Update UILabel with results from results of dispatch_async NSURLSession call

Question

I started Objective-C programming a couple weeks ago, so my understanding of how all these pieces fit together & the order they're all happening is still confusing to me. I'm trying to make a JSON API call to one of my apps using NSURLSession. That all works flawlessly, but I want to update a label with a piece of the data that's returned & anytime I look at/try to update the label, I get null.

Some of the SO posts I've found that are similar to my problem include: this, this, this, and this. Coming from the world of Ruby on Rails, I haven't had to deal with async concepts at all, but I know I'm close.

Here's the relevant snippet of code in question:

        if (!jsonError) {
            NSDictionary *skillBuildData = [NSJSONSerialization JSONObjectWithData:data options:0 error:nil];

            dispatch_async(dispatch_get_main_queue(), ^{
                    NSLog(@"%@:", skillBuildNameLabel.text);  // should say "Build Name" but returns null
                    NSLog(@"%@", skillBuildData[@"name"]);    // correctly prints the result
                    NSLog(@"%@:", skillBuildNameLabel.text);  // should have contents of skillBuildData[@"name"] but returns null
                    skillBuildNameLabel.text = skillBuildData[@"name"]; // obviously results in null, but I don't know why.
            });
        }

EDIT:

Not sure if it's relevant, but here's the bulk of my ViewController.h to give you an idea of the outlets & actions in this very simple app. One button, one method, the IBOutlet that links the button & JSON call method, and a label:

@interface ViewController : UIViewController {
    IBOutlet UILabel *skillBuildNameLabel;
    IBOutlet UIButton *getSkillBuildDataButton;
}

- (void)getSkillBuildDataById:(int) skillBuildId;

- (IBAction)buttonPressed;

It seems like I'm very close, I just can't see the link I'm missing. Thank you so much in advance!

EDIT 2:

Check out Ben Kreeger's comment to the response I marked as the answer. I didn't connect the actual label in my storyboard to the outlet I created in my ViewController.h. I had no idea you could drag the line from the element in a storyboard to an actual line of code. That was the missing piece. Looks like I have a lot more to learn about Xcode & Objective-C. Thanks to all who helped me out!

Answer 1

You may have more luck declaring your IBOutlets as properties (@property) instead of as instance variables (see this answer for why weak instead of strong).

@interface ViewController : UIViewController

@property (weak, nonatomic) IBOutlet UILabel *skillBuildNameLabel;
@property (weak, nonatomic) IBOutlet UIButton *getSkillBuildDataButton;

...

@end

Then you'll be able to reference them as self.skillBuildNameLabel and self.getSkillBuildDataButton in your implementation.

Beware that this self.x notation inside of a callback like that may lead to what's called a retain cycle. If this is the case, Xcode will warn you about this. Here's a bit on retain cycles for you.

Footnote: I rarely ever see (and never write) this syntax anymore for declaring instance variables.

@interface ViewController : UIViewController {
    IBOutlet UILabel *skillBuildNameLabel;
    IBOutlet UIButton *getSkillBuildDataButton;
}

Use properties instead.

Answer 2

You are doing the logging before setting the text. move

skillBuildNameLabel.text = skillBuildData[@"name"];

to the top of the async block, above the NSLog statements.