Question

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Objective C: what is a “(id) sender”?

I have some questions about the following:

- (IBAction)leftButtonPressed:(id)sender 
{
     UIButton *button = (UIButton*)sender;
     _label.text = button.titleLabel.text;
}

What exactly does (UIButton)*sender do? I mean especially the (UIButton*) with the *. Is it something similar to UIButton *myButton ?

As far as I understand is it some kind of pointer to the button which was pressed, but why is that? And why couldn’t I just write

_label.text = sender.titleLabel.text;

since it “is” the button? And how do I know what can be send anyway? I mean could I write something like:

-(void)leftButtonPressed:(color)sender {...}

in order to pass the color of the button?

Answer 1

(UIButton *)sender this is typecasting your id sender to UIButton.

Yes you can write -(void)leftButtonPressed:(color)sender {...} But color should be a valid Class Type and it should be pointer like -(void)leftButtonPressed:(NSColor *)sender {...}