LAContext *myContext = [[LAContext alloc] init];
NSError *authError = nil;
NSString *myLocalizedReasonString = <#String explaining why app needs authentication#>;

if ([myContext canEvaluatePolicy:LAPolicyDeviceOwnerAuthenticationWithBiometrics error:&authError]) {
    [myContext evaluatePolicy:LAPolicyDeviceOwnerAuthenticationWithBiometrics
                  localizedReason:myLocalizedReasonString
                            reply:^(BOOL success, NSError *error) {
            if (success) {
                // User authenticated successfully, take appropriate action
            } else {
                // User did not authenticate successfully, look at error and take appropriate action
            }
        }];
} else {
    // Could not evaluate policy; look at authError and present an appropriate message to user
}