Touch ID causing app to become non-responsive

Question

I Added ios-8's new touchID API to my app. It usually works as expected, BUT when entering app while my finger is already on home-button - API's success callback is called but pop-up still appears on screen. after pressing CANCEL UI becomes non-responsive.

Answer 1

I also encountered the same issue, and the solution was to invoke the call to the Touch ID API using a high priority queue, as well as a delay:

// Touch ID must be called with a high priority queue, otherwise it might fail.
// Also, a dispatch_after is required, otherwise we might receive "Pending UI mechanism already set."
dispatch_queue_t highPriorityQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, 0.75 * NSEC_PER_SEC), highPriorityQueue, ^{
  LAContext *context = [[LAContext alloc] init];
  NSError *error = nil;

  // Check if device supports TouchID
  if ([context canEvaluatePolicy:LAPolicyDeviceOwnerAuthenticationWithBiometrics error:&error]) {
      // TouchID supported, show it to user
      [context evaluatePolicy:LAPolicyDeviceOwnerAuthenticationWithBiometrics
              localizedReason:@"Unlock Using Touch ID"
                        reply:^(BOOL success, NSError *error) {
                            if (success) {
                                // This action has to be on main thread and must be synchronous
                                dispatch_async(dispatch_get_main_queue(), ^{
                                    ...
                                });
                            }
                            else if (error) {
                                ...
                            }
                        }];
  }
});

When testing our app, we found a delay of 750ms to be optimal, but your mileage may vary.

Update (03/10/2015): Several iOS developers, like 1Password for example, are reporting that iOS 8.2 have finally fixed this issue.