UINavigationController is nil after pop

Question

My project is complex (a bit too much probably) but the question is simple, here's the code

...
self.vc = [[ClassName alloc] init];
if ([[self.navigationController.viewControllers lastObject] isKindOfClass:[ClassName class]]) {
    [self backAndGo:self.vc title:@"Title"];
}
else {
    [self vai:self.vc title:@"title"];
}
...

in this code i've to go directly in the next view if the current view is different than ClassName, otherwise go back of 1 view and the go to the next.

- (void)backAndGo:(id)view title:(NSString *)title

- (void)backAndGo:(id)view title:(NSString *)title
{
    NSLog(@"before %@,%d",[self.navigationController viewControllers],[[self.navigationController viewControllers] count]);

    [self.navigationController popViewControllerAnimated:NO];

    ALCParentViewController *viewController = (ALCParentViewController *)view;
    [self.navigationController pushViewController:viewController animated:YES];

    NSLog(@"after %@,%d",[self.navigationController viewControllers],[[self.navigationController viewControllers] count]);
}

ALCParentViewController is a parent class of vc, in this method the first log of navigatin controller is correct with all the stack of viewcontrollers, and the popViewController is executed but in the second log navigationController is null with 0 element obviously, and the pushViewController method is not executed, why? any ideas?

- (void)vai:(id)view title:(NSString *)title

- (void)vai:(id)view title:(NSString *)title
{       
    ALCParentViewController *viewController = (ALCParentViewController *)view;
    [self.navigationController pushViewController:viewController animated:YES];
}

This is instead the other method to reach directly the second view, and it works correctly, another courius thing (almost for me) is that dispite the nil pushviewcontroller, navigation (push and pop) in other view works correctly...

EDIT 1:

As lucaslt89 said i've put a breakpoint in the second log and make "po self.navigationController" in the consolle, here the result

(lldb) po self.navigationController
$0 = 0x00000000 <nil>
(lldb)

it's nil, but i can see that in the log without breakpoint...

EDIT 2:

(lldb) po self.navigationController
$0 = 0x099787d0 <UINavigationController: 0x99787d0>
(lldb) po auxNavController
$1 = 0x099787d0 <UINavigationController: 0x99787d0>
(lldb)

after operations suggested by lucaslt89, the two address are the same

Answer 1

According to your last edit, one solution is to hold a reference to the navigation controller. Your backAndGo method should be like that

- (void)backAndGo:(id)view title:(NSString *)title
{
    NSLog(@"before %@,%d",[self.navigationController viewControllers],[[self.navigationController viewControllers] count]);

    UINavigationController *auxNavController = self.navigationController;

    [self.navigationController popViewControllerAnimated:NO];

    ALCParentViewController *viewController = (ALCParentViewController *)view;
    [auxNavController pushViewController:viewController animated:YES];

    NSLog(@"after %@,%d",[auxNavController viewControllers],[[auxNavController viewControllers] count]);
}

If you debug that, the address of self.navigationController prior to the pop should be the same as the auxNavController after the pop. Try that and tell us your results!

Answer 2

Ok, even when the question is closed, i'd like to make an explanation for future visitors. According to the documentation: ...This property is nil if the view controller is not embedded inside a navigation controller.

That's exactly what you do with the pop instruction. So, basically, before loosing a reference to the navigation controller because of the pop, make sure to store a reference to it.

Answer 3

On your backAndGo method: First you pop a viewController, and then you push another ViewController.

The promblem is, after you pop a viewController, then the "self" is nil. So if you get the navigationController by using "self.navigationController" it will return nothing.

The reason is beacause "self" is nil, instead of the navigationController is empty. That is why the answer of lucastl89 is effective. He hold a navigationController object by a reference, not search it by using "self".