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

This accepted answer does not address the underlying cause of the problem: invoking evaluatePolicy() twice, the second time while the first invocation is in progress. So the current solution only works sometimes by luck, as everything is timing dependent.

The brute-force, straightforward way to work around the problem is a simple boolean flag to prevent subsequent calls from happening until the first completes.

AppDelegate *delegate = [[UIApplication sharedApplication] delegate];
if ( NSClassFromString(@"LAContext") && ! delegate.touchIDInProgress ) {
    delegate.touchIDInProgress = YES;
    LAContext *localAuthenticationContext = [[LAContext alloc] init];
    __autoreleasing NSError *authenticationError;
    if ([localAuthenticationContext canEvaluatePolicy:LAPolicyDeviceOwnerAuthenticationWithBiometrics error:&authenticationError]) {
        [localAuthenticationContext evaluatePolicy:LAPolicyDeviceOwnerAuthenticationWithBiometrics localizedReason:kTouchIDReason reply:^(BOOL success, NSError *error) {
            delegate.touchIDInProgress = NO;
            if (success) {
                ...
            } else {
                ...
            }
        }];
    }