What goes into the "Controller" in "MVC"?

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.

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The problem with MVC is that people think the view, the controller, and the model have to be as independent as possible from each other. They do not - a view and controller are often intertwined - think of it as M(VC).

The controller is the input mechanism of the user interface, which is often tangled up in the view, particularly with GUIs. Nevertheless, view is output and controller is input. A view can often work without a corresponding controller, but a controller is usually far less useful without a view. User-friendly controllers use the view to interpret the user's input in a more meaningful, intuitive fashion. This is what it makes it hard separate the controller concept from the view.

Think of an radio-controlled robot on a detection field in a sealed box as the model.

The model is all about state and state transitions with no concept of output (display) or what is triggering the state transitions. I can get the robot's position on the field and the robot knows how to transition position (take a step forward/back/left/right. Easy to envision without a view or a controller, but does nothing useful

Think of a view without a controller, e.g. someone in a another room on the network in another room watching the robot position as (x,y) coordinates streaming down a scrolling console. This view is just displaying the state of the model, but this guy has no controller. Again, easy to envision this view without a controller.

Think of a controller without a view, e.g. someone locked in a closet with the radio controller tuned to the robot's frequency. This controller is sending input and causing state transitions with no idea of what they are doing to the model (if anything). Easy to envision, but not really useful without some sort of feedback from the view.

Most user-friendly UI's coordinate the view with the controller to provide a more intuitive user interface. For example, imagine a view/controller with a touch-screen showing the robot's current position in 2-D and allows the user to touch the point on the screen that just happens to be in front of the robot. The controller needs details about the view, e.g. the position and scale of the viewport, and the pixel position of the spot touched relative to the pixel position of the robot on the screen) to interpret this correctly (unlike the guy locked in the closet with the radio controller).

Have I answered your question yet? :-)

The controller is anything that takes input from the user that is used to cause the model to transition state. Try to keep the view and controller a separated, but realize they are often interdependent on each other, so it is okay if the boundary between them is fuzzy, i.e. having the view and controller as separate packages may not be as cleanly separated as you would like, but that is okay. You may have to accept the controller won't be cleanly separated from the view as the view is from the model.

... 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?

I say a linked view and controller should interact freely without going through the model. The controller take the user's input and should do the validation (perhaps using information from the model and/or the view), but if validation fails, the controller should be able to update its related view directly (e.g. error message).

The acid test for this is to ask yourself is whether an independent view (i.e. the guy in the other room watching the robot position via the network) should see anything or not as a result of someone else's validation error (e.g. the guy in the closet tried to tell the robot to step off the field). Generally, the answer is no - the validation error prevented the state transition. If there was no state tranistion (the robot did not move), there is no need to tell the other views. The guy in the closet just didn't get any feedback that he tried to cause an illegal transition (no view - bad user interface), and no one else needs to know that.

If the guy with the touchscreen tried to send the robot off the field, he got a nice user friendly message asking that he not kill the robot by sending it off the detection field, but again, no one else needs to know this.

If other views do need to know about these errors, then you are effectively saying that the inputs from the user and any resulting errors are part of the model and the whole thing is a little more complicated ...

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The MVC pattern merely wants you to separate the presentation (= view) from the business logic (= model). The controller part is there only to cause confusion.

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Here is a good article on the basics of MVC.

It states ...

Controller - The controller translates interactions with the view into actions to be performed by the model.

In other words, your business logic. The controller responds to actions by the user taken the in the view and responds. You put validation here and select the appropriate view if the validation fails or succeeds (error page, message box, whatever).

There is another good article at Fowler.

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Practically speaking, I've never found the controller concept to be a particularly useful one. I use strict model/view separation in my code but there's no clearly-defined controller. It seems to be an unnecessary abstraction.

Personally, full-blown MVC seems like the factory design pattern in that it easily leads to confusing and over-complicated design. Don't be an architecture astronaut.

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Controller is really part of the View. Its job is to figure out which service(s) are needed to fulfill the request, unmarshal values from the View into objects that the service interface requires, determine the next View, and marshal the response back into a form that the next View can use. It also handles any exceptions that are thrown and renders them into Views that users can understand.

The service layer is the thing that knows the use cases, units of work, and model objects. The controller will be different for each type of view - you won't have the same controller for desktop, browser-based, Flex, or mobile UIs. So I say it's really part of the UI.

Service-oriented: that's where the work is done.

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Based on your question, I get the impression that you're a bit hazy on the role of the Model. The Model is fixated on the data associated with the application; if the app has a database, the Model's job will be to talk to it. It will also handle any simple logic associated with that data; if you have a rule that says that for all cases where TABLE.foo == "Hooray!" and TABLE.bar == "Huzzah!" then set TABLE.field="W00t!", then you want the Model to take care of it.

The Controller is what should be handling the bulk of the application's behavior. So to answer your questions:

Do I put the public void actionPerformed(ActionEvent e) in the View with just a call to a method in the Controller?

I'd say no. I'd say that should live in the Controller; the View should simply feed the data coming from the user interface into the Controller, and let the Controller decide which methods ought to be called in response.

