获取touchID验证以再次弹出

Question

我目前正在实现一个仅接受触摸ID作为进入下一个viewcontroller的方式的应用程序，但作为一个新手，我不完全确定如何正确地执行此操作。我目前的代码如下：

-(void)viewWillAppear:(BOOL)animated {
LAContext *myContext = [[LAContext alloc] init];
NSError *authError = nil;
NSString *myLocalizedReasonString = @"Please Enter Touch ID";

if ([myContext canEvaluatePolicy:LAPolicyDeviceOwnerAuthenticationWithBiometrics error:&authError]) {
    [myContext evaluatePolicy:LAPolicyDeviceOwnerAuthenticationWithBiometrics
              localizedReason:myLocalizedReasonString
                        reply:^(BOOL success, NSError *error) {
                            if (success) {
                                dispatch_async(dispatch_get_main_queue(), ^{
                                    [self performSegueWithIdentifier:@"showList" sender:nil];
                                });
                            } else {
                                dispatch_async(dispatch_get_main_queue(), ^{
                                    UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"Error"
                                                                                        message:@"Authentication is verified through touch only."
                                                                                       delegate:self
                                                                              cancelButtonTitle:@"Close"
                                                                              otherButtonTitles:nil, nil];
                                    [alertView show];
                                    // Rather than show a UIAlert here, use the error to determine if you should push to a keypad for PIN entry.
                                });
                            }
                        }];
}

}

每当用户点击“取消”按钮时，我的应用程序就会崩溃，但我希望它能够重新显示touchID验证。我怎么能这样做？谢谢你的帮助！