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Callback (computer programming)
 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. AnalogyTo 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. UseA 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. Event handlingA 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. Asynchronous actionA 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. PolymorphismA callback can be used to implement polymorphism. In the following pseudocode, def WriteStatus(string message):
Write(stdout, message)
def WriteError(string message):
Write(stderr, message)
def SayHi(write):
write("Hello world")
Conditional actionA callback can be used to implement conditional behavior. In the following pseudocode, if logging is enabled, def Log(getMessage):
if isLoggingEnabled:
message = getMessage();
WriteLine(message);
ImplementationThe 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[1][2] 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),[3] C++ (11+), Java (since 8),[4] 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. [5] Example codeCCallbacks have a wide variety of uses, for example in error signaling: a Unix program might not want to terminate immediately when it receives SIGTERM, 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: Xlib allows custom predicates to be specified to determine whether a program wishes to handle an event. In the following C code, function #include <stdio.h>
#include <stdlib.h>
void PrintNumber(int (*getNumber)(void)) {
int val = getNumber();
printf("Value: %d\n", val);
}
int GetAnswerToMostImportantQuestion(void) {
return 42;
}
int main(void) {
PrintNumber(GetAnswerToMostImportantQuestion);
return 0;
}
C++In C++, functor can be used in addition to function pointer. C#In the following C# code,
method 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");
}
}
KotlinIn the following Kotlin code, function 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()}")
}
JavaScriptIn the following JavaScript code, function 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
alert(calculate(10, 2, sum));
The collection method $("li").each(function(index) {
console.log(index + ": " + $(this).text());
});
Deferred callbacks are commonly used for handling events from the user, the client and timers. Examples can be found in In addition to using callbacks in JavaScript source code, C functions that take a function are supported via js-ctypes.[7] Red and REBOLThe following REBOL/Red code demonstrates callback use.
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
RustRust have the 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);
LuaA color tweening example using the Roblox engine that takes an optional .done callback: wait(1)
local DT = wait()
function tween_color(object, finish_color, fade_time)
local step_r = finish_color.r - object.BackgroundColor3.r
local step_g = finish_color.g - object.BackgroundColor3.g
local step_b = finish_color.b - object.BackgroundColor3.b
local total_steps = 1/(DT*(1/fade_time))
local completed;
coroutine.wrap(function()
for i = 0, 1, DT*(1 / fade_time) do
object.BackgroundColor3 = Color3.new (
object.BackgroundColor3.r + (step_r/total_steps),
object.BackgroundColor3.g + (step_g/total_steps),
object.BackgroundColor3.b + (step_b/total_steps)
)
wait()
end
if completed then
completed()
end
end)()
return {
done = function(callback)
completed = callback
end
}
end
tween_color(some_object, Color3.new(1, 0, 0), 1).done(function()
print "Color tweening finished!"
end)
PythonIn the following Python code, function def square(val):
return val ** 2
def calculate(operate, val):
return operate(val)
# outputs: 25
calculate(square, 5)
JuliaIn the following Julia code, function 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
See alsoReferences
External links
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