Question

first post don’t hurt me 🙂

I am using a BorderLayout with the usual North, West, Center, South Panels. On the West ContentPanel, I’ve got a Tree. If an event (OnClick)occurs I want a particular dialog box displayed on the Center ContentPanel.

What is the best way for me to do this? Currently I’m using a function called returnPanel() that returns the center ContentPanel. In the event handler I call this function (MainWindow.returnPanel().add(myDialog)).

Answer 1

The way you are doing it is intuitive and works, but will start causing hell when the application grows, because different parts of the application are strongly coupled. The solutions to this problems are the MVC design pattern and the observer design pattern.

Ideally, using the MVC pattern, you don’t want any widget to ‘know’ of any other widget. There is only class that knows all the widgets, which is the Controller. Anytime one widget needs to message/signal another widget, it tells it to the Controller class, which relays the message in the appropriate way to the appropriate widget. In this way, the two widgets are decpoupled and one can change without breaking the other. You may want to use an enum to enumerate all possible actions to which the controller has to responsd.

If your widget has to call only the Controller when an event occurs, you may simply call an aptly named (static) method on it and be done with it. However, as soon as multiple other classes needs to be informed of an event, you are better of using the Observer pattern, which allows you to signal multiple other classes, without changing your class. It simply calls notifyPObservers() in the eventHandler and that’s it. How many listeners there are, and what type they are, is irrelevant. This way, you also decouple a class from it’s listeners. Even if only the Controller listens, it may be advisable to use the pattern, as it clearly seperated the ‘call back’ code from the other code in the classes.

BTW, this has nothing to do with GWT or even Java in particular.