- (void)viewDidLoad {
    [super viewDidLoad];

    [self testGCD];
}

- (void)testGCD {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSLog(@"1");
        return;
    });
    NSLog(@"2");
}

- (void)testGCD {
    static dispatch_queue_t queue = dispatch_queue_create("com.example.MyQueue", NULL);
    static bool firstRun = YES;

    dispatch_sync(queue, ^{
       if (firstRun) {
           NSLog(@"1);
           firstRun = NO;
       } else {
           NSLog(@"2");
       }
    });
}

- (void)testGCD {
    static BOOL didTrigger = NO;

    if(didTrigger) {
        NSLog(@"2");
    }
    else {
        didTrigger = YES;
        NSLog(@"1");
    }
}

- (void)testGCD {
    static dispatch_once_t onceToken;
    static BOOL didInvokeOnceBlock = NO;
    static BOOL didPassSkippedBlock = NO;
    dispatch_once(&onceToken, ^{
        NSLog(@"1");
        didInvokeOnceBlock = YES;
    });
    if(didInvokeOnceBlock && didPassSkippedBlock) {
        NSLog(@"2");
    }
    didPassSkippedBlock = YES;
}

- (void)testGCD {
    static dispatch_once_t onceToken;
    __block NSString *text = @"2";
    dispatch_once(&onceToken, ^{
        text = @"1";
    });
    NSLog(@"%@", text);
}