Using an event system

In many of our daily systems and programming languages, the interaction of different instances is handled by events and event listeners (these are also called event handlers or callbacks). Usually, one part of an application or system triggers a certain event and another part of the application or systems—that listens for this event—executes the event listener for the specified event.

Let's think about a simple example in JavaScript. We want to call a resize function whenever a resize event of the browser occurs. Therefore, we have to create an event listener that waits (listens) for the resize event of the browser window and executes the resize() event listener whenever the event occurs. This function is as follows:

window.addEventListener('resize', function(event){
  resize();
});

This looks quite simple, and it is very easy to understand. The only thing we have to know is which events exist on which elements (in which browsers). You have already learned in a previous...