Need some clarifications on the Model side of MVC

I guess 开发者_运维百科I would need some really good explanation on some Model related concepts.

1. In general does the model, as described by frameworks like Robotlegs play the role of an application state holder, or a domain state holder? I originally thought that models are entirely domain based, i.e UserModel, LocationModel, which play the same role that DAO classes play on the server. The more source code I am looking at though, the more I see stuff like UserAccountModel, ShoppingCartModel, etc, full of properties and methods related to the state of the client application, not the domain state.

2. I see that the people do not bother to add complex relationships to the VO classes, i.e. if a User has a lot of photos, the photos collection is obviously omitted from the UserVO class. Instead, a bunch of PhotoVO objects are loaded from the server whenever necessary, based on a service call with the user ID. Is that some sort of a rule of thumb - in general keeping VOs as "bare" as possible? Doesn't that increase the possible number of calls that must be made to the server to fetch all the data? Moreover, doesn't that fragment the domain model in general? (an entity User class on the server will always have a photos property)

3. With so many calls to the server, it is normal to fetch some objects that might be already on the client storage. does it make sense to make a client side cache, and check if the object that is going to be fetched is already there, or in general, the overhead of getting it once again will be paid off by the benefits of getting a fully synced object from the server. Otherwise, every object stored on the client side cache must be cared for when a change occurs. I personally think that the overhead of getting an object from the server, which might have already been picked up before is not as big. Better have fresh and synced data I'd say.

I do not believe your question is answerable, because so many of the answers are "it depends." It depends on the application you're building and the needs of the UI.

I don't really understand your distinction between "Domain State" and "Application State." However, I believe that any "Value Object" style classes implemented in the UI should focus on holding the state of specific views. It is extremely rare that a single view is a one to one relationship to database tables. As such, my UI Data Objects may not be identical to server side data objects. Although, it is very common that I will map UI objects to server side objects using AMF. But, it doesn't mean that every object in the UI is implemented server side and every server object is implemented on the UI.

I see that the people do not bother to add complex relationships to the VO classes,

I'm not sure where you see that; I will often do exactly this. However, it depends what the view is supposed to display. IF the view is not displaying a lot of photos related to the user, then I won't make a remote call to retrieve the user information with all their photos.

With so many calls to the server, it is normal to fetch some objects that might be already on the client storage.

It depends. I would say that the apps I write, the calls to the server are done as needed; and attempts are made to limit them as appropriate. If I already fetched data and have it cached on the client, then I am going to try to use that cache instead of retrieving the data again.

I'll restate my original assessment: I think the answers to most of your questions depend on the situation, and depend on the app. You seem to start with overally broad generalizations about how things are done. However, I Do not believe they are universal truths. Developer's fight about application architecture issues all the time.