From what I have learned so far: In Objective-C you can send any message to any object. If the object does implement the right method it will be executed otherwise nothing will happen. This is because before the message is send Objective-C will perform respondsToSelector.

I hope I am right so far.

I did a little program for testing where an action is invoked every time a slider is moved. Also for testing I set the sender to NSButton but in fact it is an NSSlider. Now I asked the object if it will respond to setAlternateTitle. While a NSButton will do and NSSlider will not. If I run the code and do respondsToSelector myself it will tell me the object will not respond to that selector. If I test something else like intValue, it will respond. So my code is fine so far.

- (IBAction)sliderDidMove:(id)sender
{
    NSButton *slider = sender;

    BOOL responds =
    [slider respondsToSelector:@selector(setAlternateTitle)];

    if(responds == YES)
    {
        NSLog(@"YES");        
    }
    else
    {
        NSLog(@"NO");
    }

    [slider setAlternateTitle:@"Hello World"];
}

But when I actually send the setAlternateTitle message the program will crash and I am not exactly sure why. Shouldn't it do a respondsToSelector before sending the message?