Editing Callback (computer programming) (section)

== {{Anchor|TYPES}}Use ==

A blocking callback runs in the [[Execution (computing)|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 (computing)|thread]]. As such, a deferred callback can be used for synchronization and delegating work to another thread.

=== Event handling ===

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 [[computer mouse|mouse]] clicks and [[computer keyboard|key]] presses.

=== Asynchronous action ===

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.

=== Polymorphism ===

A callback can be used to implement [[Polymorphism (computer science)|polymorphism]]. In the following pseudocode, {{code|say_hi}} can take either {{code|write_status}} or {{code|write_error}}.

<syntaxhighlight lang="python">
def write_status(message: str):
    write(stdout, message)

def write_error(message: str):
    write(stderr, message)

def say_hi(write):
    write("Hello world")
</syntaxhighlight>