindState<>*` to a `BindStateBase*`. This is safe as
-long as this `BindStateBase` pointer is only used with the stored `Run()`
-pointer.
+`Bind()` creates the `BindState<>` that has the full static type, and erases the
+target function type as well as the types of the bound parameters. It does this
+by storing a pointer to the specific `Run()` function, and upcasting the state
+of `BindState<>*` to a `BindStateBase*`. This is safe as long as this
+`BindStateBase` pointer is only used with the stored `Run()` pointer.
 
 To `BindState<>` objects are created inside the `Bind()` functions.
 These functions, along with a set of internal templates, are responsible for
 
  - Unwrapping the function signature into return type, and parameters
  - Determining the number of parameters that are bound
  - Creating the BindState storing the bound parameters
  - Performing compile-time asserts to avoid error-prone behavior
  - Returning an `Callback<>` with an arity matching the number of unbound
    parameters and that knows the correct refcounting semantics for the
    target object if we are binding a method.
 
-The `Bind` functions do the above using type-inference, and template
-specializations.
+The `Bind` functions do the above using type-inference and variadic templates.
 
 By default `Bind()` will store copies of all bound parameters, and attempt to
 refcount a target object if the function being bound is a class method. These
 copies are created even if the function takes parameters as const
 references. (Binding to non-const references is forbidden, see bind.h.)
 
 To change this behavior, we introduce a set of argument wrappers (e.g.,
 `Unretained()`, and `ConstRef()`).  These are simple container templates that
 are passed by value, and wrap a pointer to argument.  See the file-level comment
 in base/bind_helpers.h for more info.
 
-These types are passed to the `Unwrap()` functions, and the `MaybeRefcount()`
-functions respectively to modify the behavior of `Bind()`.  The `Unwrap()` and
-`MaybeRefcount()` functions change behavior by doing partial specialization
-based on whether or not a parameter is a wrapper type.
+These types are passed to the `Unwrap()` functions to modify the behavior of
+`Bind()`.  The `Unwrap()` functions change behavior by doing partial
+specialization based on whether or not a parameter is a wrapper type.
 
 `ConstRef()` is similar to `tr1::cref`.  `Unretained()` is specific to Chromium.
 
-### Why Not Tr1 Function/Bind?
-
-Direct use of `tr1::function` and `tr1::bind` was considered, but ultimately
-rejected because of the number of copy constructors invocations involved in the
-binding of arguments during construction, and the forwarding of arguments during
-invocation.  These copies will no longer be an issue in C++0x because C++0x will
-support rvalue reference allowing for the compiler to avoid these copies.
-However, waiting for C++0x is not an option.
-
-Measured with valgrind on gcc version 4.4.3 (Ubuntu 4.4.3-4ubuntu5), the
-`tr1::bind` call itself will invoke a non-trivial copy constructor three times
-for each bound parameter.  Also, each when passing a `tr1::function`, each bound
-argument will be copied again.
-
-In addition to the copies taken at binding and invocation, copying a
-`tr1::function` causes a copy to be made of all the bound parameters and state.
-
-Furthermore, in Chromium, it is desirable for the `Callback` to take a reference
-on a target object when representing a class method call. This is not supported
-by tr1.
-
-Lastly, `tr1::function` and `tr1::bind` has a more general and flexible
-API. This includes things like argument reordering by use of
-`tr1::bind::placeholder`, support for non-const reference parameters, and some
-limited amount of subtyping of the `tr1::function` object (e.g.,
-`tr1::function<int(int)>` is convertible to `tr1::function<void(int)>`).
-
-These are not features that are required in Chromium. Some of them, such as
-allowing for reference parameters, and subtyping of functions, may actually
-become a source of errors. Removing support for these features actually allows
-for a simpler implementation, and a terser Currying API.
-
-### Why Not Google Callbacks?
-
-The Google callback system also does not support refcounting.  Furthermore, its
-implementation has a number of strange edge cases with respect to type
-conversion of its arguments. In particular, the argument's constness must at
-times match exactly the function signature, or the type-inference might
-break. Given the above, writing a custom solution was easier.
-
 ### Missing Functionality
- - Invoking the return of `Bind`.  `Bind(&foo).Run()` does not work;
  - Binding arrays to functions that take a non-const pointer.
    Example:
 ```cpp
 void Foo(const char* ptr);
 void Bar(char* ptr);
 Bind(&Foo, "test");
 Bind(&Bar, "test");  // This fails because ptr is not const.
 ```
 
 If you are thinking of forward declaring `Callback` in your own header file,
 please include "base/callback_forward.h" instead.