Programmatically navigating in iOS - iphone

Question

I'm in the process of porting an existing Android app to iOS, and I'm pretty inexperienced in this environment. The examples I've read so far for navigating multiple views all use some kind of visual user control for triggering the loading and unloading of views (tab bar, nav bar). This application's navigation needs to be pretty strict and not allow the user to freely move around between the three views.
The app needs to have a full screen splash view, a main view that the user interacts with, and a third view for data collection. The splash screen should appear, and the user should be navigated to the main view when tapping on the splash image. There will be custom logic in the main view controller for determining if data is required at which point it should navigate to the data collection view. Once valid data is entered and the user clicks OK, it should navigate back to the main view.
What I've read so far is that all Views should have an associated UIViewController class, and the easy way to do this is to create the XIB and UIViewController class in one shot and link them together (I have plenty examples/books/tutorials that I can reference for that part). I believe what I've read is that the app should have a root UIViewController that handles loading the others and navigating between them.
My questions are:
What class should I derive from for my main view controller that I use to load the others?
How do I wire that up to the app so that it knows to load this as the main controller?
What is the accepted standard way of having a navigation controller in the app and allowing the other views to obtain a reference to it? Should my UIViewControllers hold a reference to their parent controller, or should they ask the UIApplication for a reference to it when needed? How do I make sure I don't instantiate extra copies of the views and their controllers as the user navigates?

Answer 1

What class should I derive from for my
main view controller that I use to
load the others?
UIViewController
How do I wire that up to the app so
that it knows to load this as the main
controller?
Read the "Defining Your Subclass" section of View Controller Programming Guide for iOS. Scratch that -- read the whole thing. It's all important, you might as well start to learn it now. Also read App Programming Guide for iOS. Again, read the whole thing, but the application lifecycle part is the most relevant to your question.
What is the accepted standard way of
having a navigation controller in the
app and allowing the other views to
obtain a reference to it?
Again, this is explained well in View Controller Programming Guide. Views should never care about the navigation controller, but any view controllers that are part of a navigation stack have direct access to the nav controller via their respective navigationController properties.
Should my UIViewControllers hold a
reference to their parent controller,
or should they ask the UIApplication
for a reference to it when needed?
A view controller already has a reference to its parent controller in its (surprise!) parentController property. It's best for a controller to avoid assuming too much about its
parent, though. If the controller expects its parent to be a certain type or respond to certain messages, it becomes more difficult to reuse that controller or reorganize your application. Try to give the controller what it needs to do its thing when you create it. If the controller will need to ask for additional data or something like that, delegation is a good way to go.
How do I make sure I don't instantiate
extra copies of the views and their
controllers as the user navigates?
Exercise caution. There's not much danger of creating extra copies of views in a properly structured application because each view controller should take care of its own views. If you find yourself loading or otherwise creating views outside the context of the view controller that owns them, stop that.

Answer 2

It sounds like you can accomplish what you need with a couple of basic calls. To programmatically call a view controller:
- (void)showController {
MyViewController *myController = [[MyViewController alloc] initWithNibName:#"MyViewControllerXIB" bundle:nil];
[self.navigationController pushViewController:myController animated:YES];
[myController release];
}
To return to the previous view just call from any view controller:
- (void)goBack {
[self.navigationController popViewControllerAnimated:YES];
}
Read up on the documentation for the UINavigationController for more ways to move through views. This method is just one of many ways to do this and may not be suitable for all situations.

Answer 3

Not quite right — each UIViewController should know how to trigger its children. Apple's preferred navigation path through views is a branching tree, with the caveat of tab bars that collapse multiple view controllers into a single node on the tree.
You don't explicitly handle loading. Normally you have a sufficient relationship between your NIBs that the container classes are loaded automatically. Cocoa will then load the views whenever they're needed but not yet loaded (which is the purpose of loadView and viewDidLoad), and keep them unless and until a low memory warning requires them to be purged (leading to viewDidUnload). It's relatively rare that you explicitly load a NIB yourself (though table view cells are an obvious example where programmatically loading a NIB is quite common).
So you'd probably have:
a splash screen or preview of the first view controller, as the Default.png
a view controller that probably displays Default.png, and has two outlets going to the data collection controller and the main controller
when the user taps the button on the main screen, ask the model whether data collection is necessary. If so then navigate to the data collection controller, otherwise navigate to the main controller
give the data collection controller an outlet to the main controller and let it perform a navigation there at the appropriate moment
You get a MainWindow.xib for free when creating a new view based project. Probably the easiest thing to do is to put references to the three UIViewController subclasses in there, but set each of them to load from other files. Set the links between them in MainWindow.xib, set the links to things within the relevant views within the relevant XIBs.
That will prevent you from keeping multiple instances of any controllers about, and the built-in Cocoa loading mechanisms will ensure that the stuff that occupies significant amounts of memory — the views — is loaded only on demand and kept for no longer than space allows.
There's no need to link to parent view controllers. Every view controller already knows who presented it, via the parentViewController property. So if a view controller wants to dismiss itself and return to whoever presented it, you can just issue:
[self.parentViewController dismissModalViewControllerAnimated:YES];
Because the model is ideally a separate sovereign thing, all controllers really need to know is which other controllers they can present, how to populate themselves from the model and how to push data back to the model. You rarely end up with particularly complicated links between view controllers.

Answer 4

I think you should have the view loading/unloading in the application delegate and then each view should send notifications to the application delegate.
Here is the official introduction from Apple:
http://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/Notifications/Introduction/introNotifications.html

Answer 5

You can set before which ViewController to load first
If the application is navigation based use the following code:
AppDelegate.m
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
MainViewController *mainViewController = [[MainViewController alloc] initWithNibName:#"MainViewController" bundle:nil];
self.nav = [[UINavigationController alloc] initWithRootViewController:mainViewController];
[_window addSubview:nav.view];
[_window makeKeyAndVisible];
}
If the application is View based use the following code:
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
self.viewController = [[ViewController alloc] initWithNibName:#"ViewController" bundle:nil];
self.window.rootViewController = self.viewController;
[self.window makeKeyAndVisible];
return YES;
}