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Callback (computer programming)

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</style><div role="note" class="hatnote navigation-not-searchable">For a discussion of callback with computer <a href="Modem" title="Modem">modems</a>, see <a href="Callback_(telecommunications)" title="Callback (telecommunications)">Callback (telecommunications)</a>.</div>

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In <a href="Computer_programming" title="Computer programming">computer programming</a>, a callback is a <a href="Function_(computer_programming)" title="Function (computer programming)">function</a> that is stored as data (a <a href="Reference_(computer_science)" title="Reference (computer science)">reference</a>) and designed to be called by another function – often back to the original <a href="Abstraction_(computer_science)" title="Abstraction (computer science)">abstraction layer</a>.A function that accepts a callback <a href="Parameter_(computer_programming)" title="Parameter (computer programming)">parameter</a> may be designed to call back before <a href="Return_statement" title="Return statement">returning</a> to its caller which is known as <a href="Synchronization_(computer_science)" title="Synchronization (computer science)">synchronous</a> 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, <a href="Non-blocking_algorithm" title="Non-blocking algorithm">non-blocking</a> or deferred.

<a href="Programming_languages" class="mw-redirect" title="Programming languages">Programming languages</a> support callbacks in different ways such as <a href="Function_pointers" class="mw-redirect" title="Function pointers">function pointers</a>, <a href="Lambda_(programming)" class="mw-redirect" title="Lambda (programming)">lambda expressions</a> and <a href="Block_(programming)" title="Block (programming)">blocks</a>.A callback can be likened to leaving instructions with a tailor for what to do when a suit is ready, such as calling a specific phone number or delivering it to a given address.These instructions represent a callback: a function provided in advance to be executed later, often by a different part of the system and not necessarily by the one that received it.The term callback can be misleading, as it does not necessarily imply a return to the original caller, unlike a <a href="Callback_(telecommunications)" title="Callback (telecommunications)">telephone callback</a>.Mesa programming language formalised the callback mechanism used in Programming Languages. By passing a procedure as a parameter, Mesa essentially delegated the execution of that procedure to a later point in time when a specific event occurred, similar to how callbacks are implemented in modern programming languages.<a href="#cite_note-1">[1]</a>

A blocking callback runs in the <a href="Execution_(computing)" title="Execution (computing)">execution</a> context of the function that passes the callback. A deferred callback can run in a different context such as during <a href="Interrupt" title="Interrupt">interrupt</a> or from a <a href="Thread_(computing)" title="Thread (computing)">thread</a>. As such, a deferred callback can be used for synchronization and delegating work to another thread.

<div class="mw-heading mw-heading3"><h3 id="Event_handling">Event handling</h3></div>

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 <a href="Graphical_user_interface" title="Graphical user interface">graphical user interface</a> (GUI) of a program that runs in a <a href="Windowing_system" title="Windowing system">windowing system</a>. 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 <a href="Computer_mouse" title="Computer mouse">mouse</a> clicks and <a href="Computer_keyboard" title="Computer keyboard">key</a> presses.

<div class="mw-heading mw-heading3"><h3 id="Asynchronous_action">Asynchronous action</h3></div>

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.

<div class="mw-heading mw-heading3"><h3 id="Polymorphism">Polymorphism</h3></div>

A callback can be used to implement <a href="Polymorphism_(computer_science)" title="Polymorphism (computer science)">polymorphism</a>. In the following pseudocode, <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">say_hi</code> can take either <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">write_status</code> or <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">write_error</code>.

<div class="mw-highlight mw-highlight-lang-python mw-content-ltr" dir="ltr"><pre>def write_status(message: str):

write(stdout, message)

def write_error(message: str):

write(stderr, message)

def say_hi(write):

write("Hello world")

</pre></div>

<div class="mw-heading mw-heading2"><h2 id="Implementation">Implementation</h2></div>

The callback technology is implemented differently by <a href="Programming_language" title="Programming language">programming language</a>.

