How to Build an Agent's View

In this section, we explain how a view of the agent is created. All our agents follow the same approach. If you want to see the code while reading the notes, go to  TreeViewerAgent View.

Let's go back to our example MyBrowserAgent (see How To Build An Agent).

1. Create a view package in the mybrowser package.
2. Create the following classes MyBrowser (I/F), MyBrowserComponent, MyBrowserModel, MyBrowserControl, and MyBrowserUI. If you browse the source code, you will notice that we usually have a class used as a toolbar and a class used as a status bar. Both classes are initialised by the BrowserUI. For clarity, they have been omitted in the following diagram.
3. Create a MyBrowserFactory. This class keeps trach of the MyBrowser instances created and not yet discarded. A component is only created if none of the tracked ones is displaying the data otherwise the existing component is recycled.

Typical life-cycle of an Agent View

The object is first created using the MyBrowserFactory

//Somewhere in the MyBrowserFactory code

    /** The sole instance. */
    private static final MyBrowserFactory  singleton = new MyBrowserFactory();
    
    /**
     * Returns a viewer to display the specified images.
     * 
     * @param images The <code>ImageData</code> objects.
     */
    public static MyBrowser getViewer(Set<ImageData> images)
    {
        MyBrowserModel model = new MyBrowserModel(images);
        return singleton.getViewer(model);
    }


    /**
     * Creates or recycles a viewer component for the specified 
     * <code>model</code>.
     * 
     * @param model The component's Model.
     * @return A {@link MyBrowser} for the specified <code>model</code>.  
     */
    private MyBrowser getViewer(MyBrowserModel model)
    {
        Iterator v = viewers.iterator();
        MyBrowserComponent comp;
        while (v.hasNext()) {
            comp = (MyBrowserComponent) v.next();
            if (model.isSameDisplay(comp.getModel())) {
            	comp.refresh(); //refresh the view.
            	return comp;
            }
        }
        comp = new MyBrowserComponent(model);
        comp.initialize();
        comp.addChangeListener(this);
        viewers.add(comp);
        return comp;
    }

After creation, the object is in the MyBrowser#NEW state and is waiting for the MyBrowser#activate() method to be called. Such a call usually triggers of the objects on the server. The object is now in the MyBrowser#LOADING state. After all the data have been retrieved, the object is the MyBrowser#READY state and the data display is built and set on screen.

When the user quits the window, the MyBrower#discard() method is invoked and the object transitions to the MyBrowser#DISCARDED state. At which point, all clients should de-reference the component to allow for garbage collection.