Pushing a new VC into the navigationController seems to have no effect - ios

Question

This problem sounds quite basic but I don’t understand what I am overlooking.
I am trying to push a new view controller into a navigation controller, however the topViewController remains unaffected.
#import "TNPViewController.h"
#interface TNCViewController : UIViewController <UICollectionViewDataSource, UICollectionViewDelegateFlowLayout>
#implementation TNCViewController
-(void)userDidSelectNewsNotification:(NSNotification*)note
{
TNPViewController *nextViewController = [[TNPViewController alloc] init];
[[self navigationController] pushViewController:nextViewController animated:YES];
UIViewController *test = [[self navigationController] topViewController];
}
The test shows an instance of TNCViewController instead of TNPViewController. How is this possible?
UPDATE
Thanks for everyone's participation. The method name indicating notifications is a red herring. I found the problem, as Stuart had mentioned previously but deleted later on. (As I have high reputation score, I still can see his deleted post).
My initial unit test was this:
-(void)testSelectingNewsPushesNewViewController
{
[viewController userDidSelectNewsNotification:nil];
UIViewController *currentTopVC = navController.topViewController;
XCTAssertFalse([currentTopVC isEqual:viewController], #"New viewcontroller should be pushed onto the stack.");
XCTAssertTrue([currentTopVC isKindOfClass:[TNPViewController class]], #"New vc should be a TNPViewController");
}
And it failed. Then I set a breakpoint and tried the test instance above and it still was showing the wrong topviewcontroller.
At least the unit test works if I change
[[self navigationController] pushViewController:nextViewController animated:YES];
to
[[self navigationController] pushViewController:nextViewController animated:NO];
A better solution is to use an ANIMATED constant for unit tests to disable the animations.

Answer 1

This doesn't really answer your question about why your navigationController is not pushing your VC. But it is a suggestion about another possible approach.
You could instead add a new VC on the Storyboard and simply activate the segue when the userDidSelectNewsNotification method is activated. Then change the information accordingly to the event in the VC, specially since you are initializing it every time anyway.

Answer 2

This is something of a stab in the dark, but the issue is hard to diagnose without more information.
I see you're trying to push the new view controller in response to a notification. Are you sure this notification is being handled on the main thread? UI methods such as pushing new view controllers will fail (or at least behave unpredictably) when not performed on the main thread. This may also go some way to explaining the odd behaviour of topViewController returning an unexpected view controller instance.*
Ideally, you should guarantee these notifications are posted on the main thread, so they will be received on that same thread. If you cannot guarantee this (for example if you're not responsible for posting the notifications elsewhere in your code), then you should dispatch any UI-related code to the main thread:
- (void)userDidSelectNewsNotification:(NSNotification *)note
{
dispatch_async(dispatch_get_main_queue(), ^{
TNPViewController *nextViewController = [[TNPViewController alloc] initWithNibName:#"TNPViewController" bundle:nil];
[self.navigationController pushViewController:nextViewController animated:YES];
});
}
Also, it appears you are not initialising TNPViewController using the designated initialiser (unless in your subclass you are overriding init and calling through to initWithNibName:bundle: from there?). I wouldn't expect this to cause the transition to fail entirely, but may result in your view controller not being properly initialised.
In general, you might be better creating your view controllers in a storyboard and using segues to perform your navigation transitions, as #Joze suggests in his answer. You can still initiate these storyboard segues in code (e.g. in response to your notification) with performSegueWithIdentifier:, but again, be sure to do so on the main thread. See Using View Controllers in Your App for more details on this approach.
*I originally wrote an answer trying to explain the unexpected topViewController value as being a result of deferred animated transitions. While it is true that animated transitions are deferred, this does not prevent topViewController from being set to the new view controller immediately.