In <a href="Assembly_language" title="Assembly language">assembly</a>, <a href="C_(programming_language)" title="C (programming language)">C</a>, <a href="C%2B%2B" title="C++">C++</a>, <a href="Pascal_(programming_language)" title="Pascal (programming language)">Pascal</a>, <a href="Modula2" class="mw-redirect" title="Modula2">Modula2</a> and other languages, a callback function is stored internally as a <a href="Function_pointer" title="Function pointer">function pointer</a>. Using the same storage allows different languages to directly share callbacks without a <a href="Program_lifecycle_phase" title="Program lifecycle phase">design-time or runtime</a> <a href="Interoperability" title="Interoperability">interoperability</a> <a href="Abstraction_layer" title="Abstraction layer">layer</a>. For example, the <a href="Windows_API" title="Windows API">Windows API</a> is accessible via multiple languages, compilers and assemblers.C++ also allows objects to provide an implementation of the function call operation. The <a href="Standard_Template_Library" title="Standard Template Library">Standard Template Library</a> accepts these objects (called <a href="Function_object" title="Function object">functors</a>) as parameters.Many <a href="Dynamic_programming_language" title="Dynamic programming language">dynamic languages</a>, such as <a href="JavaScript" title="JavaScript">JavaScript</a>, <a href="Lua_(programming_language)" class="mw-redirect" title="Lua (programming language)">Lua</a>, <a href="Python_(programming_language)" title="Python (programming language)">Python</a>, <a href="Perl" title="Perl">Perl</a><a href="#cite_note-2">[2]</a><a href="#cite_note-3">[3]</a> and <a href="PHP" title="PHP">PHP</a>, allow a function object to be passed.<a href="List_of_CLI_languages" title="List of CLI languages">CLI languages</a> such as <a href="C_Sharp_(programming_language)" title="C Sharp (programming language)">C#</a> and <a href="VB.NET" class="mw-redirect" title="VB.NET">VB.NET</a> provide a <a href="Type_safety" title="Type safety">type-safe</a> encapsulating function reference known as <a href="Delegate_(CLI)" title="Delegate (CLI)">delegate</a>.Events and <a href="Event_handlers" class="mw-redirect" title="Event handlers">event handlers</a>, as used in .NET languages, provide for callbacks.Functional languages generally support <a href="First-class_functions" class="mw-redirect" title="First-class functions">first-class functions</a>, which can be passed as callbacks to other functions, stored as data or returned from functions.

Many languages, including Perl, Python, <a href="Ruby_(programming_language)" title="Ruby (programming language)">Ruby</a>, <a href="Smalltalk" title="Smalltalk">Smalltalk</a>, <a href="C%2B%2B" title="C++">C++</a> (11+), C# and VB.NET (new versions) and most functional languages, support <a href="Lambda_(programming)" class="mw-redirect" title="Lambda (programming)">lambda expressions</a>, unnamed functions with inline syntax, that generally acts as callbacks..In some languages, including <a href="Scheme_(programming_language)" title="Scheme (programming language)">Scheme</a>, <a href="ML_(programming_language)" title="ML (programming language)">ML</a>, JavaScript, Perl, Python, Smalltalk, PHP (since 5.3.0),<a href="#cite_note-4">[4]</a> C++ (11+), Java (since 8),<a href="#cite_note-5">[5]</a> and many others, a lambda can be a <a href="Closure_(computer_science)" class="mw-redirect" title="Closure (computer science)">closure</a>, i.e. can access variables locally defined in the context in which the lambda is defined.In an <a href="Object-oriented_programming" title="Object-oriented programming">object-oriented programming</a> language such as <a href="Java_(programming_language)" title="Java (programming language)">Java</a> 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 <a href="PL/I" title="PL/I">PL/I</a> and <a href="ALGOL_60" title="ALGOL 60">ALGOL 60</a> 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. <a href="#cite_note-6">[6]</a>

<div class="mw-heading mw-heading2"><h2 id="Example_code">Example code</h2></div>

Callbacks have a wide variety of uses, for example in error signaling: a <a href="Unix" title="Unix">Unix</a> program might not want to terminate immediately when it receives <a href="SIGTERM" class="mw-redirect" title="SIGTERM">SIGTERM</a>, so to make sure that its termination is handled properly, it would register the cleanup function as a callback. Callbacks may also be used to control whether a function acts or not: <a href="Xlib" title="Xlib">Xlib</a> allows custom predicates to be specified to determine whether a program wishes to handle an event.In the following <a href="C_(programming_language)" title="C (programming language)">C</a> code, function <code>print_number</code> uses parameter <code>get_number</code> as a blocking callback. <code>print_number</code> is called with <code>get_answer_to_most_important_question</code> which acts as a callback function. When run the output is: "Value: 42".

<div class="mw-highlight mw-highlight-lang-c mw-content-ltr" dir="ltr"><pre>#include <stdio.h>

#include <stdlib.h>

void print_number(int (*get_number)(void)) {

int val = get_number();

printf("Value: %d\n", val);

}

int get_answer_to_most_important_question(void) {

return 42;

}

int main(void) {

print_number(get_answer_to_most_important_question);

return 0;

}

</pre></div>

In C++, <a href="Function_object" title="Function object">functor</a> can be used in addition to function pointer.

