Event objects

Flash Player 9 and later, Adobe AIR 1.0 and later

Event objects serve two main purposes in the new event-handling system. First, event objects represent actual events by storing information about specific events in a set of properties. Second, event objects contain a set of methods that allow you to manipulate event objects and affect the behavior of the event-handling system.

To facilitate access to these properties and methods, the Flash Player API defines an Event class that serves as the base class for all event objects. The Event class defines a fundamental set of properties and methods that are common to all event objects.

This section begins with a discussion of the Event class properties, continues with a description of the Event class methods, and concludes with an explanation of why subclasses of the Event class exist.

Understanding Event class properties

The Event class defines a number of read-only properties and constants that provide important information about an event object.The following are especially important:

  • Event object types are represented by constants and stored in the Event.type property.

  • Whether an event’s default behavior can be prevented is represented by a Boolean value and stored in the Event.cancelable property.

  • Event flow information is contained in the remaining properties.

Event object types

Every event object has an associated event type. Event types are stored in the Event.type property as string values. It is useful to know the type of an event object so that your code can distinguish objects of different types from one another. For example, the following code specifies that the clickHandler() listener function should respond to any mouse click event objects that are passed to myDisplayObject:

myDisplayObject.addEventListener(MouseEvent.CLICK, clickHandler);

Some two dozen event types are associated with the Event class itself and are represented by Event class constants, some of which are shown in the following excerpt from the Event class definition:

package flash.events 
{ 
    public class Event 
    { 
        // class constants 
        public static const ACTIVATE:String = "activate"; 
        public static const ADDED:String= "added"; 
        // remaining constants omitted for brevity 
    } 
}

These constants provide an easy way to refer to specific event types. You should use these constants instead of the strings they represent. If you misspell a constant name in your code, the compiler will catch the mistake, but if you instead use strings, a typographical error may not manifest at compile time and could lead to unexpected behavior that could be difficult to debug. For example, when adding an event listener, use the following code:

myDisplayObject.addEventListener(MouseEvent.CLICK, clickHandler);

rather than:

myDisplayObject.addEventListener("click", clickHandler);

Default behavior information

Your code can check whether the default behavior for any given event object can be prevented by accessing the cancelable property. The cancelable property holds a Boolean value that indicates whether or not a default behavior can be prevented. You can prevent, or cancel, the default behavior associated with a small number of events using the preventDefault() method. For more information, see Canceling default event behavior under Understanding Event class methods.

Event flow information

The remaining Event class properties contain important information about an event object and its relationship to the event flow, as described in the following list:

  • The bubbles property contains information about the parts of the event flow in which the event object participates.

  • The eventPhase property indicates the current phase in the event flow.

  • The target property stores a reference to the event target.

  • The currentTarget property stores a reference to the display list object that is currently processing the event object.

The bubbles property

An event is said to bubble if its event object participates in the bubbling phase of the event flow, which means that the event object is passed from the target node back through its ancestors until it reaches the Stage. The Event.bubbles property stores a Boolean value that indicates whether the event object participates in the bubbling phase. Because all events that bubble also participate in the capture and target phases, any event that bubbles participates in all three of the event flow phases. If the value is true, the event object participates in all three phases. If the value is false, the event object does not participate in the bubbling phase.

The eventPhase property

You can determine the event phase for any event object by investigating its eventPhase property. The eventPhase property contains an unsigned integer value that represents one of the three phases of the event flow. The Flash Player API defines a separate EventPhase class that contains three constants that correspond to the three unsigned integer values, as shown in the following code excerpt:

package flash.events 
{ 
    public final class EventPhase 
    { 
        public static const CAPTURING_PHASE:uint = 1; 
        public static const AT_TARGET:uint = 2; 
        public static const BUBBLING_PHASE:uint= 3; 
    } 
}

These constants correspond to the three valid values of the eventPhase property. You can use these constants to make your code more readable. For example, if you want to ensure that a function named myFunc() is called only if the event target is in the target stage, you can use the following code to test for this condition:

if (event.eventPhase == EventPhase.AT_TARGET) 
{ 
    myFunc(); 
}

The target property

The target property holds a reference to the object that is the target of the event. In some cases, this is straightforward, such as when a microphone becomes active, the target of the event object is the Microphone object. If the target is on the display list, however, the display list hierarchy must be taken into account. For example, if a user inputs a mouse click on a point that includes overlapping display list objects, Flash Player and AIR always choose the object that is farthest away from the Stage as the event target.

