Java – What goes into the “Controller” in “MVC”

javamodel-view-controller

I think I understand the basic concepts of MVC – the Model contains the data and behaviour of the application, the View is responsible for displaying it to the user and the Controller deals with user input. What I'm uncertain about is exactly what goes in the Controller.

Lets say for example I have a fairly simple application (I'm specifically thinking Java, but I suppose the same principles apply elsewhere). I organise my code into 3 packages called app.model, app.view and app.controller.

Within the app.model package, I have a few classes that reflect the actual behaviour of the application. These extends Observable and use setChanged() and notifyObservers() to trigger the views to update when appropriate.

The app.view package has a class (or several classes for different types of display) that uses javax.swing components to handle the display. Some of these components need to feed back into the Model. If I understand correctly, the View shouldn't have anything to do with the feedback – that should be dealt with by the Controller.

So what do I actually put in the Controller? Do I put the public void actionPerformed(ActionEvent e) in the View with just a call to a method in the Controller? If so, should any validation etc be done in the Controller? If so, how do I feedback error messages back to the View – should that go through the Model again, or should the Controller just send it straight back to View?

If the validation is done in the View, what do I put in the Controller?

Sorry for the long question, I just wanted to document my understanding of the process and hopefully someone can clarify this issue for me!

Best Solution

In the example you suggested, you're right: "user clicked the 'delete this item' button" in the interface should basically just call the controller's "delete" function. The controller, however, has no idea what the view looks like, and so your view must collect some information such as, "which item was clicked?"

In a conversation form:

View: "Hey, controller, the user just told me he wants item 4 deleted."
Controller: "Hmm, having checked his credentials, he is allowed to do that... Hey, model, I want you to get item 4 and do whatever you do to delete it."
Model: "Item 4... got it. It's deleted. Back to you, Controller."
Controller: "Here, I'll collect the new set of data. Back to you, view."
View: "Cool, I'll show the new set to the user now."

In the end of that section, you have an option: either the view can make a separate request, "give me the most recent data set", and thus be more pure, or the controller implicitly returns the new data set with the "delete" operation.