Challenge 1

Find and fix the problem.

@interface XYZWaitController : UIViewController

@property (strong, nonatomic) UILabel *alert;

@end

@implementation XYZWaitController

- (void)viewDidLoad
{
    CGRect frame = CGRectMake(20, 200, 200, 20);
    self.alert = [[UILabel alloc] initWithFrame:frame];
    self.alert.text = @"Please wait 10 seconds...";
    self.alert.textColor = [UIColor whiteColor];
    [self.view addSubview:self.alert];

    NSOperationQueue *waitQueue = [[NSOperationQueue alloc] init];
    [waitQueue addOperationWithBlock:^{
        [NSThread sleepUntilDate:[NSDate dateWithTimeIntervalSinceNow:10]];
        self.alert.text = @"Thanks!";
    }];
}

@end

@implementation XYZAppDelegate

- (BOOL)application:(UIApplication *)application
  didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    self.window.rootViewController = [[XYZWaitController alloc] init];
    [self.window makeKeyAndVisible];
    return YES;
}

// The problem here:
NSOperationQueue *waitQueue = [[NSOperationQueue alloc] init];
[waitQueue addOperationWithBlock:^{
[NSThread sleepUntilDate:[NSDate dateWithTimeIntervalSinceNow:10]];
self.alert.text = @"Thanks!";
}];

// You should access the UI throw main thread like this to grantee the block will execute on the main thread:
NSOperationQueue *waitQueue = [[NSOperationQueue alloc] init];
[waitQueue addOperationWithBlock:^{
[NSThread sleepUntilDate:[NSDate dateWithTimeIntervalSinceNow:10]];
[[NSOperationQueu mainQueue] addOperationWithBlock:^{
self.alert.text = @"Thanks!";
}];
}];

The problem is that ui elements must changed in main queue then you can write:

NSOperationQueue *waitQueue = [NSOperationQueue mainQueue];
[waitQueue addOperationWithBlock:^{
[NSThread sleepUntilDate:[NSDate dateWithTimeIntervalSinceNow:10]];
self.alert.text = @"Thanks!";
}];