UINavigationController - ask for confirmation on Back

Didn't expect this to be such a problem, but once you tap into the UINavigationController things become hairy.

Requirement:

When user leaves a screen by tapping on a "back" in navigation bar and there are changed data in the screen I should ask for a confirmation and keep user at the current UIViewController if she decided to continue editing the data.

Solution.

You may run into this: http://stackoverflow.com/questions/1214965/setting-action-for-back-button-in-navigation-controller/19132881#19132881 (particularly this: https://github.com/onegray/UIViewController-BackButtonHandler).

Once, I was trying to solve the keyboard accessory to be shown each time for each UITextField on shouldBeginEditing by writing a category for a UITextField. And here is something I learned in a hard way:

When you plan or see any category re-writing the existing framework method, STOP! Simple as this and go read on what can turn wrong with this.

The solution mentioned above use this:

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@implementation UINavigationController (ShouldPopOnBackButton) - (BOOL)navigationBar:(UINavigationBar *)navigationBar shouldPopItem:(UINavigationItem *)item { if([self.viewControllers count] < [navigationBar.items count]) {

No go for me, no need to study, excuse for being abrupt :). But one part of code from this solution turned actually to be useful.

One of the comments on another post: http://stackoverflow.com/questions/20327165/popviewcontroller-strange-behaviour got me here: http://blog.macca.tech/2013/11/ios-prevent-back-button-navigating-to.html

And was not I lucky? It really makes sense, no private APIs, framework's UIViewController gets the chance to do its stuff always. What I wanted to improve though was that "safeDelegate" property and the way it is established. So I added a new method (into UISafeNavigationController.m):

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-(id<UISafeNavigationDelegate>) popDelegate { UIViewController *topController = [self topViewController]; if ([topController conformsToProtocol:@protocol(UISafeNavigationDelegate)]) { return (id<UISafeNavigationDelegate>)topController; } return nil; }

And then you can just substitute safeDelegate with popDelegate:

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- (UIViewController *)popViewControllerAnimated:(BOOL)animated { if (self.popDelegate && ![self.popDelegate navigationController:self shouldPopViewController:[self.viewControllers lastObject] pop:^{ [super popViewControllerAnimated:animated]; }]) { if (self.navigationBar) { [self restoreViewsForNavigationBar:self.navigationBar]; } return nil; } return [super popViewControllerAnimated:animated]; }

Also note lines 7-9 where I call a new method (borrowed from the first stackoverflow solution that I actually criticize :)):

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-(void) restoreViewsForNavigationBar: (UINavigationBar *) navigationBar { for(UIView *subview in [navigationBar subviews]) { if(0. < subview.alpha && subview.alpha < 1.) { [UIView animateWithDuration:.25 animations:^{ subview.alpha = 1.; }]; } } }

This is to avoid the back arrow in the navigation bar looking as disabled when answer from our controller to the shouldPop is NO.

Then protocol method in the related view controller may look like:

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- (BOOL)navigationController:(UINavigationController *)navigationController shouldPopViewController:(UIViewController *)controller pop:(void(^)())pop { if (!_item.id) { UIAlertController *alert = [UIAlertController alertControllerWithTitle:LSSTRING(@"Save the item?") message:LSSTRING(@"You are closing this screen by using Back button and have not saved the item.") preferredStyle:UIAlertControllerStyleAlert]; [alert addAction:[UIAlertAction actionWithTitle:LSSTRING(@"Save and close") style:UIAlertActionStyleDefault handler:^(UIAlertAction *action) { [self done:nil]; }]]; [alert addAction:[UIAlertAction actionWithTitle:LSSTRING(@"Don't save and close") style:UIAlertActionStyleDestructive handler:^(UIAlertAction *action) { [self doCancellationCleanup]; if (pop) { pop(); } }]]; [alert addAction:[UIAlertAction actionWithTitle:LSSTRING(@"Cancel") style:UIAlertActionStyleCancel handler:^(UIAlertAction *action) { }]]; [alert show]; return false; } return true; }

Hope this can help someone! All the credit goes to Hong Kong Web Entrepreneur guy! Once again, here: http://blog.macca.tech/2013/11/ios-prevent-back-button-navigating-to.html

[UPDATE] In the end I had to rework the final part presented here - showing the confirmation as the way it is presented was not releasing the controller correctly and was not cleaning up the views as well. Quite bigger effort is required to get it right and it is not that generic in the end. But this is a good start! :).