关注点分离; MVC; 为什么？

Question

I'm currently reading up on OO before I embark upon my next major project. To give you some quick background, I'm a PHP developer, working on web applications.

One area that particularly interests me is the User Interface; specifically how to build this and connect it to my OO "model".

I've been doing some reading on this area. One of my favourites is this: Building user interfaces for object-oriented systems

"All objects must provide their own UI"

Thinking about my problem, I can see this working well. I build my "user" object to represent someone who has logged into my website, for example. One of my methods is then "display_yourself" or similar. I can use this throughout my code. Perhaps to start with this will just be their name. Later, if I need to adjust to show their name+small avatar, I can just update this one method and hey-presto, my app is updated. Or if I need to make their name a link to their profile, hey-presto I can update again easily from one place.

In terms of an OO system; I think this approach works well. Looking on other StackOverflow threads, I found this under "Separation of Concerns": Soc

"In computer science, separation of concerns (SoC) is the process of breaking a computer program into distinct features that overlap in functionality as little as possible. A concern is any piece of interest or focus in a program. Typically, concerns are synonymous with features or behaviors. Progress towards SoC is traditionally achieved through modularity and encapsulation, with the help of information hiding."

To my mind I have achieved this. My user object hides all it's information. I don't have any places in my code where I say $user->get_user_name() before I display it.

However, this seems to go against what other people seem to think of as "best practice".

To quote the "selected" (green one) answer from the same question:

"The separation of concerns is keeping the code for each of these concerns separate. Changing the interface should not require changing the business logic code, and vice versa. Model-View-Controller (MVC) design pattern is an excellent example of separating these concerns for better software maintainability."

Why does this make for better software maintainability? Surely with MVC, my View has to know an awful lot about the Model? Read the JavaWorld article for a detailed discussion on this point: Building user interfaces for object-oriented systems

Anyway... getting to the actual point, finally!

1. Can anyone recommend any books that discuss this in detail? I don't want an MVC book; I'm not sold on MVC. I want a book that discusses OO / UI, the potential issues, potential solutuions etc.. (maybe including MVC) Arthur Riel's Object-Oriented Design Heuristics

touches on it (and is an excellent book as well!), but I want something that goes into more detail.

2. Can anyone put forward an argument that is as well-explained as Allen Holub's JavaWorld article that explains why MVC is a good idea?

Many thanks for anyone who can help me reach a conclusion on this.

Answer 1

This is a failure in how OOP is often taught, using examples like rectangle.draw() and dinosaur.show() that make absolutely no sense.

You're almost answering your own question when you talk about having a user class that displays itself.

"Later, if I need to adjust to show their name+small avatar, I can just update this one method and hey-presto, my app is updated."

Think about just that little piece for moment. Now take a look at Stack Overflow and notice all of the places that your username appears. Does it look the same in each case? No, at the top you've just got an envelope next to your username followed by your reputation and badges. In a question thread you've got your avatar followed by your username with your reputation and badges below it. Do you think that there is a user object with methods like getUserNameWithAvatarInFrontOfItAndReputationAndBadgesUnderneath() ? Nah.

An object is concerned with the data it represents and methods that act on that data. Your user object will probably have firstName and lastName members, and the necessary getters to retrieve those pieces. It might also have a convenience method like toString() (in Java terms) that would return the user's name in a common format, like the first name followed by a space and then the last name. Aside from that, the user object shouldn't do much else. It is up to the client to decide what it wants to do with the object.

Take the example that you've given us with the user object, and then think about how you would do the following if you built a "UI" into it:

1. Create a CSV export showing all users, ordered by last name. E.g. Lastname, Firstname.
2. Provide both a heavyweight GUI and a Web-based interface to work with the user object.
3. Show an avatar next to the username in one place, but only show the username in another.
4. Provide an RSS list of users.
5. Show the username bold in one place, italicized in another, and as a hyperlink in yet another place.
6. Show the user's middle initial where appropriate.

If you think about these requirements, they all boil down to providing a user object that is only concerned with the data that it should be concerned with. It shouldn't try to be all things to everyone, it should just provide a means to get at user data. It is up to each of the many views you will create to decide how it wants to display the user data.

Your idea about updating code in one place to update your views in many places is a good one. This is still possible without mucking with things at a too low of a level. You could certainly create widget-like classes that would encapsulate your various common views of "stuff", and use them throughout your view code.