Multiple view controllers on screen at once?

Question

Hello! I am trying to wrap my head around controllers in Cocoa Touch. The main problem is that I would like to have more then one controller “on screen” at once – I want to have a large view (with controller A) composed of smaller views controlled by their own controllers (say B). I’d like to have it this way because the division makes the code much cleaner. What’s bad is that the additional controllers (of type B) are not “first-class citizens” on the screen, for example they do not receive the autorotation queries and notifications. (And cannot easily display modal controllers, they have to send the presentModal… message to their parent controller.)

What is the difference between the A and B controllers from Cocoa viewpoint? Does the system keep some kind of pointer to the “frontmost controller”, a privileged one to which it sends notifications and such stuff? Why don’t the other controllers receive them, even though their views are on the screen? Is having multiple controllers “on screen” considered a hack? Or is it supported and I am just missing some point? Thank you.

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More about the problem I am trying to solve: I am writing a simple photo browser. Photos are displayed in full screen, user can swipe left or right to change photos. The A controller takes care of the scrolling part and the B controllers take care of each photo itself.

Isolating B seemed like a good idea, since the photos are loaded from network and there is a lot that can happen, like the network might be down et cetera. In the B controller the code is fairly simple, since B only works with one particular photo. If I moved the code to the A controller, things would get messy.

The only thing I don’t like about the current solution is that I have to manually work around B not being a “first-class” controller. I have to pass some calls manually through A to B and when B wants to display a modal dialog, it has to send the presentModal… to A. Which is ugly.

Answer 1

First, and this is important, view controllers don't get "on screen" -- views do. Your "top level" controller can certainly pass along the kinds of messages you're describing to its "sub-view-controllers". In fact, this is how most apps work. Consider an app that has a tab bar, and where the views use navigation controllers. You actually have several view controllers "running" at the same time, each with its own view on screen at once -- your "root" view controller will be an instance (or subclass) of UITabBarController, which then has several nested UINavigationControllers, each which will display nested view controllers (like an instance or a subclass of UITableViewController).

You might want to read up a bit on how responder chains work. Consider a touch event. It will be generated for the view closest to the top of the stack, that can receive events, which is also underneath the tap. If that view can't handle it, it gets passed up the view hierarchy food chain until something deals with it (and then eats it).

As to the specifics of your question, on the whole, I'm not sure exactly what the strategy you describe is really doing to benefit you in terms of complexity. It depends on how exactly you're implementing things, but having separate view controllers for each little subview may require more bookkeeping code than just having one view controller that knows about all its sub-view components.

Answer 2

It's not closely related to the original question but important. Apple clearly states in View Controller Programming Guide that a view controller is responsible for controlling exactly one screen's content:

"Each custom view controller object you create is responsible for managing exactly one screen’s worth of content. The one-to-one correspondence between a view controller and a screen is a very important consideration in the design of your application. You should not use multiple custom view controllers to manage different portions of the same screen. Similarly, you should not use a single custom view controller object to manage multiple screens worth of content.

Note: If you want to divide a single screen into multiple areas and manage each one separately, use generic controller objects (custom objects descending from NSObject) instead of view controller objects to manage each subsection of the screen. Then use a single view controller object to manage the generic controller objects. The view controller coordinates the overall screen interactions but forwards messages as needed to the generic controller objects it manages."

However in iPad Programming Guide they also say that there may be container view controllers:

"A view controller is responsible for a single view. Most of the time, a view controller’s view is expected to fill the entire span of the application window. In some cases, though, a view controller may be embedded inside another view controller (known as a container view controller) and presented along with other content. Navigation and tab bar controllers are examples of container view controllers."

Up to my current knowledge I would not use sub-view controllers in a view controller but try to subclass NSObject and send messages to them from my main view controller.

Also check this thread: MGSplitViewController discussion