Question

I am accessing a dispatched notification like so:

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(handleUnpresent:) name:UNPRESENT_VIEW object:nil];

...

-(void)handleUnpresent:(NSNotification *)note;
{
    NSLog(@"%@", note.object.footer);
    //property 'footer' not found on object of type 'id'
}

Some of the incoming note.object objects have a “footer” and some don’t. However, I don’t want to go through to trouble of making a class that only has a property called footer just to make this work. I even tried ((NSObject *)note.object).footer) which works in some languages, but apparently not obj-c. What can I do?

Answer 1

Checking the isKindOfClass is certainly the more robust option. However, if you have multiple unrelated classes that return the property you need, there is another way: respondsToSelector. Just ask if the object has a footer method, and you can safely call it.

-(void)handleUnpresent:(NSNotification *)note;
{
    id noteObject = [note object];
    if ([note respondsToSelector:@selector(footer)])
    {
        NSLog(@"Footer = %@", [noteObject footer]);
    }
}

That respondsToSelector method is powerful and handy in the right places, but don’t go wild with it. Also, it can’t tell you anything about the return type, so the footer you get may not be of the class you were expecting.

The syntax for noteObject.footer and [noteObject footer] are easy to treat as equivalent. However, when the class of noteObject is unknown, the compiler will accept the latter but not the former. If noteObject has a defined class that doesn’t usually respond to footer, it will give a warning, but still compile and run. In these cases, it is your responsibility to guarantee that the method will indeed exist when needed, and therefore that the method call won’t crash at run time.