Building Reusable Java Widgets

An introduction to writing pluggable do-it-yourself widgets for the Java programmer.
Example 3. WindowBar

The window bar is a component needed for the collapsing pane example below. You click on the window bar and the pane collapses. Click on it again and it opens up. To keep it simple I will use a button for the bar, rather than creating a fancy visual bar. The bar must broadcast events to signal that the pane should collapse or restore.

3.1. Layout

The layout of this widget is intentionally trivial, to keep the example simple.

public WindowBar() {
        super() ;
        _closer = new Button( "Collapse" ) ;
        _closer.addActionListener( this ) ;
        add( _closer ) ;
3.2. Create Event Classes

For this widget we need an event with states of “collapse” and “restore”. This will be very similar to our e-mail entry widget. Here is the PaneSwitchEvent:

class PaneSwitchEvent extend AWTEvent
  public static final int COLLAPSE = 1 ;
  public static final int
  RESTORE =2 ; private int _type ;
  public PaneSwitchEvent( Object source, int t )
        super( source , 0 ) ; _type = t ;
  public boolean isRestore()
        return _type == RESTORE ;

The pane switch event needs only to maintain the type of event that it represents.

3.3. Create a Listener Interface

Let's continue down a familiar road and look at the PaneSwitchEventListener. Here it is:

interface PaneSwitchListener extends EventListener
  public void restore( PaneSwitchEvent e ) ;
  public void collapse( PaneSwitchEvent e ) ;

Your listener defines the two methods invoked when the pane switch event occurs. In this case it is restore and collapse.

3.4. Event Multicaster

The multicaster is very similar as well. Just as in the e-mail example, you must create the add and remove methods that accept pane switch listeners as arguments. Then create the collapse and restore methods. Note that you do not need to create a new multicaster for each event class you create. You may choose to have a single multicaster class for all of your widgets. I have chosen to combine the events from both examples into one multicaster class. See Listing 4 for details.

3.4. Hooking it Up

Finally, we need to complete the WindowBar widget. This widget will simply change its text from “collapse” to “restore” and back again when clicked. In addition it sends the corresponding event. Take a look at Listing 6 to see how it's done.

3.6. Exercises for the Reader
  • A visual window bar: Create a subclass of WindowBar that renders itself graphically, instead of as a button. Read up on mouse listeners and mouse events; you will need to listen for them.

  • A more complete window bar: Consider additional actions such as maximize, close, etc. What classes change and in what way?

Example 4. Collapsing Pane

The collapsing pane widget is a container containing exactly one component. It provides a window bar across the top, and the contained component takes up the rest of the area. When the bar is clicked, the widget collapses to display just the window bar. When the bar is clicked again, the component is restored. This widget does not need to generate events. However, it does need to listen for events from the window bar and take action based on them.

4.1. Layout

The CollapsingPane class is a subclass of Panel. I install BorderLayout as its layout manager. The component that will be collapsible is installed in the center area, and the window bar is installed in the north.

class CollapsingPane extends Panel implements
        SwitchPaneListener {
        public CollapsingPane( Component c ) {
                setLayout( new BorderLayout() ) ;
                WindowBar bar = new WindowBar() ;
                add( "North", bar ) ;
                add( "Center", c ) ;
                bar.addCollapseListener( this ) ;

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