About event listeners

The following key points apply to handling events for ActionScript 3.0 components:

All events are broadcast by an instance of a component class. The component instance is the broadcaster.
You register an event listener by calling the addEventListener() method for the component instance. For example, the following line of code adds a listener for the MouseEvent.CLICK event to the Button instance aButton:
```
aButton.addEventListener(MouseEvent.CLICK, clickHandler);
```
The second parameter of the addEventListener() method registers the name of the function, clickHandler, to be called when the event occurs. This function is also referred to as a callback function.

You can register multiple listeners to one component instance.

aButton.addEventListener(MouseEvent.CLICK, clickHandler1);
aButton.addEventListener(MouseEvent.CLICK, clickHandler2);

You can register one listener to multiple component instances.

aButton.addEventListener(MouseEvent.CLICK, clickHandler1);
bButton.addEventListener(MouseEvent.CLICK, clickHandler1);

The event handler function is passed to an event object that contains information about the event type and the instance that broadcast the event. For more information, see About the event object.
The listener remains active until the application terminates or you explicitly remove it using the removeEventListener() method. For example, the following line of code removes the listener for the MouseEvent.CLICK event on aButton:
```
aButton.removeEventListener(MouseEvent.CLICK, clickHandler);
```