Question

I’ve written a asynchronous call that has a completion block to return a UIImage, then in the completion block the view controller sets a UIImageView to use that image. My question is what happens if that view controller is popped off UINavigationController stack and is no longer alive before the completion block executes?

[MyAPI getImage:imageID completionBlock:^(MyAPIStatus status, id result) {
        if (status == kSuccessful) {
            self.ImageView.image = [UIImage imageWithData:result];
        }
    }];

Answer 1

Because the block passed to your API captures(retains) self, self will be alive.
So if you use retain/release properly or use ARC, this is harmless.