In computer programming, a callback, also known as a "call-after" function, is any executable
code that is passed as an argument to other code that is expected to call back (execute)
the argument at a given time.
This execution may be immediate as in a synchronous callback, or it might happen at a later time
as in an asynchronous callback.
In all cases, the intention is to specify a function or subroutine as an entity that
is, depending on the language, more or less similar to a variable (see first-class functions).
Programming languages support callbacks in different ways, often implementing them with
subroutines, lambda expressions, blocks, or function pointers.
== Design == There are two types of callbacks, differing
in how they control data flow at runtime: blocking callbacks (also known as synchronous
callbacks or just callbacks) and deferred callbacks (also known as asynchronous callbacks).
While blocking callbacks are invoked before a function returns (in the C example below,
which illustrates a blocking callback, it is function main), deferred callbacks may
be invoked after a function returns.
Deferred callbacks are often used in the context of I/O operations or event handling, and are
called by interrupts or by a different thread in case of multiple threads.
Due to their nature, blocking callbacks can work without interrupts or multiple threads,
meaning that blocking callbacks are not commonly used for synchronization or delegating work
to another thread.
Callbacks are used to program applications in windowing systems.
In this case, the application supplies (a reference to) a specific custom callback function
for the operating system to call, which then calls this application-specific function in
response to events like mouse clicks or key presses.
A major concern here is the management of privilege and security: whilst the function
is called from the operating system, it should not run with the same privilege as the system.
A solution to this problem is using rings of protection.
== Implementation == The form of a callback varies among programming
languages:
In assembly, C, C++, Pascal, Modula2 and similar languages, a machine-level pointer to a function
may be passed as an argument to another (internal or external) function.
This is supported by most compilers and provides the advantage of using different languages
together without special wrapper libraries or classes.
One example may be the Windows API that is directly (more or less) accessible by many
different languages, compilers and assemblers.
C++ allows objects to provide their own implementation of the function call operation.
The Standard Template Library accepts these objects (called functors), as well as function
pointers, as parameters to various polymorphic algorithms.
Many dynamic languages, such as JavaScript, Lua, Python, Perl and PHP, simply allow a
function object to be passed through.
CLI languages such as C# and VB.NET provide a type-safe encapsulating reference, a "delegate",
to define well-typed function pointers.
These can be used as callbacks.
Events and event handlers, as used in .NET languages, provide generalized syntax 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.
Some languages, such as Algol 68, Perl, Python, Ruby, Smalltalk, C++11 and later, newer versions
of C# and VB.NET as well as most functional languages, allow unnamed blocks of code (lambda
expressions) to be supplied instead of references to functions defined elsewhere.
In some languages, e.g. Scheme, ML, JavaScript, Perl, Smalltalk, PHP (since 5.3.0), C++11
and later, Java (since 8), and many others, such functions can be closures, i.e. they
can access and modify variables locally defined in the context in which the function was defined.
Note that Java cannot, however, modify the local variables in the enclosing scope.
In object-oriented programming languages without function-valued arguments, such as in Java
before its 8 version, callbacks can be simulated by passing an instance of an abstract class
or interface, of which the receiver will call one or more methods, while the calling end
provides a concrete implementation.
Such objects are effectively a bundle of callbacks, plus the data they need to manipulate.
They are useful in implementing various design patterns such as Visitor, Observer, and Strategy.
== Use ==
=== C === Callbacks 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.
The following C code demonstrates the use of callbacks to display two numbers.
This should provide output similar to:
125185 and 89187225 9084 and 9441
42 and 42
Note how this is different from simply passing the output of the callback function to the
calling function, PrintTwoNumbers() - rather than printing the same value twice, the PrintTwoNumbers
calls the callback as many times as it requires.
This is one of the two main advantages of callbacks.
The other advantage is that the calling function can pass whatever parameters it wishes to
the called functions (not shown in the above example).
This allows correct information hiding: the code that passes a callback to a calling function
does not need to know the parameter values that will be passed to the function.
If it only passed the return value, then the parameters would need to be exposed publicly.Another
example:
The output after compilation:
This information hiding means that callbacks can be used when communicating between processes
or threads, or through serialised communications and tabular data.
=== C# === A simple callback in C#:
=== JavaScript === Callbacks are used in the implementation of
languages such as JavaScript, including support of JavaScript functions as callbacks through
js-ctypes and in components such as addEventListener.
However, a native example of a callback can be written without any complex code:
First a function someEvent is defined with a parameter intended for callback: callbackFunction.
Then a function that can be used as a callback to someEvent is defined, calcProduct.
Other functions may be used for callbackFunction, like calcSum.
In this example, someEvent() is invoked twice, once with calcProduct as a callback and once
with calcSum.
The functions return the product and sum, respectively, and then the alert will display
them to the screen.
In this primitive example, the use of a callback is primarily a demonstration of principle.
One could simply call the callbacks as regular functions, calcProduct(x, y).
Callbacks are generally used when the function needs to perform events before the callback
is executed, or when the function does not (or cannot) have meaningful return values
to act on, as is the case for Asynchronous JavaScript (based on timers) or XMLHttpRequest
requests.
Useful examples can be found in JavaScript libraries such as jQuery where the .each()
method iterates over an array-like object, the first argument being a callback that is
performed on each iteration.
=== Lua === A color tweening example using the ROBLOX
engine that takes an optional .done callback:
=== Python === A classic use of callbacks in Python (and
other languages) is to assign events to UI elements.
Here is a very trivial example of the use of a callback in Python.
First define two functions, the callback and the calling code,
then pass the callback function into the calling code.
== See also