Қайта шақыру функциясы: түсіндірме және қолданылуы

A function passed as data to and to be called by another function
a discussion of callback with computer modems

In computer programming, a callback is a function that is stored as data (a reference) and designed to be called by another function often back to the original abstraction layer. A function that accepts a callback parameter may be designed to call back before returning to its caller which is known as synchronous or blocking. The function that accepts a callback may be designed to store the callback so that it can be called back after returning which is known as asynchronous, non blocking or deferred. Programming languages support callbacks in different ways such as function pointers, lambda expressions and blocks.

To aid understanding the concept, the following is an analogy from real life. A customer visits a store to place an order. This is like the first call. The customer gives to a clerk a list of items, a check to cover their cost and delivery instructions. These are the parameters of the first call including the callback which is the delivery instructions. It is understood that the check will be cashed and that the instructions will be followed. When the staff are able, they deliver the items as instructed which is like calling the callback. Notably, the delivery need not be made by the clerk who took the order. A callback need not be called by the function that accepted the callback as a parameter. Also, the delivery need not be made directly to the customer. A callback need not be to the calling function. In fact, a function would generally not pass itself as a callback. Some find the use of back to be misleading since the call is (generally) not back to the original caller as it is for a telephone call.

A blocking callback runs in the execution context of the function that passes the callback. A deferred callback can run in a different context such as during interrupt or from a thread. As such, a deferred callback can be used for synchronization and delegating work to another thread.

A callback can be used for event handling. Often, consuming code registers a callback for a particular type of event. When that event occurs, the callback is called. Callbacks are often used to program the graphical user interface (GUI) of a program that runs in a windowing system. The application supplies a reference to a custom callback function for the windowing system to call. The windowing system calls this function to notify the application of events like mouse clicks and key presses.

A callback can be used to implement asynchronous processing. A caller requests an action and provides a callback to be called when the action completes which might be long after the request is made.

The callback technology is implemented differently by programming language. In assembly, C, C++, Pascal, Modula2 and other languages, a callback function is stored internally as a function pointer. Using the same storage allows different languages to directly share callbacks without a design time or runtime interoperability layer. For example, the Windows API is accessible via multiple languages, compilers and assemblers. C++ also allows objects to provide an implementation of the function call operation. The Standard Template Library accepts these objects (called functors) as parameters. Many dynamic languages, such as JavaScript, Lua, Python, Perl and PHP, allow a function object to be passed. CLI languages such as C# and VB. NET provide a type safe encapsulating function reference known as delegate. Events and event handlers, as used in NET languages, provide for callbacks. Functional languages generally support first class functions, which can be passed as callbacks to other functions, stored as data or returned from functions. Many languages, including Perl, Python, Ruby, Smalltalk, C++ (11+), C# and VB. NET (new versions) and most functional languages, support lambda expressions, unnamed functions with inline syntax, that generally acts as callbacks

In some languages, including Scheme, ML, JavaScript, Perl, Python, Smalltalk, PHP (since 5.3.0), C++ (11+), Java (since 8), and many others, a lambda can be a closure, i. e. can access variables locally defined in the context in which the lambda is defined. In an object oriented programming language such as Java versions before function valued arguments, the behavior of a callback can be achieved by passing an object that implements an interface. The methods of this object are callbacks. In PL/I and ALGOL 60 a callback procedure may need to be able to access local variables in containing blocks, so it is called through an entry variable containing both the entry point and context information.

In C++, functor can be used in addition to function pointer.