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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 .
For developers, it provides an example of creating a .deb package, either traditionally or using debhelper, and the version of hello used, GNU Hello, serves as an example of writing a GNU program. [15] Variations of the "Hello, World!" program that produce a graphical output (as opposed to text output) have also been shown.
This includes functional programming languages such as Lisp and ML, and many modern, multi-paradigm languages, such as Julia, Python, and Rust. Closures are also often used with callbacks , particularly for event handlers , such as in JavaScript , where they are used for interactions with a dynamic web page .
The term comes from the specification for Common Lisp, which explicitly refers to the programming language feature enabling for inter-language calls as such; [citation needed] the term is also often used officially by the interpreter and compiler documentation for Haskell, [1] Rust, [2] PHP, [3] Python, and LuaJIT [4] [5]: 35 . [6]
A typical use of a function object is in writing callback functions. A callback in procedural languages, such as C, may be performed by using function pointers. [2] However it can be difficult or awkward to pass a state into or out of the callback function. This restriction also inhibits more dynamic behavior of the function.
For example, in user-interface (UI) programming, a routine can set up dialog box fields and pass these, along with a continuation function, to the UI framework. This call returns right away, allowing the application code to continue while the user interacts with the dialog box.
Events can be implemented through various mechanisms such as callbacks, message objects, signals, or interrupts, and events themselves are distinct from the implementation mechanisms used. Event propagation models, such as bubbling, capturing, and pub/sub, define how events are distributed and handled within a system.
ReactiveX is an API for asynchronous programming with observable streams. [1] Asynchronous programming allows programmers to call functions and then have the functions "callback" when they are done, usually by giving the function the address of another function to execute when it is done.