Question

I am trying to learn more about dispatch queues. If I put three methods in a dispatch queue as in the code below, do they execute one after the other or all at once ?

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(queue, ^  {  

    [activeModel release];
    [mainViewController showSceneList];
    [mainViewController removeTidyUpScreen];

});

How would I specify that the next should not run until the previous one is completed ?

Answer 1

Think of a block — the code you submit to a dispatch queue as you have here — as an anonymous function. So, the code you have in your block here executes in order just as if you were calling a function that contained the same calls, one method, then the next, and so on.

In your particular example, it looks like you may be doing some operations with the UI on a queue that is not the main queue. You MUST do UI operations on the main queue, because it has access to the UI. You might use dispatch_get_main_queue() instead, to be sure you’re getting that queue. If you have something you want to run in the background that will not touch the UI, then using a global queue is fine, and preferred especially if not stalling the UI is important.