In the following <a href="C_Sharp_(programming_language)" title="C Sharp (programming language)">C#</a> code,

method <code>Helper.Method</code> uses parameter <code>callback</code> as a blocking callback. <code>Helper.Method</code> is called with <code>Log</code> which acts as a callback function. When run, the following is written to the console: "Callback was: Hello world".

<div class="mw-highlight mw-highlight-lang-c# mw-content-ltr" dir="ltr"><pre>public class MainClass

{

static void Main(string[] args)

{

Helper helper = new Helper();

helper.Method(Log);

}

static void Log(string str)

{

Console.WriteLine($"Callback was: {str}");

}

}

public class Helper

{

public void Method(Action<string> callback)

{

callback("Hello world");

}

}

</pre></div>

<div class="mw-heading mw-heading3"><h3 id="Kotlin">Kotlin</h3></div>

In the following <a href="Kotlin_(programming_language)" title="Kotlin (programming language)">Kotlin</a> code, function <code>askAndAnswer</code> uses parameter <code>getAnswer</code> as a blocking callback. <code>askAndAnswer</code> is called with <code>getAnswerToMostImportantQuestion</code> which acts as a callback function. Running this will tell the user that the answer to their question is "42".

<div class="mw-highlight mw-highlight-lang-kotlin mw-content-ltr" dir="ltr"><pre>fun main() {

print("Enter the most important question: ")

val question = readLine()

askAndAnswer(question, ::getAnswerToMostImportantQuestion)

}

fun getAnswerToMostImportantQuestion(): Int {

return 42

}

fun askAndAnswer(question: String?, getAnswer: () -> Int) {

println("Question: $question")

println("Answer: ${getAnswer()}")

}

</pre></div>

<div class="mw-heading mw-heading3"><h3 id="JavaScript">JavaScript</h3></div>

In the following <a href="JavaScript" title="JavaScript">JavaScript</a> code, function <code>calculate</code> uses parameter <code>operate</code> as a blocking callback. <code>calculate</code> is called with <code>multiply</code> and then with <code>sum</code> which act as callback functions.

<div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>function calculate(a, b, operate) {

return operate(a, b);

}

function multiply(a, b) {

return a * b;

}

function sum(a, b) {

return a + b;

}

// outputs 20

alert(calculate(10, 2, multiply));

// outputs 12

</pre></div>

The collection method <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">.each()</code> of the <a href="JQuery" title="JQuery">jQuery</a> <a href="JavaScript_libraries" class="mw-redirect" title="JavaScript libraries">library</a> uses the function passed to it as a blocking callback. It calls the callback for each item of the collection. For example:

<div class="mw-highlight mw-highlight-lang-javascript mw-content-ltr" dir="ltr"><pre>$("li").each(function(index) {

console.log(index + ": " + $(this).text());

});

</pre></div>

Deferred callbacks are commonly used for handling events from the user, the client and timers. Examples can be found in <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">addEventListener</code>, <a href="Ajax_(programming)" title="Ajax (programming)">Ajax</a> and <code><a href="XMLHttpRequest" title="XMLHttpRequest">XMLHttpRequest</a></code>.

In addition to using callbacks in JavaScript source code, C functions that take a function are supported via js-ctypes.<a href="#cite_note-8">[8]</a>

<div class="mw-heading mw-heading3"><h3 id="Red_and_REBOL">Red and REBOL</h3></div>

The following <a href="REBOL" class="mw-redirect" title="REBOL">REBOL</a>/<a href="Red_(programming_language)" title="Red (programming language)">Red</a> code demonstrates callback use.

<ul><li>As alert requires a string, form produces a string from the result of calculate</li>

<li>The get-word! values (i.e., :calc-product and :calc-sum) trigger the interpreter to return the code of the function rather than evaluate with the function.</li>

<li>The datatype! references in a block! [float! integer!] restrict the type of values passed as arguments.</li></ul>

<div class="mw-highlight mw-highlight-lang-red mw-content-ltr" dir="ltr"><pre>Red [Title: "Callback example"]

calculate: func [

num1 [number!]

num2 [number!]

callback-function [function!]

][

callback-function num1 num2

]

calc-product: func [

num1 [number!]

num2 [number!]

][

num1 * num2

]

calc-sum: func [

num1 [number!]

num2 [number!]

