Event propagation

When events are triggered, there are three phases in which Flex checks whether there are event listeners. These phases occur in the following order:

First, capturing
Next, targeting
Finally, bubbling
During each of these phases, the nodes have a chance to react to the event. For example, assume the user clicks a Button control that is inside a VBox container. During the capturing phase, Flex checks the Application object and the VBox for listeners to handle the event. Flex then triggers the Button's listeners in the target phase. In the bubbling phase, the VBox and then the Application are again given a chance to handle the event now in the reverse order from the order in which they were checked in the capturing phase.

In ActionScript 3.0, you can register event listeners on a target node and on any node along the event flow. Not all events, however, participate in all three phases of the event flow. Some types of events are dispatched directly to the target node and participate in neither the capturing nor the bubbling phases. All events can be captured unless they are dispatched from the top node.

Other events may target objects that are not on the display list, such as events dispatched to an instance of the Socket class. These event objects flow directly to the target node, without participating in the capturing or bubbling phases. You can also cancel an event as it flows through the event model so that even though it was supposed to continue to the other phases, you stopped it from doing so. You can do this only if the cancelable property is set to true.

Capturing and bubbling happen as the Event object moves from node to node in the display list: parent-to-child for capturing and child-to-parent for bubbling. This process has nothing to do with the inheritance hierarchy. Only DisplayObject objects (visual objects such as containers and controls) can have a capturing phase and a bubbling phase in addition to the targeting phase.

Mouse events and keyboard events are among those that bubble. Any event can be captured, but no DisplayObject objects listen during the capturing phase unless you explicitly instruct them to do so. In other words, capturing is disabled by default.

When a faceless event dispatcher, such as a Validator, dispatches an event, there is only a targeting phase, because there is no visual display list for the Event object to capture or bubble through.