Question

I have a computational process that takes quite a bit of time to perform so a UIActivityIndicatorView seems appropriate. I have a button to initiate the computation.

I’ve tried putting the command [calcActivity startAnimating]; at the beginning of the computation in an IBAction and [calcActivity stopAnimating]; at the end of the computation but nothing shows.

Next, I created a new IBAction to contain the starting and stopping with a call to the computation IBAction and a dummy for loop just to give the startAnimating a little chance to get started between the two. This doesn’t work either.

The skeletal code looks like this:

- (IBAction)computeNow:(id)sender {
    [calcActivity startAnimating];
    for (int i=0; i<1000; ++i) { }
    [self calcStats];
    [calcActivity stopAnimating];
    }

- (IBAction)calcStats {
    // do lots of calculations here
    return;
    }

Answer 1

Ok, as I commented, you should never performe complex calculations in your main thread. It not only leads to situations like yours, your app might also be rejected from the store.

Now, the reason for the UIActivityIndicatorView not being updated is, that the UI doesn’t actually update itself e.g. when you call [calcActivity startAnimating]; Instead, it gets updated after your code ran through. In your case, that means that startAnimating and stopAnimating are getting called at once, so nothing really happens.

So, the ‘easy’ solution: Start a new thread, using either this techniques or, probably better, GCD.