// Copyright (c) 2012 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_CALLBACK_H_
#define BASE_CALLBACK_H_
#include "base/callback_forward.h"
#include "base/callback_internal.h"
#include "base/template_util.h"
// NOTE: Header files that do not require the full definition of Callback or
// Closure should #include "base/callback_forward.h" instead of this file.
// -----------------------------------------------------------------------------
// Introduction
// -----------------------------------------------------------------------------
//
// The templated Callback class is a generalized function object. Together
// with the Bind() function in 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 (base::Callback<void()>)
// is called a base::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.
//
// 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.
//
// The reason to pass via a const-reference is to avoid unnecessary
// AddRef/Release pairs to the internal state.
//
//
// -----------------------------------------------------------------------------
// Quick reference for basic stuff
// -----------------------------------------------------------------------------
//
// BINDING A BARE FUNCTION
//
// int Return5() { return 5; }
// base::Callback<int()> func_cb = base::Bind(&Return5);
// LOG(INFO) << func_cb.Run(); // Prints 5.
//
// 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.
//
// class Ref : public base::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);
// 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.
//
// RUNNING A CALLBACK
//
// Callbacks can be run with their "Run" method, which has the same
// signature as the template argument to the callback.
//
// void DoSomething(const base::Callback<void(int, std::string)>& callback) {
// 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).
//
// void DoSomething(const base::Callback<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:
//
// void MyFunc(int i, const std::string& str) {}
// base::Callback<void(int, const std::string&)> cb = base::Bind(&MyFunc);
// cb.Run(23, "hello, world");
//
// PASSING BOUND INPUT PARAMETERS
//
// Bound parameters are specified when you create thee 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.
//
// void MyFunc(int i, const std::string& str) {}
// base::Callback<void()> cb = base::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:
//
// base::Closure cb = base::Bind(&MyFunc, 23, "hello world");
//
// When calling member functions, bound parameters just go after the object
// pointer.
//
// base::Closure cb = base::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.
//
// void MyFunc(int i, const std::string& str) {}
// base::Callback<void(const std::string&)> cb = base::Bind(&MyFunc, 23);
// cb.Run("hello world");
//
// When calling a function bound parameters are first, followed by unbound
// parameters.
//
//
// -----------------------------------------------------------------------------
// Quick reference for advanced binding
// -----------------------------------------------------------------------------
//
// BINDING A CLASS METHOD WITH WEAK POINTERS
//
// base::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
//
// base::Bind(&MyClass::Foo, base::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
//
// MyClass* myclass = new MyClass;
// base::Bind(&MyClass::Foo, base::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.
//
// IGNORING RETURN VALUES
//
// Sometimes you want to call a function that returns a value in a callback
// that doesn't expect a return value.
//
// int DoSomething(int arg) { cout << arg << endl; }
// base::Callback<void(int)> cb =
// base::Bind(base::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).
//
// PASSING PARAMETERS OWNED BY THE CALLBACK
//
// void Foo(int* arg) { cout << *arg << endl; }
// int* pn = new int(1);
// base::Closure foo_callback = base::Bind(&foo, base::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 scoped_ptr
//
// void TakesOwnership(scoped_ptr<Foo> arg) {}
// scoped_ptr<Foo> f(new Foo);
// // f becomes null during the following call.
// base::Closure cb = base::Bind(&TakesOwnership, base::Passed(&f));
//
// Ownership of the parameter will be with the callback until the it is run,
// when 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
//
// void TakesOneRef(scoped_refptr<Foo> arg) {}
// scoped_refptr<Foo> f(new Foo)
// base::Closure cb = base::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.
//
// PASSING PARAMETERS BY REFERENCE
//
// Const references are *copied* unless ConstRef is used. Example:
//
// 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));
// 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!
//
//
// -----------------------------------------------------------------------------
// 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.
//
//
// HOW THE IMPLEMENTATION WORKS:
//
// There are three main components to the system:
// 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 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.
//
// 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.
//
// 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.
//
// 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:
// void Foo(const char* ptr);
// void Bar(char* ptr);
// Bind(&Foo, "test");
// Bind(&Bar, "test"); // This fails because ptr is not const.
namespace base {
// First, we forward declare the Callback class template. This informs the
// compiler that the template only has 1 type parameter which is the function
// signature that the Callback is representing.
//
// After this, create template specializations for 0-7 parameters. Note that
// even though the template typelist grows, the specialization still
// only has one type: the function signature.
//
// If you are thinking of forward declaring Callback in your own header file,
// please include "base/callback_forward.h" instead.
namespace internal {
template <typename Runnable, typename RunType, typename... BoundArgsType>
struct BindState;
} // namespace internal
template <typename R, typename... Args>
class Callback<R(Args...)> : public internal::CallbackBase {
public:
// MSVC 2013 doesn't support Type Alias of function types.
// Revisit this after we update it to newer version.
typedef R RunType(Args...);
Callback() : CallbackBase(nullptr) { }
template <typename Runnable, typename BindRunType, typename... BoundArgsType>
explicit Callback(
internal::BindState<Runnable, BindRunType, BoundArgsType...>* bind_state)
: CallbackBase(bind_state) {
// Force the assignment to a local variable of PolymorphicInvoke
// so the compiler will typecheck that the passed in Run() method has
// the correct type.
PolymorphicInvoke invoke_func =
&internal::BindState<Runnable, BindRunType, BoundArgsType...>
::InvokerType::Run;
polymorphic_invoke_ = reinterpret_cast<InvokeFuncStorage>(invoke_func);
}
bool Equals(const Callback& other) const {
return CallbackBase::Equals(other);
}
R Run(typename internal::CallbackParamTraits<Args>::ForwardType... args)
const {
PolymorphicInvoke f =
reinterpret_cast<PolymorphicInvoke>(polymorphic_invoke_);
return f(bind_state_.get(), internal::CallbackForward(args)...);
}
private:
using PolymorphicInvoke =
R(*)(internal::BindStateBase*,
typename internal::CallbackParamTraits<Args>::ForwardType...);
};
} // namespace base
#endif // BASE_CALLBACK_H_