If so, should any validation etc be done in the Controller?

The bulk of your validation really ought to be done by the Controller; it should answer the question of whether or not the data is valid, and if it isn't, feed the appropriate error messages to the View. In practice, you may incorporate some simple sanity checks into the View layer for the sake of improving the user experience. (I'm thinking primarily of web environments, where you might want to have an error message pop up the moment the user hits "Submit" rather than wait for the whole submit -> process -> load page cycle before telling them they screwed up.) Just be careful; you don't want to duplicate effort any more than you have to, and in a lot of environments (again, I'm thinking of the web) you often have to treat any data coming from the user interface as a pack of filthy filthy lies until you've confirmed it's actually legitimate.

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?

You should have some protocol set up where the View doesn't necessarily know what happens next until the Controller tells it. What screen do you show them after the user whacks that button? The View might not know, and the Controller might not know either until it looks at the data it just got. It could be "Go to this other screen, as expected" or "Stay on this screen, and display this error message".

In my experience, direct communication between the Model and the View should be very, very limited, and the View should not directly alter any of the Model's data; that should be the Controller's job.

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

See above; the real validation should be in the Controller. And hopefully you have some idea of what should be put in the Controller by now. :-)

It's worth noting that it can all get a little blurry around the edges; as with most anything as complex as software engineering, judgment calls will abound. Just use your best judgment, try to stay consistent within this app, and try to apply the lessons you learn to the next project.

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Here is a rule of thumb that I use: if it is a procedure that I will be using specifically for an action on this page, it belongs in the controller, not the model. The model should provide only a coherent abstraction to the data storage.

I've come up with this after working with a large-ish webapp written by developers who thought they were understood MVC but really didn't. Their "controllers" are reduced to eight lines of calling static class methods that are usuall called nowhere else :-/ making their models little more than ways of creating namespaces. Refactoring this properly does three things: shifts all the SQL into the data access layer (aka model), makes the controller code a bit more verbose but a lot more understandable, and reduces the old "model" files to nothing. :-)

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The controller is primarily for co-ordination between the view and the model.

Unfortunately, it sometimes ends up being mingled together with the view - in small apps though this isn't too bad.

I suggest you put the:

public void actionPerformed(ActionEvent e)

in the controller. Then your action listener in your view should delegate to the controller.

As for the validation part, you can put it in the view or the controller, I personally think it belongs in the controller.

I would definitely recommend taking a look at Passive View and Supervising Presenter (which is essentially what Model View Presenter is split into - at least by Fowler). See:

http://www.martinfowler.com/eaaDev/PassiveScreen.html

http://www.martinfowler.com/eaaDev/SupervisingPresenter.html

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also note that each Swing widget is can be considered to contain the three MVC components: each has a Model (ie ButtonModel), a View (BasicButtonUI), and a Control (JButton itself).

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You are essentially right about what you put in the controller. It is the only way the Model should interact with the View. The actionperformed can be placed in the View, but the actual functionality can be placed in another class which would act as the Controller. If you're going to do this, I recommend looking into the Command pattern, which is a way of abstracting all of the commands that have the same receiver. Sorry for the digression.

Anyway, a proper MVC implementation will have the following interactions only: Model -> View View -> Controller Controller -> View

The only place where there may be another interaction is if you use an observer to update the View, then the View will need to ask the Controller for the information it needs.

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As I understand it, the Controller translates from user-interface actions to application-level actions. For instance, in a video game the Controller might translate "moved the mouse so many pixels" into "wants to look in such and such a direction. In a CRUD app, the translation might be "clicked on such and such a button" to "print this thing", but the concept is the same.

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We do it thusly, using Controllers mainly to handle and react to user-driven input/actions (and _Logic for everything else, except view, data and obvious _Model stuff):

(1) (response, reaction - what the webapp "does" in response to user) Blog_Controller

->main()

->handleSubmit_AddNewCustomer()

->verifyUser_HasProperAuth()

(2) ("business" logic, what and how the webapp "thinks") Blog_Logic

->sanityCheck_AddNewCustomer()

->handleUsernameChange()

->sendEmail_NotifyRequestedUpdate()

(3) (views, portals, how the webapp "appears") Blog_View

->genWelcome()

->genForm_AddNewBlogEntry()

->genPage_DataEntryForm()

(4) (data object only, acquired in _construct() of each Blog* class, used to keep all webapp/inmemory data together as one object) Blog_Meta

(5) (basic data layer, reads/writes to DBs) Blog_Model

->saveDataToMemcache()

->saveDataToMongo()

->saveDataToSql()

->loadData()

Sometimes we get a little confused on where to put a method, in the C or the L. But the Model is rock solid, crystal clear, and since all in-memory data resides in the _Meta, it's a no-brainer there, too. Our biggest leap forward was adopting the _Meta use, by the way, as this cleared out all the crud from the various _C, _L and _Model objects, made it all mentally easy to manage, plus, in one swoop, it gave us what's being called "Dependency Injection", or a way to pass around an entire environment along with all data (whose bonus is easy creation of "test" environment).

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