where to store main table view data? (appDelegate or rootViewController)

Question

Any advice on where to store the main list data for an iPhone application like the following?

• NavigationController based
• Level 1 (main screen) is a List of Items. Hence it uses a table view (table of items)
• Level 2_EDIT: Is a view you can get to from the main screen by clicking EDIT. Here you can add text to be added to the main view list
• Level 2_DETAIL: Is a view you can get to from the main screen by clicking on a cell.

Now assuming the implementation is (rough overview): * MainView - appDelegate (holds UIWindow & UINavigationController) * RootViewController - table view of the main items list (? variables here ?) * EditViewController - input text to add to main list * DetailViewController - shows detail of record

Question - Where to hold the NSArray that keeps the main list of items? Should it be in the RootViewController where the Table View exists that displays it? Or should it be higher up in the ApplicationDelegate? I note that when you go from RootViewController to EditViewController, then in this edit view you would have to ADD items to the array, so would it be easier for the code in the EditViewController to access the main array from the AppDelegate (as opposed to the RootViewController)?

(Note - still haven't made an app that has a specific model object, re MVC, yet, so not sure if this should come into the picture.)

Answer 1

Your data becomes the table view's data source, and although it doesn't have to, the view controller that contains the table view often does the data source's role. The view controller can also be a delegate for the EditViewController, so the EditViewController sends a message to it so it can update the array.

Apple's CoreDataBooks sample project show similar architecture. You may want to take a look.

Having the array in the application delegate is often not a great idea. Although it can give you a little bit of convenience, now your classes totally depend on your application delegate unnecessarily.

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Your table view shows your data. This corresponds to View in the MVC design pattern. I assume RootViewController is the view controller of the table view, which acts as a Controller in the pattern. Your data, the location of which is not decided yet, corresponds to Model. The role of RootViewController becomes connecting the Model and View.

The ideal, or the reason of the MVC pattern is to isolate the model and the view, so the model can work with other views with appropriate controllers, and the view can also work with other models with appropriate controllers. For example, your RootViewController will provide the table view with data. It will specify the data in the language of table view, e.g. the number of sections and rows, the contents of cells, etc. If you want to present the data in a different way, for example a graph, your controller will access the same data (model) and provide the graph view with a different representation of the same data. The model need not change, neither do the views. You write only the controller, for each combination of a model and a view.

Ideally, therefore, you will have a different class for the model. In this class, you will store the array, and provide a general interface for controllers can interact with the data.

However, it's often not that necessary, either because it's unlikely that you will use the model class often again, or because the model itself is way too simple so it can be easily implemented anywhere. For example, if your data for the table is a simple array, an NSArray object is often sufficient for the model's role. Therefor, here comes an idea that to combine the controller and the model into a single object.

This is why it makes sense that your table view controller often acts as the data source of the table view.

However, storing the data in the application delegate is a completely different idea. Now the application delegate becomes your model, but which does not make sense, because the application delegate is only used for the specific application. Why would you have a separate model object that totally depends on a single application? Also, if your table view directly interacts with the application delegate, it means that now your view cannot work for other applications either because it now depends on the specific application's application delegate.

Often the reason why people are tempted to have data in the application delegate is, that the application delegate is easily accessible by any objects in the application by using [UIApplication sharedApplication].delegate. The M-V-C relationship is not always very simple. For example, your EditViewController also need to access the same model. To do this, you have to write some code to make the model accessible by both the table view and the edit view. If you have the data in the application delegate, you don't need to do anything, because you can magically access the array by accessing the application delegate.

But that's all. A few minute's saving in your coding time, for the price of ruining your software architecture. I'm not a fundamentalist, and I do sometimes use application delegate to store some data when I'm absolutely sure that it's not worth to provide well-formed interfaces, but it's rare.

So how should you connect your edit view to the data merged into the table view controller? There could be multiple ways. What I suggested before is let the edit view controller has a weak reference to the table view controller (delegate) and send a defined message, for example - (void)editViewController:(EditViewController *editViewController) didFinishEditing:(id) someData. In this way, you can use this edit view controller with some other view controllers as long as they use the same protocol. But others may implement different interfaces for it.

Answer 2

You need to store in the appDelegate,as you can acees the data in anyviewcontroller then.But if you use rootviewController,then you can only send data to next ViewController then,not to the 2nd or 3rd ViewController directly. Hope it will help you.

Good Luck