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Advanced Buttons and Event Handling in Adobe Flash CS5

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This chapter introduces events, listeners, and functions used to respond to events. Next, it explores the simplest class for creating interactivity: the SimpleButton class.

Creating graphics and animation in Flash is only half the story. The other half is interactivity, which involves giving the viewer control of those graphics and animation. What makes a movie interactive? It’s the back-and-forth communication between the user and the movie. Mouse movements, button clicks, or keypresses are examples of things that happen, called events. Events form the basis of interactivity. There are many kinds of events—some are user driven whereas others are not. You’ll learn to make Flash listen for and respond to these events (event handling).

This chapter first introduces events, listeners, and functions used to respond to events. Next, it explores the simplest class for creating interactivity: the SimpleButton class. You’ll learn about invisible buttons, animated buttons, and more complex buttons. You’ll also learn about the classes and events that are involved in keyboard input and the context menu. Additionally, you’ll learn an important event known as the ENTER_FRAME event, which you’ll rely on to create continuously running actions. Understanding these classes and event handling is essential to creating Flash interactivity because these elements are the scaffold on which you’ll hang virtually all your ActionScript.

Listening for Events

Events are things that happen that Flash can recognize and respond to. A mouse click is an event, as are mouse movements and keypresses. Events can also be things that the user doesn’t initiate. The completion of a sound, for example, is an event. Anytime an event happens, an object of the Event class is created. When the mouse button is clicked, a MouseEvent object (a subclass of the Event class) is created. When a key on the keyboard is pressed, a KeyboardEvent object (another subclass of the Event class) is created. It may seem a little strange that an object represents an event, but remember Flash objects can be very abstract!

With all these events happening, you need a way to detect and respond to them. You detect an event by creating an event handler. An event handler simply tells Flash what to do when a specific kind of event happens. Creating an event handler is a two-part operation: first, you add, or “register,” a listener to detect the event and trigger a function, and second, you create the function that tells Flash how to respond. (It doesn’t matter if you register the listener first or create the function first. As long as they are both in the same block of code, the event handler will work.)

For example, if you want to listen for a mouse click on top of a particular button, you add an event listener to that object as follows:

myButton_btn.addEventListener(MouseEvent.CLICK, reportClick);

The addEventListener() method takes two parameters. The first is the specific kind of event that you want to detect. All the event objects have properties (like MouseEvent.CLICK), which give more specificity to the event. The second parameter is the name of your function, which is triggered when the event is detected.

Next, add a function as the response to the event. Create the function with a parameter strictly typed to the MouseEvent object, like so:

function reportClick(myevent:MouseEvent):void {
   // do something in response
}

Between the curly braces of the function, you add actions as the response. The word myevent in this example is the parameter name that you make up that refers to the event.

The actual object that receives the event (in this example, it is the button called myButton_btn) can be referenced in the function by using the property target. In the preceding example, the expression myevent.target references myButton_btn.

When you no longer need to listen for an event, you can delete the listener with the method removeEventListener(). The method takes two parameters, which are identical to the ones in the addEventListener() method.

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