Events & reactions

You tap an icon and the app opens. You press a key and a letter appears. Your phone buzzes because a message arrives. Behind all of this is the same idea: the program waits for an event and reacts to it. Without this principle every app would just be a rigid movie, with it the app becomes something that listens to you.

What is an event?

An event is something that happens and that a program can react to: a click, a key press, an incoming message, a timer that runs out, or a sensor value that changes. For each event the program can have a rule: if this event occurs, then run this reaction. Such a rule is also called an event handler. It has the same if-then shape as a condition, only it is not the program that decides when its turn comes, but the event.

Apps are collections of rules

You can picture a whole app as a collection of such event rules: when the start button is pressed, start the game. When a message arrives, show a notification. When the battery is almost empty, dim the screen. Most of the time the app does nothing at all, it just waits. Only when an event occurs does the matching rule run.

Different from step by step

So far you knew programs that work through a fixed list of steps from top to bottom, like a recipe. Event driven programs are different: it is not fixed what happens next, because you decide that with your clicks and inputs. The order of the reactions depends on which events occur in which order.