Updating UI element without dispatching to the main queue

Question

I have a dispatch_block_t which is passed to another function , and this block will be called when the function finishes the asynchronous task. But the problem is that I don't know which thread this block will be called.

I want to update my UI in the main thread , hence I want to use

dispatch_async(dispatch_get_main_queue(), ^{...})

to update my UI. But I am afraid that this will cause a deadlock if such occasion happens

dispatch_queue_t queue = dispatch_queue_create("my.label", DISPATCH_QUEUE_SERIAL);
dispatch_async(queue, ^{
    dispatch_async(queue, ^{
        // outer block is waiting for this inner block to complete,
        // inner block won't start before outer block finishes
        // => deadlock
    });

    // this will never be reached
}); 

Is there a way to prevent the deadlock ? Like updating the UI element without using the dispatch queue . Is it possible to create a weak reference to self in order to update the UI?

Answer 1

Try running your example with NSLogs and you'll notice that deadlock doesn't occur. This is due to the fact that using dispatch_async just submits a block to the queue without waiting for it to finish execution (in contrary to dispatch_sync ).

So running this code:

dispatch_queue_t queue = dispatch_queue_create("my.label", DISPATCH_QUEUE_SERIAL);

dispatch_async(queue, ^{
    NSLog(@"1");
    dispatch_async(queue, ^{
        NSLog(@"2");
    });
    NSLog(@"3");
});

Will produce the following log:

Testtt[32153:2250572] 1
Testtt[32153:2250572] 3
Testtt[32153:2250572] 2

Moreover, I'm concerned that using dispatch_async(dispatch_get_main_queue(), ^{...}) here is commonly-used technique which ensures that the consumer gets the result on the main thread (ie consumer doesn't 'care' about threading).

Why, though, you use dispatch_block_t to pass a completion block? In my opinion, it's a bit confusing to use something like that on the consumer side - I would pass an anonymous (without typedef) block or create my own typedef for these simple completion blocks.