Question

I am using a NSFetchedResultsController to populate a UITableView. The fetch take some time so I would like to present a spinning wheel to the user while the fetch is under way.

How would I do that?

Answer 1

You should start your spinner in the Main thread and push the “heavy work” for a secondary thread. When the work is done, stop the spinner. You can achieve that with something like this:

    // Start the spinning here.
    [mySpinner startAnimating];
    // Declare the queue
    dispatch_queue_t workingQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    // it's not, so we will start a background process to calculate it and not block the UI
    dispatch_async(workingQueue, 
                   ^{
                      // Some heavy work here.

                       dispatch_async(dispatch_get_main_queue(), ^{ 
                          // stop the spinner here
                          [mySpinner stopAnimating];
                       });
                   });

Doing the following in the Main thread wont make you accomplish what you want:

Start Spinner => Heavy work => Stop Spinner

When the Heavy work begins, it will block your UI thread, so you won’t actually see the UIActivityMonitor animating.

To finish I would advise you using this as spinner.