Does dispatch_async(dispatch_get_main_queue(), ^{…}); wait until done?

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No it doesn’t wait and the way you are doing it in that sample is not good practice.

dispatch_async is always asynchronous. It’s just that you are enqueueing all the UI blocks to the same queue so the different blocks will run in sequence but parallel with your data processing code.

If you want the update to wait you can use dispatch_sync instead.

// This will wait to finish
dispatch_sync(dispatch_get_main_queue(), ^{
    // Update the UI on the main thread.
});

Another approach would be to nest enqueueing the block. I wouldn’t recommend it for multiple levels though. 

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    // Background work

    dispatch_async(dispatch_get_main_queue(), ^{
        // Update UI

        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            // Background work

            dispatch_async(dispatch_get_main_queue(), ^{
                // Update UI
            });
        });
    });
});

If you need the UI updated to wait then you should use the synchronous versions. It’s quite okay to have a background thread wait for the main thread. UI updates should be very quick.

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