How can I pop a view from a UINavigationController and replace it with another in one operation?

Question

I have an application where I need to remove one view from the stack of a UINavigationController and replace it with another. The situation is that the first view creates an editable item and then replaces itself with an editor for the item. When I do the obvious solution within the first view:

MyEditViewController *mevc = [[MYEditViewController alloc] initWithGizmo: gizmo];

[self retain];
[self.navigationController popViewControllerAnimated: NO];
[self.navigationController pushViewController: mevc animated: YES];
[self release];

I get very strange behavior. Usually the editor view will appear, but if I try to use the back button on the nav bar I get extra screens, some blank, and some just screwed up. The title becomes random too. It is like the nav stack is completely hosed.

What would be a better approach to this problem?

Thanks, Matt

Answer 1

This UINavigationController instance method might work...

Pops view controllers until the specified view controller is the top view controller and then updates the display.

• (NSArray *)popToViewController:(UIViewController *)viewController animated:(BOOL)animated

Answer 2

From experience, you're going to have to fiddle with the UINavigationController's viewControllers property directly. Something like this should work:

MyEditViewController *mevc = [[MYEditViewController alloc] initWithGizmo: gizmo];

[[self retain] autorelease];
NSMutableArray *controllers = [[self.navigationController.viewControllers mutableCopy] autorelease];
[controllers removeLastObject];
self.navigationController.viewControllers = controllers;
[self.navigationController pushViewController:mevc animated: YES];

Note: I changed the retain/release to a retain/autorelease as that's just generally more robust - if an exception occurs between the retain/release you'll leak self, but autorelease takes care of that.

Answer 3

After much effort (and tweaking the code from Kevin), I finally figured out how to do this in the view controller that is being popped from the stack. The problem that I was having was that self.navigationController was returning nil after I removed the last object from the controllers array. I think it was due to this line in the documentation for UIViewController on the instance method navigationController "Only returns a navigation controller if the view controller is in its stack."

I think that once the current view controller is removed from the stack, its navigationController method will return nil.

Here is the adjusted code that works:

UINavigationController *navController = self.navigationController;
MyEditViewController *mevc = [[MYEditViewController alloc] initWithGizmo: gizmo];

NSMutableArray *controllers = [[self.navigationController.viewControllers mutableCopy] autorelease];
[controllers removeLastObject];
navController.viewControllers = controllers;
[navController pushViewController:mevc animated: YES];

Answer 4

I've discovered you don't need to manually mess with the viewControllers property at all. Basically there are 2 tricky things about this.

1. self.navigationController will return nil if self is not currently on the navigation controller's stack. So save it to a local variable before you lose access to it.
2. You must retain (and properly release) self or the object who owns the method you are in will be deallocated, causing strangeness.

Once you do that prep, then just pop and push as normal. This code will instantly replace the top controller with another.

// locally store the navigation controller since
// self.navigationController will be nil once we are popped
UINavigationController *navController = self.navigationController;

// retain ourselves so that the controller will still exist once it's popped off
[[self retain] autorelease];

// Pop this controller and replace with another
[navController popViewControllerAnimated:NO];
[navController pushViewController:someViewController animated:NO];

In that last line if you change the animated to YES, then the new screen will actually animate in and the controller you just popped will animate out. Looks pretty nice!

Answer 5

Thanks, this was exactly what I needed. I also put this in an animation to get the page curl:

        MyEditViewController *mevc = [[MYEditViewController alloc] initWithGizmo: gizmo];

    UINavigationController *navController = self.navigationController;      
    [[self retain] autorelease];

    [UIView beginAnimations:nil context:NULL]; [UIView setAnimationDuration: 0.7];
    [UIView setAnimationTransition:<#UIViewAnimationTransitionCurlDown#> forView:navController.view cache:NO];

    [navController popViewControllerAnimated:NO];
    [navController pushViewController:mevc animated:NO];

    [UIView commitAnimations];

0.6 duration is fast, good for 3GS and newer, 0.8 is still a bit too fast for 3G..

Johan