Java SWT update canvas from dialog

Question

I am working on a GUI application with Java and SWT.
I have createad a main window with a canvas in which I load an image (well it's a PDF page converted to image).
I have attached a PaintListener to the canvas and when i drag the mouse onte the canvas I am able to draw a rectangle.
When i release the left button on mouse, I want a dialog window to came out to "fine setting" the rectangle area, so I made a dialog with 4 spinner (x, y, width and height).
Now I want to click on the spinners and the canvas redraw the changed rectangle area.
I tried to pass the canvas object to the dialog window and attach a second paint listener to it (So I have one paintlistener from MainWindow.java and another from Dialog.java), but it's no working.
The problem is that I have multiple rectangle drawn on the canvas, and if I call canvas.redraw() from dialog window, the rectangles already drawn on canvas "disappeared".

What is the best practise in such situation?
I thinks to put the dialog window "in the toolbar", that is put the 4 spinners in the toolbar (or another area in the main window) so I have only one paint listener and get rid of the dialog, but I prefer the dialog 'cause it is impratical to drag a rectangle onto canvas and then move the mouse to click on toolbar.

Thanks in advance, Mauro

Kuba Kuba · Accepted Answer · 2015-09-28T12:58:32

Your main window should implement custom listener for example:

public interface PreferencesListener {
    public void updatePreference(PreferencesEvent e)
}

Then create a manager which control updates:

    public class PreferencesController {

        private static PreferencesController instance = new PreferencesController();
        private List<PreferencesListener> preferencesListeners;

        private PreferencesController() {
            preferencesListeners = new ArrayList<PreferencesListener>();
        }

        public static PreferencesController getInstance() {
            return instance;
        }

        public void addPreferenceListener(PreferencesListener listener) {
            preferencesListeners.add(listener);
        }

        public void notify(int x, int y, int w, int h) {
            PreferencesEvent e = new PreferencesEvent(x, y, w, h);
            for(final PreferencesListener listener: preferencesListeners) {
                listener.updatePreference(e);
            }
        }
    }

At the moment of creation main window register it in controller:

PreferencesController.getInstance().addPreferenceListener(this);

When you change value in dialog trigger notify:

PreferencesController.getInstance().notify(x,y,w,h);

And then finally implement updatePreference(PreferencesEvent e) from PreferencesListener. From this moment whenever values x y w h are changed your window will be notified. From my point of view it's a good solution because your window and dialog don't even know each other exists, and you could easily add another component that react to preference change.

Java SWT update canvas from dialog

1 Answers