How to handle service and eventbus instances in GWT MVP Architecture?

Question

We are developing an app using the MVP pattern, as described in this guide:

http://code.google.com/webtoolkit/articles/mvp-architecture.html

When creating the controller instance we do the following:

appController = new AppController(service, eventBus);
appController.go(RootPanel.get("SOME_SLOT"));

Now, when the controller creates a certain presenter, it does something like this:

sthPresenter = new SthPresenter(service, eventBus, new SthView());
sthPresenter.go();

The presenter than saves the eventBus and the service to a private field variable, and uses either as needed.

As the application grows, we have more and more presenters and views, so the question is can we use a different method of obtaining the service and the eventBus in the presenters, without passing a reference via the constructor of each presenter.

For example, creating a static field in the controller and just calling it with something like AppController.getService(). Maybe a singleton pattern.

Would a static field in the controller (or somewhere else) be a bad idea for this design. Keep in mind that the code is compiled to javascript, if that makes any difference.

Answer 1

I'd highly suggest dependency injection (DI). It allows you to avoid boilerplate code (singletons, etc.), global state and in general leads to a more testable code. Misko Hevery has some very interesting posts, including the very informative guide to writing testable code .

For DI in GWT you should use Gin - a wrapper around the popular Guice DI framework . I've been using it for a rather complex project and just using DI/Gin (and thinking how it should be applied most effectively) has definitely lead to a more "clean", testable code.

Answer 2

Use a singleton and an Observer pattern. Make sure you only use high level events for your notifications, if not you will end up with a nightmare.

The code compiling to JavaScript is really transparent to you.

Answer 3

This is a situation very often encountered in GWT MVP applications. In my application, I use dependency injection (with GIN ) to inject the event bus into presenters. Presenters themselves are singletons and can be eagerly instantiated if needed. Doing so, however, will greatly reduce the scalability of your application since a large app will require you to instantiate many presenters as soon as it starts up.

Correctly solving this problem yourself can be a bit involved. I suggest you take a look at the GWT-platform framework , which handles many of the more difficult problems associated with a GWT MVP app, including lazy instantiation of presenters and views, history management, efficient code splitting, etc.