MVC: why the separation of model, view, and contro

问题:

Other than the “philosophical” aspects of it, is it a bad idea to have my controller also be my model?

It seems to save some programming time. I don’t have to create logic between the controller and the model, since it’s the same thing. And I can directly interact with the view.

What’s the point of separating the M and the C? Is modularity — that is, the ability to swap one model and controller set for another — the only reason to separate them? It seems to me that “swapping” modules out happens a lot less than (for example) having to update both the model and the controller because something in the model is changing.

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It seems odd that a simple calculator, according to the MVC concept, should have both a controller and a view for its settings (like default settings, or something). I know this is a simple example, but it seems to apply to all cases (except maybe frameworks).

回答1:

The primary reason is for reusability of code. If you’re only ever going to write one program in your professional life, then perhaps it doesn’t matter. If you plan to make a career of it, having reusable pieces is valuable. Well-designed model, controller and view classes are very easy to drop into other programs. I do this all the time.

Consider UITableViewController, which is a Controller. Now imagine if it were designed exclusively to handle music tracks (the Model), and you needed to create a completely different table-management class when you wanted to handle something else. Avoiding this nightmare is why MVC is heavily used in Cocoa.

There are other ways to split things up. Some languages subclass heavily rather than delegating. But in Cocoa, the primary means of splitting up programs is MVC, and it works very well.

EDIT: Just some more reasons from the world of developing commercial apps.

• Memory handling is much easier in MVC. You can hold on to your model objects and throw away your view objects (and many of your controller objects) when they go offscreen.

• It’s easier to serialize model objects that aren’t wrapped up with controllers and views, and it’s much easier to display the same data in multiple ways. Even in a “simple” text editor, you may want to be able to do split-screen, or have multiple windows showing the same document. In MVC that’s very easy.

If you need no flexibility now or in the future, you don’t need much architecture. But most real projects aren’t so simple. MVC grew out of Xerox’s experience with writing large programs and the difficulties encountered when everything was thrown together.

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EDIT 2: I was looking at your earlier edit: “It seems odd that a simple calculator, according to the MVC concept, should have both a controller and a view for its settings (like default settings, or something).”

This is exactly the reason for MVC. It would seem crazy to have to re-code all of the things required for saving user settings specially for a Calculator app. You’d want a generic “please save these user settings” that was completely separate from the UI and that you could reuse. On OS X it’s called NSUserDefaults, and the Calculator app stores its configuration in exactly this way.

回答2:

MVC is a standard pattern that is well understood in the development community, and for good reasons. The separation really makes things easy to read, easy to troubleshoot, easy to find, and easy to test, as individual components, each with its own area of responsibility.

Do you have to use it? of course not. But keeping the parts separate is generally considered a good idea.

回答3:

The controller knows how to link a specific view to your model. The separation of model and controller, apart from improving documentation and maintainability, has the immediate benefit of allowing multiple views to display the same information from the model without adding any complexity to either.

That applies not just to multiple views in the same application, but also to the multiple variations in views you'll have across multiple versions of your application. Your model is insulated and logically clean.

Combining model and controller is a classic false economy in my opinion. It may feel like it saves a few minutes, but it costs significantly as an application develops and grows.

回答4:

If it works for you then it works. Period. The reason for separation of Models, Views, and Controllers revolves around the idea that most development for enterprise applications is done by a team of developers.

Imagine 10 developers trying to work on your controller. But all they want to do is add something to the model. Now your Controller broke? What did they do?

回答5:

The Models are usually separate components which can be re-used between Controllers. If you are absolutely certain you won't be re-using Models in multiple Controller, I don't really see a problem with blending these concerns.

I guess one could argue why even use MVC design if you are planning on deviating. Maybe there is a more suitable pattern to follow for your situation. Can you give us an example of something you've done where the Controller is the Model? It would help us understand what you are trying to do better.

回答6:

MVC is all about management (separation of data, representation and business logic). So it's like this: if you run a small company, having a MS-sized management would be a real drag. But if you are a giant corporation, not having big middle management is impossible.

Honestly, in most of my college progamming assignments, I combined the models and controllers, because I didn't see the need for the separation. But working on big projects? The deficiency would be pretty obvious if you try to not separate. Just do what you feel right.

回答7:

The model depends on neither the view nor the controller. This is one the key benefits of the separation. This separation allows the model to be built and tested independent of the visual presentation.