Event loop

In computer science, the event loop, message dispatcher, message loop, message pump, or run loop is a programming construct that waits for and dispatches events or messages in a program. It works by making a request to some internal or external "event provider" (that generally blocks the request until an event has arrived), and then it calls the relevant event handler ("dispatches the event"). The event-loop may be used in conjunction with a reactor, if the event provider follows the file interface, which can be selected or 'polled' (the Unix system call, not actual polling). The event loop almost always operates asynchronously with the message originator.

When the event loop forms the central control flow construct of a program, as it often does, it may be termed the main loop or main event loop. This title is appropriate, because such an event loop is at the highest level of control within the program.

Message pumps are said to 'pump' messages from the program's message queue (assigned and usually owned by the underlying operating system) into the program for processing. In the strictest sense, an event loop is one of the methods for implementing inter-process communication. In fact, message processing exists in many systems, including a kernel-level component of the Mach operating system. The event loop is a specific implementation technique of systems that use message passing.

This approach is in contrast to a number of other alternatives:

Due to the predominance of graphical user interfaces, most modern applications feature a main loop. The get_next_message() routine is typically provided by the operating system, and blocks until a message is available. Thus, the loop is only entered when there is something to process.

...
Wikipedia