<mx:Button id="someButton" click="handleClick( event );"/>

Now when someone clicks on someButton, it calls the function handleClick(). Although not strictly best practice, it really is a nice little shorthand for adding an event listener to a Flex component. And since the Adobe components contain this functionality, people are used to it. If you have any plans of releasing your component into the wild, people will expect it to act like the components they already know and love.

Well, here’s how you do it: with the [Event] metadata tag.

1- First of all, the tag goes before the class declaration because it applies to the entire class. You can actually have more than one of these tags, for different events that you might broadcast. In my example, I have two, and they both broadcast ModelEvents.

// create the [Event] tags
[Event(name="onLoad", type="com.pixelwelders.events.ModelEvent")]
[Event(name="onLoadError", type="com.pixelwelders.events.ModelEvent")]
 
// declare your class
public class DataModel extends EventDispatcher implements IModel { ...

2- The type attribute refers to the actual type of event that this class broadcasts, package and all.

3- The name attribute refers to the string you use when you create a new Event of this type. You have to use an actual literal here; you can’t use a variable like ModelEvent.LOADED. So if you are dispatching an event like either of these:

dispatchEvent( new ModelEvent( ModelEvent.LOADED ) );
dispatchEvent( new ModelEvent( ModelEvent.LOAD_ERROR ) );

…you’ll need to check in your Event class to see what the actual value of LOADED or LOAD_ERROR is. In my ModelEvent class, I have:

public static const LOADED				: String = "onLoad";
public static const LOAD_ERROR			: String = "onLoadError";

Thus, my complete Event metadata tags are:

[Event(name="onLoad", type="com.pixelwelders.events.ModelEvent")]
[Event(name="onLoadError", type="com.pixelwelders.events.ModelEvent")]

Of course, that’s only the first part of the equation. The second part is listening for the event, which thanks to our [Embed] tags, is now a piece of figurative cake. Here’s my Flex tag:

<model:DataModel
		id="dataModel"
		xmlURL="fileName.xml" 
		onLoad="handleModelLoaded( event );"
		onLoadError=”handleModelError( event );”/>

That onLoad and onLoadError attributes have the same value as your name attributes that you specified in step 3 above. Now put your two handlers in your Script tag, and they will be called any time those events are fired. Voilà.

Creating a Singleton through MXML

Okay, so I’ve been putting together a little image-viewer component in Flex, and I wanted it to load one data model that all other components would share. Sounds like a job for a Singleton, right? However, I also wanted to instantiate said model from MXML, which adds some kinks. In fact, I’ve heard it said that it’s not possible at all. And after some trial and error, I found that apparently it is possible- you just have to hack around a little bit.

If you instantiate your class (or “component”) through MXML, Flex will automatically call its constructor. Typically, this is a bad thing because you don’t call constructors directly in Singletons- instead of calling a method that returns a new object (i.e. a constructor), you call a method that returns a reference to an already-created object (getInstance() or instance()). Flex won’t do that for you. However, this is not a deal-breaker. To make that class a Singleton, you just need to make sure that the constructor is only called once.

So, set up your Singleton like normal:

private static var _instance: DataModel = null;
public static function get instance(): DataModel
{
	return _instance;
}

Then, in the constructor of said Singleton, add the following:

if ( !_instance )
{
	_instance = this;
} else {
	throw new Error( "DataModel is a Singleton: only one instance allowed." );
}

All I’ve done here is move the creation of the Singleton from the get instance() method to the constructor. And if the constructor is called more than once, it throws an error. Now you can instantiate the class through MXML, just like any other component.

<model:DataModel
		id="dataModel"
		xmlURL="fileName.xml" />

Meanwhile in Actionscriptland, you can refer to this instance of your class just like it was a normal Singleton:

	trace( DataModel.instance ); // traces "[object DataModel]"

Plus, if you try to create another instance, be it through MXML or through ActionScript, you’ll get a good ol’ runtime error. You’ll have to wait until runtime to see it, but that’s certainly better than nothing, right?