Flash CS3 Documentation

Programming ActionScript 3.0 > Getting started with ActionScript > Working with objects > Events > Examining the event-handling process

Examining the event-handling process

The following is a step-by-step description of the process that happens when you create an event listener. In this case, it's an example of creating a listener function that is called when an object named myButton is clicked.

The actual code written by the programmer is as follows:

function eventResponse(event:MouseEvent):void
{
    // Actions performed in response to the event go here.
}

myButton.addEventListener(MouseEvent.CLICK, eventResponse);

Here is how this code would actually work when it's running in Flash Player:

1. When the SWF file loads, Flash Player makes note of the fact that there's a function named eventResponse().

   
   
   

2. Flash Player then runs the code (specifically, the lines of code that aren't in a function). In this case that's only one line of code: calling the addEventListener() method on the event source object (named myButton) and passing the eventResponse function as a parameter.

   
   
   

   1. Internally, myButton has a list of functions that are listening to each of its events, so when its addEventListener() method is called, myButton stores the eventResponse() function in its list of event listeners.

      
      
      

3. At some point, the user clicks the myButton object, triggering its click event (identified as MouseEvent.CLICK in the code).

   
   
   

   At that point, the following occurs:

   1. Flash Player creates an object, an instance of the class associated with the event in question (MouseEvent in this example). For many events this will be an instance of the Event class; for mouse events it will be a MouseEvent instance; and for other events it will be an instance of the class that's associated with that event. This object that's created is known as the event object, and it contains specific information about the event that happened: what type of event it is, where it happened, and other event-specific information if applicable.

      
      
      

   2. Flash Player then looks at the list of event listeners stored by myButton. It goes through these functions one by one, calling each function and passing the event object to the function as a parameter. Since the eventResponse() function is one of myButton's listeners, as part of this process Flash Player calls the eventResponse() function.

      
      
      

   3. When the eventResponse() function is called, the code in that function runs, so your specified actions are carried out.

      
      
      

Flash CS3