How event-driven methodology facilitates GUI programming?

Answer 1

The first computers had to use polling to determine events that happened, such as key presses, mouse movement, and joystick movement. A long-running process might cause the screen to appear unresponsive. As the number of elements on the screen increased, and particularly with the advent of multi-tasking, it was no longer appropriate for the application to maintain its own state through polling.

Instead, all modern GUIs advise running programs of events that would interest that program. Keypresses, mouse events, and so on are no longer checked many times per second, which would cause extreme lag on even the fastest computers. A typical web browser, for example, might have hundreds or thousands of "windows", including buttons, text, and so on. If each window had a thread that had to run full-time just to detect if it were doing anything, it would result in massive slowdown as tens of thousands of mini-programs competed for limited system resources.