code inside dispatch_get_main_queue block

Question

I have few codes like below:

- (void)viewDidLoad {
    [super viewDidLoad];

    NSLog(@"1");

    dispatch_async(dispatch_get_main_queue(), ^{
        NSLog(@"3");
    });

    NSLog(@"2");
}

In console, the console output are: 1, 2, 3. At first I thought it should be 1, 3, 2 cause dispatch_get_main_queue is the main queue, and outside it is main queue as well. Why the output is different?

Answer 1

you dispatch the code in NSLog(3) using dispatch_async which will make it be executed asynchronously after the the current synchronous function is done.

---

I'd recommend reading a tutorial on Threading in General and GCD in particular since SO cant&shouldnt fully cover this.