diff options
Diffstat (limited to 'third_party/chromium/base/callback.h')
| -rw-r--r-- | third_party/chromium/base/callback.h | 411 |
1 files changed, 411 insertions, 0 deletions
diff --git a/third_party/chromium/base/callback.h b/third_party/chromium/base/callback.h new file mode 100644 index 0000000..00669dd --- /dev/null +++ b/third_party/chromium/base/callback.h @@ -0,0 +1,411 @@ +// 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(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(void)> 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(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(void)> cb = base::Bind(&MyFunc, 23, "hello world"); +// cb.Run(); +// +// A callback with no unbound input parameters (base::Callback<void(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(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. +template <typename Sig> +class Callback; + +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: + typedef R(RunType)(Args...); + + Callback() : CallbackBase(NULL) { } + + // Note that this constructor CANNOT be explicit, and that Bind() CANNOT + // return the exact Callback<> type. See base/bind.h for details. + template <typename Runnable, typename BindRunType, typename BoundArgsType> + 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: + typedef R(*PolymorphicInvoke)( + internal::BindStateBase*, + typename internal::CallbackParamTraits<Args>::ForwardType...); +}; + +// Syntactic sugar to make Callback<void(void)> easier to declare since it +// will be used in a lot of APIs with delayed execution. +typedef Callback<void(void)> Closure; + +} // namespace base + +#endif // BASE_CALLBACK_H_ |