Why a separate instance of ViewController Class is affecting previous instance?

Question

UPDATE - Cause found!... please read below & suggest the solution:

While creating a video to show this issue, I have found why does that happen...

Any Control/Element that is defined between #imports & @implementation DetailViewController in .m file is lost by the original detVC when a new instance is created of VC.

Example: I am including a video that re-creates this issue. As shown, all controls work except a UISegmentedControl & a UILabel.. both of which are defined in DetailViewController.m as:

#import "DetailViewController.h"

UISegmentedControl *sortcontrol;
UILabel *incrementLabel;

@implementation DetailViewController

Video Link - http://www.youtube.com/watch?v=2ABdK0LkGiA

I think we are pretty close.. Waiting for the answer!!

P.S.: I think this info is enough can lead us to the solution, but if needed I can share the code too.

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EARLIER:

There's a detailViewController (detVC) in our app which is normally 'pushed' from a UITableViewController.

Now we are also implementing Push Notifications which would 'modally' present the same detVC whenever the notification arrives, which in turn can happen over any view controller.

It works all fine until the detVC through notification is presented while another instance of detVC was already the active view controller (pushed earlier through TableView).

The problem happens when the presented detVC is dismissed - the pushed detVC 'looses' control of about everything - It is not pointing to the same data as earlier & even the UISegmentedControl on Navigation Toolbar points to -1 index on pressing any index!

Why does such thing happen when we are creating a separate instance of detVC? Why does it affect the earlier detVC? How can it be resolved / What about Objective C needs to be learned here? (FYI, we are using ARC in the project)

Thanks

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EDIT - Some code:

When Pushed:

ProductDetailViewController *detailViewController = [[ProductDetailViewController alloc] init];    
detailViewController.hidesBottomBarWhenPushed = YES;  
[self.navigationController pushViewController:detailViewController animated:YES];
detailViewController.serverOffset=serverOffset;
detailViewController.prdctIndex = indexPath.row;

When presented through Notification:

- (void)displaySingleProduct:(NSDictionary*)userInfo{
    ProductDetailViewController *onePrdVC = [[ProductDetailViewController alloc] init];    
    UINavigationController *addNav = [[UINavigationController alloc] initWithRootViewController:onePrdVC];
    onePrdVC.justOne=YES;
    onePrdVC.onePrdIDtoLoad=[[[userInfo valueForKey:@"prd"] valueForKey:@"id"] intValue];
    [self.navigationController presentModalViewController:addNav animated:YES];
}

There is a Product class in detVC which gets the data from the values stored at the prdctIndex row in the SQLite table.

After dismissing the presented detVC through notification, the Product class of the pushed detVC starts pointing to the row which was used for Product class in presented detVC. And there is no such code in viewDidAppear which would alter Product class.

So, this, UISegmentedControl issue mentioned above & some other problems are causing the pain! Basically, the whole detVC acts weird - which re-works if we just pop & re-push it!

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EDIT 2

I have more info on it which could lead to the cause. If I run viewDidLoad on viewDidAppear like this:

if (!justOne) {
    aProduct = [allProducts getProduct:(prdctIndex+1) one:NO];
    [self viewDidLoad];
    [self populateDetails];
}

the pushed detVC regains the control & every element/control starts re-working as expected (but not in the ideal way). So, it is quite clear that the separate instance of presented detVC does messes up the earlier pushed detVC & a re-setting up of all the controls / pointers is required through viewDidLoad.

Can any helpful conclusion can be derived from this info?

Answer 1

When in your code you write:

#import "DetailViewController.h"

UISegmentedControl *sortcontrol;
UILabel *incrementLabel;

@implementation DetailViewController

You are not defining these variables as instance variables (ivars), so they are not individual for each instance. There are three ways to define ivars. 1) The traditional way, in your .h file.

@interface DetailViewController{
    UISegmentedControl *sortcontrol;
    UILabel *incrementLabel; 
}

2) Some additions to objective-C have added support for the next two ways. Like declaring them in your .m file.

@implementation DetailViewController{
    UISegmentedControl *sortcontrol;
    UILabel *incrementLabel;
}

3) If these ivars use properties to expose them, then you can simply leave out the explicit definition of them. So in .h:

@interface DetailViewController
@property (strong, nonatomic) IBOutlet UISegmentedControl *sortcontrol;
@property (strong, nonatomic) IBOutlet UILabel *incrementLabel;

and then in the .m file:

@implementation DetailViewController
@synthesize sortcontrol;
@synthesize incrementLabel;

Answer 2

If two instances of the same class are interfering with each other, that strongly suggests that you are not storing all your state in instance variables. I would suspect that something in ProductDetailViewController stores its state in global or class data, and that your problem lives in there.

It's possible that you have done something silly like making ProductDetailViewController a forced singleton (overriding +alloc, which you should almost never do), but generally people remember when they've done that.

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Never call viewDidLoad directly. That's only for the system to call. (I know you were just testing, but don't leave that in.)

But are you calling [super viewDidAppear:animated] in your viewDidAppear:? You have to call your superclass in that method.

Answer 3

It actually depends on how your apps architecture has been created. If you are using TabBarController based application, here is the proper way to do so.

Answer 4

Are you loading the view controller from a NIB? If so, it may be that the same instance of the view controller is used each time. You can log the address of your view controller in viewDidLoad and see if this is the case.