| Flex 2 Developer's Guide > Using Flex Programming Languages > Using Events > About events | |||
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This section introduces you to the event model in Flex 2. In addition, this section describes the Event object and its subclasses, and describes the event dispatching model. For a quick start in using events in Flex, you can skip this section and see sample code in Using events.
Events let a developer know when something happens within a Flex application. They can be generated by user devices, such as the mouse and keyboard, or other external input, such as the return of a web service call. Events are also triggered when changes happen in the appearance or life cycle of a component, such as the creation or destruction of a component or when the component is resized.
Any user interaction with your application can generate events. Events can also occur without any direct user interaction, such as when data finishes loading from a server or when an attached camera becomes active. You can "handle" these events in your code by adding an event handler. Event handlers are the functions or methods that you write to respond to specific events. They are also sometimes referred to as event listeners.
The Flex event model is based on the Document Object Model (DOM) Level 3 Events Model. Although Flex does not adhere specifically to the Document Object Model standard, the implementations are very similar.
Components generate and dispatch events and consume (listen to) other events. An object that requires information about another object's events registers a listener with that object. When an event occurs, the object dispatches the event to all registered listeners by calling a function that was requested during registration. To receive multiple events from the same object, you must register your listener for each event.
Components have built-in events that you can handle in ActionScript blocks in your MXML applications. You can also take advantage of the Flex event system's dispatcher-listener model to define your own event listeners outside of your applications, and define which methods of your custom listeners will listen to certain events. You can register listeners with the target object so that when the target object dispatches an event, the listeners get called.
All visual objects, including Flex controls and containers, are subclasses of the DisplayObject class. They are in a tree of visible objects that make up your application. The root of the tree is the Stage. Below that is the SystemManager object, and then the Application object. Child containers and components are leaf nodes of the tree. That tree is known as the display list. An object on the display list is analogous to a node in the DOM hierarchical structure. The terms display list object and node are used interchangeably in this topic.
For information about each component's events, see the component's description in Using Controls or the control's entry in Adobe Flex 2 Language Reference.
For a detailed description of a component's startup life cycle, including major events in that life cycle, see Creating Advanced Visual Components in ActionScript in Creating and Extending Flex 2 Components.
Flex 2.01
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