| // 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_ |