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Concept
Event-driven programming
Summary
In computer programming, event-driven programming is a programming paradigm in which the flow of the program is determined by events such as user actions (mouse clicks, key presses), sensor outputs, or message passing from other programs or threads. Event-driven programming is the dominant paradigm used in graphical user interfaces and other applications (e.g., JavaScript web applications) that are centered on performing certain actions in response to user input. This is also true of programming for device drivers (e.g., P in USB device driver stacks).
In an event-driven application, there is generally a main loop that listens for events and then triggers a callback function when one of those events is detected. In embedded systems, the same may be achieved using hardware interrupts instead of a constantly running main loop. Event-driven programs can be written in any programming language, although the task is easier in languages that provide high-level abstractions, such as await and closures.
Event handler
Because the code for checking of events and the main loop are common amongst applications, many programming frameworks take care of their implementation and expect the user to provide only the code for the event handlers. In this simple example, there may be a call to an event handler called that includes an argument with a string of characters, corresponding to what the user typed before hitting the ENTER key. To add two numbers, storage outside the event handler must be used. The implementation might look like below.
globally declare the counter K and the integer T.
OnKeyEnter(character C)
{
convert C to a number N
if K is zero store N in T and increment K
otherwise, add N to T, print the result and reset K to zero
}
While keeping track of history is normally trivial in a sequential program because event handlers execute in response to external events, correctly structuring the handlers to work when called in any order can require special attention and planning in an event-driven program.
Official source
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