How to dispatch_group_wait for dispatch_group_async inside an asynchronous block

asynchronous, grand-central-dispatch, ios, multithreading, objective-c

Solution

Think of this method as a static counter available to threads, so when you enter a group the counter increments, and when that block returns, decrements...

When that counter is 0, it will call a block to invoke

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_t group = dispatch_group_create();

while(someCondition)
{
    dispatch_group_enter(group);
   [SomeClassThatLoadsOffTheInternet getMyImages:^{

        // do something with these.
        dispatch_group_leave(group);

    });
}

dispatch_group_notify(group, queue, ^{
    // do something when all images have loaded
});

Is that what you were thinking of?

Problem

I have code that looks something like this: ``` [SVProgressHUD show]; [imageGenerator generateCGImagesAsynchronouslyForTimes:times completionHandler:^(CMTime requestedTime, ...) { dispatch_group_async(queueGroup, queue, ^{ // Do stuff }); }]; dispatch_group_wait(queueGroup, DISPATCH_TIME_FOREVER); [SVProgressHUD dismiss]; ``` Basically, display a loading animation HUD and start generating image thumbnails from an asset, then once it's done hide the HUD. I'm using a dispatch group since i want to make sure all the thumbnails are generated before i hide the HUD. But when i run it, the HUD gets dismissed immediately. I'm guessing this is because of the asynchronous nature of the `generateCGImagesAsynchronouslyForTimes: completionHandler:`--`dispatch_group_wait` is called before the first `dispatch_group_async` inside the completionHandler. What is a graceful way to get around this situation? Thanks.

Original source