Question

While using a UIActionSheet in an iphone app, the typical methods of matching actions to buttons seem very fragile and aesthetically unpleasant. Perhaps its due to my minimal C/C++ background (more Perl, Java, Lisp and others). Matching on button indexes just seems like too many magic numbers and too disconnected to avoid simple logical or consistency errors.

For instance,

UIActionSheet *sources = [[UIActionSheet alloc]
         initWithTitle:@"Social Networks"
              delegate:self 
     cancelButtonTitle:@"Cancel" 
destructiveButtonTitle:nil 
     otherButtonTitles:@"Twitter", @"Facebook", @"Myspace", @"LinkedIn", @"BlahBlah", nil
];

<snip>

-(void)actionSheet:(UIActionSheet *)actionSheet didDismissWithButtonIndex:(NSInteger)buttonIndex {
    if (buttonIndex == [actionSheet cancelButtonIndex]) {
        // all done
    } else if (buttonIndex == 0) {
        // Twitter
    } else if (buttonIndex == 1) {
        // Facebook
    } else if (buttonIndex == 2) {
        // LinkedIn
    } else if (buttonIndex == 3) {
        // Myspace
    }
}

Notice there’s at least two errors in the action handling code (according to the comments at least).

What I’m missing is the correct design pattern for avoiding that disconnect in Objective-C. If this were perl, I would first build an array of my button options and then probably create a quick lookup table hash that would correspond to another lookup table of objects or subroutines that did the appropriate thing for each item. In java, the original list would probably be objects in the first place with callbacks. I know I could build a Dictionary to mimic the perl hash but that feels very unwieldy and cumbersome for 3-4 options. I’ve also considered using a enum to mask the magic-ness of indexes but that’s only a minor part of the problem.

The real problem seems to be that there’s no (simple?) way to specify BOTH the list of button strings and corresponding actions in one place thereby eliminating the need to modify code in two places when adding/removing/reordering options and thus making it effectively impossible to make the kinds of mistakes that my sample code makes.

I’m not trying to start a programming language holy war, I just want to figure what the correct design pattern in this scenario (and I believe many others in Objective C) for connecting the list of button strings to the list of actions.

Answer 1

Building on Aaron’s selector suggestion, I really like the idea now of doing a simple ad-hoc dispatch method. It successfully avoids the possibility of handling the wrong option as well as providing a clean factorization of concerns. Of course, I could imagine a use case where you want to do something else first for each option, such as instantiating an object and passing it the option string much like Toro’s answer.

Here’s a simple dispatch that calls methods such as ‘actionTwitter’:

-(void)actionSheet:(UIActionSheet *)actionSheet didDismissWithButtonIndex:(NSInteger)buttonIndex {  
    if (buttonIndex == [actionSheet cancelButtonIndex]) {
        return;
    }

    NSString *methodName = [@"action" stringByAppendingString:[actionSheet buttonTitleAtIndex:buttonIndex]];
    SEL actionMethod = NSSelectorFromString(methodName);
    if ([self respondsToSelector:actionMethod]) {
        [self performSelector:actionMethod];
    } else {
        NSLog(@"Not yet implemented")
    }
}