][

num1 + num2

]

; alerts 75, the product of 5 and 15

alert form calculate 5 15 :calc-product

; alerts 20, the sum of 5 and 15

alert form calculate 5 15 :calc-sum

</pre></div>

<a href="Rust_(programming_language)" title="Rust (programming language)">Rust</a> have the <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">Fn</code>, <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">FnMut</code> and <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">FnOnce</code> traits.<a href="#cite_note-9">[9]</a>

<div class="mw-highlight mw-highlight-lang-rust mw-content-ltr" dir="ltr"><pre>fn call_with_one<F>(func: F) -> usize

where F: Fn(usize) -> usize {

func(1)

}

let double = |x| x * 2;

assert_eq!(call_with_one(double), 2);

</pre></div>

In this <a href="Lua" title="Lua">Lua</a> code, function <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">calculate</code> accepts the <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">operation</code> parameter which is used as a blocking callback. <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">calculate</code> is called with both <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">add</code> and <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">multiply</code>, and then uses an <a href="Anonymous_function" title="Anonymous function">anonymous function</a> to divide.

<div class="mw-highlight mw-highlight-lang-lua mw-content-ltr" dir="ltr"><pre>function calculate(a, b, operation)

return operation(a, b)

end

function multiply(a, b)

return a * b

end

function add(a, b)

return a + b

end

print(calculate(10, 20, multiply)) -- outputs 200

print(calculate(10, 20, add)) -- outputs 30

-- an example of a callback using an anonymous function

print(calculate(10, 20, function(a, b)

return a / b -- outputs 0.5

end))

</pre></div>

<div class="mw-heading mw-heading3"><h3 id="Python">Python</h3></div>

In the following <a href="Python_(programming_language)" title="Python (programming language)">Python</a> code, function <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">calculate</code> accepts a parameter <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">operate</code> that is used as a blocking callback. <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">calculate</code> is called with <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">square</code> which acts as a callback function.

<div class="mw-highlight mw-highlight-lang-python mw-content-ltr" dir="ltr"><pre>def square(val):

return val ** 2

def calculate(operate, val):

return operate(val)

calculate(square, 5) # outputs: 25

</pre></div>

<div class="mw-heading mw-heading3"><h3 id="Julia">Julia</h3></div>

In the following <a href="Julia_(programming_language)" title="Julia (programming language)">Julia</a> code, function <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">calculate</code> accepts a parameter <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">operate</code> that is used as a blocking callback. <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">calculate</code> is called with <code class="mw-highlight mw-highlight-lang-text mw-content-ltr" style="" dir="ltr">square</code> which acts as a callback function.

<div class="mw-highlight mw-highlight-lang-jlcon mw-content-ltr" dir="ltr"><pre>julia> square(val) = val^2

square (generic function with 1 method)

julia> calculate(operate, val) = operate(val)

calculate (generic function with 1 method)

julia> calculate(square, 5)

25

</pre></div>

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<ul><li><a href="Command_pattern" title="Command pattern">Command pattern</a></li>

<li><a href="Continuation-passing_style" title="Continuation-passing style">Continuation-passing style</a></li>

<li><a href="Event_loop" title="Event loop">Event loop</a></li>

<li><a href="Event-driven_programming" title="Event-driven programming">Event-driven programming</a></li>

<li><a href="Implicit_invocation" title="Implicit invocation">Implicit invocation</a></li>

<li><a href="Inversion_of_control" title="Inversion of control">Inversion of control</a></li>

<li><a href="Libsigc%2B%2B" title="Libsigc++">libsigc++</a>, a callback library for C++</li>

<li><a href="Signals_and_slots" title="Signals and slots">Signals and slots</a></li>

</div>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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<div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div>

<ul><li><a rel="nofollow" class="external text" href="https://learn.microsoft.com/en-us/archive/msdn-magazine/2002/december/using-net-implementing-callback-notifications-using-delegates">Basic Instincts: Implementing Callback Notifications Using Delegates</a> - <a href="MSDN_Magazine" class="mw-redirect" title="MSDN Magazine">MSDN Magazine</a>, December 2002</li>

<li><a rel="nofollow" class="external text" href="https://web.archive.org/web/20080916192721/http://www.javaworld.com/javaworld/javatips/jw-javatip10.html">Implement callback routines in Java</a></li>

<li><a rel="nofollow" class="external text" href="https://www.codeproject.com/Articles/7865/Implement-Script-Callback-Framework-in-ASP-NET-1-x">Implement Script Callback Framework in ASP.NET 1.x</a> - Code Project, 2 August 2004</li>

<li>Interfacing C++ member functions with C libraries (archived from the original on July 6, 2011)</li>

<li><a rel="nofollow" class="external text" href="http://gotw.ca/gotw/083.htm">Style Case Study #2: Generic Callbacks</a></li></ul></div><div><div class="zim-footer">

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