How to call a method when back button on a UINavigationController is pressed? (iPhone)

Question

How can I call a method when the back button on a UINavigationController is pressed. I need to be able to call a method before the previous view is displayed. Is this possible? If so please can someone advise on how to do this?

Answer 1

A fast solution is to add an implementation for the viewWillDisappear: method. It will be triggered as soon as the viewController will disappear in response to the back button pression.

- (void)viewWillDisappear:(BOOL)animated {
  //... 
  //make you stuff here
  //... 
}

Another solution is to add a custom responder to the back button. You can modify your viewController's init method as follow:

- (id)init {
    if (self = [super init]) {
        //other your stuff goes here
        //... 
        //here we customize the target and action for the backBarButtonItem
        //every navigationController has one of this item automatically configured to pop back
        self.navigationItem.backBarButtonItem.target = self;
        self.navigationItem.backBarButtonItem.action = @selector(backButtonDidPressed:);
    }
    return self;
}

and then you can use a selector method as the following. Be sure to dismiss correctly the viewController, otherwise your navigation controller won't pop as wanted.

- (void)backButtonDidPressed:(id)aResponder {
    //do your stuff
    //but don't forget to dismiss the viewcontroller
    [self.navigationController popViewControllerAnimated:TRUE];
}

Answer 2

UIButton *backButton = [[UIButton alloc] initWithFrame:CGRectMake(0, 0, 52, 31)];
[backButton setBackgroundImage:[UIImage imageNamed:@"BackButton.png"] forState:UIControlStateNormal];
//[backButton setTitle:@"CLOSE" forState:UIControlStateNormal];
[backButton setTitleColor:[UIColor whiteColor] forState:UIControlStateNormal];
[backButton.titleLabel setFont:[UIFont boldSystemFontOfSize:14.0f]];
[backButton addTarget:self action:@selector(tappedBackButton:) forControlEvents:UIControlStateHighlighted];

UIBarButtonItem* item = [[UIBarButtonItem alloc] initWithCustomView:backButton];

self.navigationItem.leftBarButtonItem = item;
[item release];

Create a custom Back Button and using that selector call a method that you want. And also use [self.navigationController popViewControllerAnimated:TRUE];

- (void)tappedBackButton:(id)button
{
    // call your method here
    [self.navigationController popViewControllerAnimated:TRUE];
} 

Answer 3

I think I found a "clean" way to do that :

First, set your view controller as complying with the UINavigationControllerDelegate protocol

@interface MyViewController () <UINavigationControllerDelegate>

Then, define it as the delegate

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view.

    // Set this View Controller as the navigation controller delegate
    self.navigationController.delegate = self;

}

Last, use this method from the UINavigationControllerDelegate protocol :

#pragma mark - Navigation controller delegate
- (void)willMoveToParentViewController:(UIViewController *)parent
{
    // If there is no parent, then it means that the view controller has been removed from the stack, which happens when the back button has been pressed
    if (!parent) {
        NSLog(@"Back button pressed");

        // it can be useful to store this into a BOOL property
        self.backButtonPressed = YES;
    }
}

Answer 4

The easiest way would be, to put the code in the ViewController that will be presented when the back button is pressed. You can use viewWillAppear

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];
     //Your code here
}

Note that this will also be executed when the view is presented for any other reason, so if you want it to happen only when the back button is pressed you have to use the delegate: UINavigationControllerDelegate