Most Flex developers have seen and used the [Bindable] tag but not many know what this tag does or even what it is. [Bindable] is what is known as a metadata tag. Metadata tags are special tags that are inserted in your source code that give the compiler information on how to compile the application. These tags aren't actually compiled into the generated SWF file but provide instructions on how the compiler should create the SWF. There are 12 documented metadata tags. This article will offer definitions of these metadata tags and give examples of their use. By the end of this article you will understand when and where to use metadata tags in your Flex 2 applications.

[ArrayElementType]
Defining an array is usually very generic in nature because the elements of the array can be any type. However, using the ArrayElementType metadata tag lets you define the data types of the array elements. Here is the sample syntax of [ArrayElementType]:

    [ArrayElementType("String")]
    public var arrayOfStrings:Array;

    [ArrayElementType("Number")]
    public var arrayOfNumbers:Array;

    [ArrayElementType("mx.core.UIComponent")]
    public var arrayOfUIComponents:Array;

[Bindable]
The Bindable metadata tag is one of the most used metadata tags because it allows for easy data synchronization within the components of your application. Bindable can be used to bind simple data, classes, complex data, and functions. To bind a simple piece of data, you must simply define the data with the metadata tag included, as shown in Listing 1. Figure 1 shows the results of Listing 1

Bindable also allows binding to events. Listing 2 shows how to bind a property using getters and setters, along with an event binding. This example has a private variable named phoneNumber, as well as a public getter and setter. The getter method uses the Bindable tag to bind to an event named phoneNumberChanged. This setter method dispatches the phoneNumberChanged even whenever its data changes. By using a setter method, the data can be manipulated before it's set to the private variable. In this example, the data is formatted only when the length of the value coming into the method is 10. When the phoneNumberChanged event is dispatched, the second TextInput component updates because its text property is bound to the phoneNumber variable.

Figure 2 and Figure 3 show the results of Listing 2.

[DefaultProperty]
The DefaultProperty metadata tag is used to set a single property as a default property of a class. This allows the property to be set within a container tag without needing to define the property name. A simple example of this would be a custom Button class. Listing 3 shows a simple Button class that has the label property set as the DefaultProperty. Listing 4 shows how the label is defined as a string within the custom Button container tags.

[Embed]
The Embed metadata tag is used to import images into your application. There are two ways to use Embed. You can either embed the image in ActionScript and assign it to a variable (as in the first example in the following code), or you can assign it directly to the component property (using the syntax shown in the second example of the following code).

[Embed(source="myIcon.gif")]
[Bindable]
public var myIcon:Class;

<mx:Button label="Icon Button 1" icon="{myIcon}"/>

<mx:Button label="Icon Button 2" icon="{myIcon}"/>

The output from the following is identical to the previous code block. The benefits of creating the myIcon class are that it can be defined one time in a single class and bound to multiple components in your application.

<mx:Button label="Icon Button 1" icon="@Embed(source=myIcon.gif')"/>

<mx:Button label="Icon Button 2" icon="@Embed(source=myIcon.gif')"/>

[Event]
The Event metadata tag is used to declare events that will be dispatched by your custom class. Adding this metadata tag to your class definition allows you to add an event handler function to the MXML tag used to instantiate your custom class. Listing 5 creates a custom Button class that will dispatch an event whenever its label property changes. The main application file shown in Listing 6 instantiates the custom Button and creates the event handler function, which dumps the new label property to a TextArea component to show the occurring changes.

Figure 4 shows the results of Listing 5 and Listing 6.

[Effect]
The Effect metadata tag is used to define a custom effect that will be dispatched when an event occurs. This can be easily demonstrated by building on the earlier Event examples. By simply changing a single line to the ButtonLabel class (Listing 7), an effect is defined that can be assigned to an Effect instance (Listing 8).