Question

I need to run some database processing when my app first starts once a day.

I elected to do this in my first view and showed an alertview with an activity indicator. I tried using NSThread detachNewThreadSelector to run the db processing. At the end of the function I used dismissWithClickedButtonIndex to dismiss the alert view.

I then called the view controller from applicationWillEnterForeground if the date permitted.

However, I’m getting an error Tried to obtain the web lock from a thread other than the main thread or the web thread. Crashing now, on the dismissWithClickedButtonIndex line.

I believe this is occurring due to a flaw in using an object like an alertview with a thread.

I need this to be iOS 4 compatible.

Can anyone point me at an alternative approach ?

Answer 1

You won’t be able to create and work directly with the UIAlertView in a background thread.

Here’s a skeletal idea of what you could do:

UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"Title" message:@"Message" delegate:self cancelButtonTitle:@"OK" otherButtonTitles:nil];
//  create your progress indicator in your alert view
[alertView show];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
      //  do your db update
      dispatch_async(dispatch_get_main_queue(), ^{
           //  update your alert view here.
      });
});

The idea is to do your processing on a background queue and message back to the main queue for objects that are main-thread only (like UIAlertView.)