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

Question

I have a scenario in my app, where I want to do some time consuming task which consists of some data processing as well as UI update, in a method. My method looks like this,

Answer 1

The good practice is: Dispatch Groups

dispatch_group_t imageGroup = dispatch_group_create();

dispatch_group_enter(imageGroup);
[uploadImage executeWithCompletion:^(NSURL *result, NSError* error){
    // Image successfully uploaded to S3
    dispatch_group_leave(imageGroup);
}];

dispatch_group_enter(imageGroup);
[setImage executeWithCompletion:^(NSURL *result, NSError* error){
    // Image url updated
    dispatch_group_leave(imageGroup);
}];

dispatch_group_notify(imageGroup,dispatch_get_main_queue(),^{
    // We get here when both tasks are completed
});