Question

How do I debug an iPhone aplication? How can I find out what’s going on in the simulator? I’m a beginner in Xcode development and don’t know what’s the problem with the code below. The app is crashing on button click.

- (void)viewDidLoad {

    myLabel = [[UILabel alloc]init];

    [myLabel setText:@"Labela"];
    myLabel.frame = CGRectMake(50.0,50.0, 100.0, 30.0);
    [self.view addSubview:myLabel];

    myButton = [UIButton buttonWithType:UIButtonTypeRoundedRect];

    [myButton addTarget:self 
                 action:@selector(buttonAction:)
       forControlEvents:UIControlEventTouchDown];

    [myButton setTitle:@"Klikni" forState:UIControlStateNormal];
    [myButton setFrame:CGRectMake(80.0, 210.0, 160.0, 40.0)];

    [self.view addSubview:myButton];
    [super viewDidLoad];

}

- (void)buttonAction {
    [myLabel setText:@"Promijenjen!"];
}

Answer 1

action:@selector(buttonAction:)

Here you specify that buttonAction selector gets 1 parameter, but it is declared to have none:

- (void)buttonAction{
...

So when button click system tries to call undefined method and that results in crash. To fix that you should either change selector name to

action:@selector(buttonAction)

or change action method declaration:

- (void)buttonAction:(id)sender{
    ...