How to set action to the backButtonItem on the navigation bar?

How to set action to the backButtonItem on the navigation bar? I have a navigation bar, when I'm pressing the back button, I need to alert some message to the user, and only after user's reaction - return to the previous view. How can I do it? Thanx!

- (void)viewDidLoad 
{
    [super viewDidLoad];

    //no one field don't changed yet
    isDirty = FALSE;

    //edited user
    //set default values
    newData = [data copy];

    //setting navigation controller rigth button
    UIBarButtonItem *rightButton = [[UIBarButtonItem alloc] initWithTitle:@"Save"
                                                                style:UIBarButtonSystemItemDone 
                                                                   target: self 
                                                                   action: @selector(saveBtnUserClick)];
    self.navigationItem.rightBarButtonItem = rightButton; 
    [rightButton release];


    UIBarButtonItem *leftButton = [[UIBarButtonItem alloc] initWithTitle:@"Back"
                                                                   style:UIBarButtonSystemItemDone 
                                                                  target: self 
                                                                  action: @selector(backBtnUserClick)];

    self.navigationItem.backBarButtonItem = leftButton;
    [leftButton release];
}

//and my method for reaction

-(IBAction) backBtnUserClick
{
    NSLog(@"\n Back pressed");

    //back to previous view
    [self.navigationController popViewControllerAnimated: TRUE];
}

This sounds like a job for UIAlertView. Instead of calling popViewControllerAnimated: in your IBAction methods, alloc/init a UIAlertView and present it. Then, when the user taps a button on the UIAlertView, dismiss the UIAlertView and call popViewControllerAnimated:.

- (IBAction)backBtnUserClicked:(id)object {
    UIAlertView *av = [[[UIAlertView alloc] initWithMessage:@"Wait!"
          delegate:self
               cancelButtonTitle:@"Ok"
               otherButtonTitles:nil] autorelease];
   [av show];
}

In your UIAlertViewDelegate methods call popViewControllerAnimated:.

- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex {
    [[self navigationController] popViewControllerAnimated:YES];
}

To set the action on the back button:

[[[self navigationController] leftBarButtonItem] setTarget:self];
[[[self navigationController] leftBarButtonItem] setAction:@selector(backBtnUserClicked:)];