Event types As mentioned above, the Event class is a model of a user interface event. Events naturally fall into categories based on the type of the event (the event type is indicated by the id data member). Table 2 lists all of the events Defined by the on the a on, sorted by category.
Window events Window events are generated in response to changes in the state of a window, frame, or dialog. Event ID WINDOW_DESTROY 201 WINDOW_EXPOSE 202 WINDOW_ICONIFY 203 WINDOW_DEICONIFY 204 WINDOW_MOVED 205 Keyboard events Keyboard events are generated in response to keys pressed and released while a component has input focus. Event ID KEY_PRESS 401 KEY_RELEASE 402 KEY_ACTION 403 KEY_ACTION_RELEASE 404 Mouse events Mouse events are generated in response to mouse actions occurring within the boundary of a component. Event ID MOUSE_DOWN 501 MOUSE_UP 502 MOUSE_MOVE 503 MOUSE_ENTER 504 MOUSE_EXIT 505 MOUSE_DRAG 506 Scroll events Scroll events are generated in response to manipulation of scrollbars. Event ID SCROLL_LINE_UP 601 SCROLL_LINE_DOWN 602 SCROLL_PAGE_UP 603 SCROLL_PAGE_DOWN 604 SCROLL_ABSOLUTE 605 List events List events are generated in response to selections made to a list. Event ID LIST_SELECT 701 LIST_DESELECT 702 Miscellaneous events Miscellaneous events are generate D in response to a variety of actions. EVENT ID ACTION_EVENT 1001 LOAD_FILE 1002 Save_File 1003 Got_focus 1004 Lost_focus 1005
Table 2: Events defined by the AWT, Sorted by category
It can be instructive to see event generation in action. The button in Figure 1, when pressed, creates an event browser that displays event information about the events the browser receives. The source code for the event browser is available here.
You NEED A JAVA-Enabled Browser to View this AppleTfigure 1: Event Generation In Action
Event dispatch and propagation Consider the applet in Figure 2. It consists of two instances of the Button class, embedded within an instance of the Panel class. This instance of the Panel class is itself embedded within another instance of the Panel class. The latter instance of the Panel class sits below an instance of class TextArea, and both instances are embedded within an instance of the Applet class. Figure 3 presents the elements that make up this applet laid out as a tree, with the TextArea and Button instances as the leaves And an applet instance as the root. (for More Information About The Hierarchical Layout of Components In a User Interface, Read last Month's Introduction to the AWT.)
You NEED A JAVA-Enabled Browser to View this applet
Figure 2: Classes Embedded within classes
Figure 3: Applet Elements Tree (Hierarchy)
When a user interacts with the applet in Figure 2, the Java run-time system creates an instance of class Event and fills its data members with information describing the action. The Java run-time system then allows the applet to handle the event. It begins with the component that initially received the event (for instance, the button that was clicked) and moves up the component tree, component by component, until it reaches the container at the top of the tree. Along the way, each component has the Opportunity to ignore the Event or To React To It in (or more) of the folloading Ways:
Modify the data members of the Event instance Take action and perform some computation based on the information contained in the event Indicate to the Java run-time system that the event should propagate no further up the treeThe Java run-time system passes event information to a Component Via the Component's Handleevent () Method. All Valid Handleevent () Methods Must Be of The Form
Public Boolean Handleevent (Event E)
.
The value returned from the handleEvent () method is important. It indicates to the Java run-time system whether or not the event has been completely handled within the event handler. A true value indicates that the event has been handled and propagation should stop. ...............
Consider the following description of an imaginary user's interaction with the applet in Figure 2. The user clicks on the button labeled "One." The Java language run-time system gathers information about the event (the number of clicks, the location of the click , the time the click occurred, and the component that received the click) and packages that information in an instance of the Event class. The Java run-time system then begins at the component that was clicked (in this case, the Button labeled " One ") and, via a call to the component's handleEvent () method, offers the component a chance to react to the event. If the component does not handle the event or handles the event incompletely (indicated by a return value of false), the Java run-time system offers the Event instance to the next higher component in the tree -. in this case an instance of the Panel class The Java run-time system continues in this manner until the event is handled or the run-time System Runs Out of Componen Ts to Try. Figure 4 illustrates the path of this event as the applet attempts to handle it.figure 4: the path of an es
Each component making up the applet in Figure 2 adds a line to the TextArea object that indicates it received an event. It then allows the event to propagate to the next component in the tree. Listing 1 contains the code for a typical handleEvent () method The Complete Source Code for this applet is available here.
Public Boolean Handleevent (Event EVT) {if (Evt.ID == Event.Action_Event) {ta.appendtext ("Panel" Str "SAW Action ... / N");} else if (evt.id == Event.mouse_down) {ta.appendtext ("Panel" Str "SAW Mouse Down ... / N");} return super.handleevent (EVT);} listing 1: a typeEpical Handleevent () Method
Event helper methods The handleEvent () method is one place a programmer can put application code for handling events. Occasionally, however, a component will only be interested in events of a certain type (for example, mouse events). In these cases, the Programmer can place the code in a helper method () method.
Here is a list of the helper method method. There are no helper method...............
Action (Event EVT, Object What) Gotfocus (Event EVT, Object What) MouseEnter (Event EVT, INT X, INT Y) MouseExit (Event EVT, INT X, INT Y) MOUSEMOVE (EVENT EVT , INT X, INT Y) MOUSEDOWN (Event EVT, INT X, INT Y) MOUSEDRAG (Event EVT, INT X, INT Y) Keydown (Event EVT, INT Key) Keyup Event EVT, INT Key
False to indeicate what the helper method.
The implementation of the handleEvent () method provided by class Component invokes each helper method. For this reason, it is important that the redefined implementations of the handleEvent () method in derived classes always end with the statement
Return super.handleevent (e);
The Code in Listing 2 Illustrates this rule.
