Trouble passing a pointer between child ViewControllers in iOS6 - pointers

Question

My problem is with an iOS 6 tabbed application. My work-in-progress has 5 tabs and several tabs are gateways to other view controllers. Most pages need access to a Model object, which contains data stored as arrays, strings, and so on. A bare-bones model is populated at runtime and the user can add to it throughout the application lifespan. For example, the code listed below is from my AppDelegate file , where it is passing a pointer to the bare-bones Model to the Project View Controller. This works fine: the tab application uses the navigation controller array stack; because I know the Project page is at index 2, I can send the model to it.
My problem has to do with the sub views of the Project page. For example, as a sub view to the Project page there is (or should be) a File_IO page where the user handles file operations. The File_IO page also needs access to the Model object. But when I try to send the Model pointer from the Project page to the File_IO page, the technique I used previously (from the AppDelegate to the Project) does not work. I point to an empty Model in the FileIO ViewController.
Example code: this is in the AppDelegate, and it works fine: the bare-bones Model in the Project ViewController is populated with the data.
//To the Project View Controller...
UINavigationController *project_NavigationController =
[[tabBarController viewControllers] objectAtIndex:2];
Project_ViewController *project_ViewController =
[[project_NavigationController viewControllers] objectAtIndex:0];
//This hides the navigation bar on the project page but it also removes the bar on any subsequent pages! They need to be programmmatically reset in ViewDidLoad.
[project_NavigationController setNavigationBarHidden:YES animated:YES];
project_ViewController.currentGeoModel = newGeoModel;
Now, my Project_ViewController is embedded in a NavigationController and has 4 child ViweControllers. One is named FileIO_ViewController. I want to pass the currentModel from the Project_ViewController to the FileIO_ViewController. Under the - (void)viewDidLoad method I have tried a number of things, which do not work. For example,
UINavigationController *project_NavigationController = (UINavigationController *) self.presentedViewController;
FileIO_ViewController *fileIO_ViewController = [[project_NavigationController viewControllers] objectAtIndex:1];
fileIO_ViewController.currentModel = currentModel;
compiles but when I try to access currentModel inside the FileIO_ViewController methods, it is empty.
If anyone can take the time to help I would be very appreciative. The best answer for me would be in the form of an explicit code example showing how to pass the pointer to an object like my Model from a ViewController to another ViewController where you do not explicitly know where in the stack the child VC lies. (In my example I used Index 1 but I do not actually know at which Index the FileIO_ViewController lives as I have three other ViewControllers under the Project_ViewController. I've tried several integers with no success.)
If you do answer this, please consider me a New Guy when it comes to iOS 6 and objective C -- climbing Mount Apple has been a long haul and I isn't anywhere near the top yet!
Thanks
Tim Redfield
Norway

Answer 1

If you have a shared single model for your app, you shouldn't proactively pass the pointer around, you should make the model available from a single location and leave it to individual objects to access this same model when they need to. A good location for your model pointer is in your Application delegate.
In the appDelegate's .h file, declare a property for your model:
//appDelegate.h
#property (nonatomic, strong) MyAppModel* appModel;
After you instantiate your model in the appDelegate, just assign it to the property:
//appDelegate.m
- (BOOL)application:(UIApplication *)application
didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
self.appModel = [[MyAppModel alloc] init];
//set up bare-bones appModel here
return YES;
}
Then you can access this property from any viewController that needs model data thus:
#import appDelegate.h;
AppDelegate* appDelegate = [UIApplication sharedApplication] delegate;
MyModel* model = appDelegate.model
(Better still, make the model object into it's own Singleton object, but using the appDelegate will get you started)
If you need to pass models around, then you need to take care that you are passing them to the right object. This is where your existing code is breaking. For example you are making many assumptions about the structure of a NavigationController stack. But whenever you move back down a stack by popping a controller off the top, you lose that top controller instance completely. When you 'return' to that controller by going forwards on the stack, in fact a new instance is created (unless you have taken care to keep a pointer hanging around and make sure to push to that pointer). If you need more help on this aspect, you will need to describe exactly the layout of your app, and how you are creating your viewControllers, navigationController stack, etc.
update (following your comments)
If your changes to the model are valid throughout the app, then I don't see why you can't just access the model when you need to from wherever you happen to be in the app via appDelegate.model. If you have concurrent versions of the model, you could look at making a singleton data manager object which could handle the details of storing an array of models (or a model history) and providing the correct model as per request. The principle is the same - rather than proactively passing model objects into your viewControllers, let them request data from a centralised source as they need it.
Regarding your description "Now, my Project_ViewController is embedded in a NavigationController and has 4 child ViewControllers.", I don't think you have quite grasped the distinction between Classes, Storyboard scenes, and instances.
You have this arrangement in a part of your storyboard:
UINavigationController
| push push push
|->UIViewController1 -----> UIViewController2 -----> UIViewController3 ----->
segue segue segue
You talk about passing data directly from VC1 (say) to VC3 by accessing the NavController's stack.
What you need to consider is that the storyboard describes a template showing how instances will interrelate when they are instantiated. They are not instantiated just because the storyboard is loaded. When you have arrived at VC1, the ability to segue to VC2 and VC3 is laid out before you in the template, but neither VC2 nor VC3 - as instances - exist until you initiate the segue. The segue process instantiates it's destinationViewController Therefore it makes no sense to pass data from VC1 directly into VC3. When you are at VC1, the navController's stack only contains one item (a VC1 instance); when you segue to VC2, it then contains instances of VC1 and VC2, and it is only when you actually segue to VC3 that the instance is created and placed in the stack.
Stepping through your code:
UINavigationController *project_NavigationController =
(UINavigationController *) self.presentedViewController;
the presentedViewController property works with modal segues, where you have a presenting, and a presented, view Controller. NavControllers on the other hand work with push segues (as they push child viewControllers onto their viewControllers stack, which is where you can obtain pointers to them from).
In this context, self.presentedViewController is nil, which you are assigning to a new variable. The code does nothing, but the compiler won't complain.
FileIO_ViewController *fileIO_ViewController =
[[project_NavigationController viewControllers] objectAtIndex:1];
project_NavigationControlle is nil, so it's properties are all nil. fileIO_ViewController gets nil.
Likewise in the last line you are sending a message to nil, which is not an error, but faintly redundant.