Question

Have mercy, newbie here.

I have a NSString value that I construct on the fly, which is the name of a UILabel instance. I want to send the label a message to update its text. But, the two data types don’t match. Here’s enough code (I think):

In header file:

IBOutlet UILabel *Clue1; // IBOutlet and IBAction are IDE flags
IBOutlet UILabel *Clue2; // IB = interface builder
IBOutlet UILabel *Clue3;

In implementation file:

- (IBAction) newPuzzle:(id)sender { // Clear all fields & get new clue

    [Clue1 setText:@""]; // Clear the fields
    [Clue2 setText:@""]; 
    [Clue3 setText:@""]; 

    // Send up a randomly chosen new clue

    NSArray *clues = [NSArray arrayWithObjects:@"222", @"333", nil];
    NSInteger randomIndex = arc4random()%[clues count];
    NSString *aClue = [clues objectAtIndex:randomIndex];

    // The clue will be split into component digits and each piece sent to a different label

    for (NSInteger charIdx = 0; charIdx < aClue.length; charIdx++) {
        NSString *cluePos = [NSString stringWithFormat:@"Clue%d", charIdx + 1];
        NSLog(@"%@", cluePos); // works
        [cluePos setText:@"test"]; // Xcode notes the type mismatch
    }
}

There are some similar questions on SO, but none are close enough for me to recognize that they apply to my case, at least as far as I can tell. Using the terminology of another language (R), I need to “coerce” the class of cluePos from NSString to UILabel. I’m on Xcode 4.2.1 and OSX 10.7.2.

TIA.

Answer 1

You can’t coerce a string into a label because they are fundamentally different. The string doesn’t have any knowledge of your view controller class or it’s properties (some of which happen to be labels).

You can however use the valueForKey: method to get a property of an object by name, where the name is specified as a string. So to get a property called Clue1 on my view controller I’d say:

UILabel *label = [self valueForKey:@"Clue1"];

Or in your case, this:

NSString *cluePos = [NSString stringWithFormat:@"Clue%d", charIdx + 1];
UILabel *label = [self valueForKey:cluePos];
label.text = @"test";

(I’m assuming ‘self’ in this case refers to the view controller, but you can call this on any object that has properties.)

Another way to do this is to turn your string into a selector using NSSelectorFromString. That would look like this:

SEL selector = NSSelectorFromString(@"Clue1");
UILabel *label = [self performSelector:selector];

For your purposes either solution works equally well, however the advantage of using the selector is that you can pass arguments to the method call (so you could call a method that returns an object, not just access a property or IBOutlet).

Note that both of these methods will raise an exception if you try to access a property or call a method that doesn’t exist. You can test if the property exists before calling it by saying:

SEL selector = NSSelectorFromString(@"Clue1");
BOOL labelExists = [self respondsToSelector:selector];
if (labelExists)
{
    UILabel *label = [self performSelector:selector];
    label.text = @"test";
}
else
{
    //do something else
}