Event-Driven Programming

About this topic

In event-driven programming, a program waits for events, such as a mouse click, key press, timer or message arrival, and runs a piece of code called an event handler or callback in response. Instead of following a fixed path from start to finish, control is guided by what the user or system does.

A main loop, sometimes called the event loop, continually checks for events and sends each to the matching handler. In a web page, you might attach a click handler to a button so that clicking it changes the text. In Scratch, the when green flag clicked and when key pressed blocks are event handlers.

Graphical apps, games, phone apps and servers all use this style because users act unpredictably. A game does not know when the player will press jump, so it simply reacts when the key event occurs. A common problem is making a handler take too long, which freezes the interface because other events must wait. Another mistake is forgetting to register the handler, so nothing happens. Keep handlers short and test with different sequences of events, including clicking things in unexpected order.