Flex Quick Starts: Building Custom Components

There is a 4-part overview for building custom components with Flex in the Flex Quick Starts area of the Adobe Flex Development Center.

1. Building components in MXML: Custom components defined as separate MXML files that are saved in a folder structure in the project. The root tag of the custom component identifies the control that is being extended, just as would be done using ActionScript. Components that contain multiple component definitions are called composite MXML components. If you want to extend the functionality of a composite component, you need to create a custom component that uses the composite component as its root tag. You can create reusable loosely coupled components by defining implicit accessors (setter and getter methods) in order to pass information back and forth. It is helpful to use the [Inspectable] metadata tag to indicate that the property should appear in the code hints of Flex Builder. It is considered a best practice to return information from a component back to the main application through the dispatching of events.
2. Building components in ActionScript: Custom components can be coded in ActionScript and stored in a folder structure just like custom components for MXML files. You can use the setStyle() method in the constructor of the custom component in order to override the default settings of the component superclass.
3. Building components using code behind: In practice you will use a combination of MXML and ActionScript to build Flex components; MXML is useful for layout of controls, and ActionScript is useful for defining the logic. With the release of Flex 2 comes a new technique for coupling the two together: code behind. You make this work by creating an MXML component and make its root tag correspond to an ActionScript class. In order for this to work, child controls must be declared as public properties in the ActionScript class. There is a thorough example provided using a best practices architecture that conforms to the Arp framework.
4. Deploying components: You can deploy components as SWC files or as part of a Runtime Shared Library (RSL) for convenience, runtime efficiency and security purposes. In order to use SWC files when building a Flex application, you specify the SWC file on the Library path tab of the Flex Build Path in the Properties window of your project in Flex Builder.