For complex SWF files, especially those in which buttons are routinely decorated with smaller child objects, the target property may not be used frequently because it will often point to a button’s child object instead of the button. In these situations, the common practice is to add event listeners to the button and use the currentTarget property because it points to the button, whereas the target property may point to a child of the button.

The currentTarget property

The currentTarget property contains a reference to the object that is currently processing the event object. Although it may seem odd not to know which node is currently processing the event object that you are examining, keep in mind that you can add a listener function to any display object in that event object's event flow, and the listener function can be placed in any location. Moreover, the same listener function can be added to different display objects. As a project increases in size and complexity, the currentTarget property becomes more and more useful.

Understanding Event class methods

There are three categories of Event class methods:

  • Utility methods, which can create copies of an event object or convert it to a string

  • Event flow methods, which remove event objects from the event flow

  • Default behavior methods, which prevent default behavior or check whether it has been prevented

Event class utility methods

There are two utility methods in the Event class. The clone() method allows you to create copies of an event object. The toString() method allows you to generate a string representation of the properties of an event object along with their values. Both of these methods are used internally by the event model system, but are exposed to developers for general use.

For advanced developers creating subclasses of the Event class, you must override and implement versions of both utility methods to ensure that the event subclass will work properly.

Stopping event flow

You can call either the Event.stopPropagation() method or the Event.stopImmediatePropagation() method to prevent an event object from continuing on its way through the event flow. The two methods are nearly identical and differ only in whether the current node’s other event listeners are allowed to execute:

  • The Event.stopPropagation() method prevents the event object from moving on to the next node, but only after any other event listeners on the current node are allowed to execute.

  • The Event.stopImmediatePropagation() method also prevents the event object from moving on to the next node, but does not allow any other event listeners on the current node to execute.

Calling either of these methods has no effect on whether the default behavior associated with an event occurs. Use the default behavior methods of the Event class to prevent default behavior.

Canceling default event behavior

The two methods that pertain to canceling default behavior are the preventDefault() method and the isDefaultPrevented() method. Call the preventDefault() method to cancel the default behavior associated with an event. To check whether preventDefault() has already been called on an event object, call the isDefaultPrevented() method, which returns a value of true if the method has already been called and false otherwise.

The preventDefault() method will work only if the event’s default behavior can be cancelled. You can check whether this is the case by referring to the API documentation for that event type, or by using ActionScript to examine the cancelable property of the event object.

Canceling the default behavior has no effect on the progress of an event object through the event flow. Use the event flow methods of the Event class to remove an event object from the event flow.

Subclasses of the Event class

For many events, the common set of properties defined in the Event class is sufficient. Other events, however, have unique characteristics that cannot be captured by the properties available in the Event class. For these events, ActionScript 3.0 defines several subclasses of the Event class.

Each subclass provides additional properties and event types that are unique to that category of events. For example, events related to mouse input have several unique characteristics that cannot be captured by the properties defined in the Event class. The MouseEvent class extends the Event class by adding ten properties that contain information such as the location of the mouse event and whether specific keys were pressed during the mouse event.

An Event subclass also contains constants that represent the event types that are associated with the subclass. For example, the MouseEvent class defines constants for several mouse event types, include the click, doubleClick, mouseDown, and mouseUp event types.

As described in the section on Event class utility methods under Event objects, when creating an Event subclass you must override the clone() and toString() methods to provide functionality specific to the subclass.