title: TouchID forward to system passcode authentication
body: <p>I want to use TouchID authenticate my own app.</p>
  
  <p>1.I want user can click 'Enter passcode' to invoke system build-in passcode screen to authenticate,if success then enter my own app. <br>
  But I don't how know to make it forward to passcode authenticate view like the following screen in <b>'case LAErrorUserFallback'</b>
  <img src="https://i.stack.imgur.com/id9E3.jpg" alt="enter image description here"></p>
  
  <p>Here is my code :</p>
  
  <pre><code>LAContext *context = [[LAContext alloc] init];
  __block  NSString *msg;
  __block  BOOL bAuth;
  // show the authentication UI with our reason string
  [context evaluatePolicy:LAPolicyDeviceOwnerAuthenticationWithBiometrics localizedReason:NSLocalizedString(@"Unlock APP With FingerPrint", nil) reply:
  ^(BOOL success, NSError *authenticationError) {
  
     if (success) {
        bAuth = YES;
        msg =[NSString stringWithFormat:NSLocalizedString(@"EVALUATE_POLICY_SUCCESS", nil)];
        dispatch_async(dispatch_get_main_queue(), ^{
           [[MYAppDelegate theDelegate] initializeAppAfterKeyVaultUnlocked];
        });
        NSLog(@"%@",msg);
     } else {
        bAuth = NO;
        switch (authenticationError.code) {
           case LAErrorAuthenticationFailed:
              msg = [NSString stringWithFormat:NSLocalizedString(@"Authentication Failed", nil)];
              // ...
              break;
  
           case LAErrorUserCancel:
              msg = [NSString stringWithFormat:NSLocalizedString(@"User pressed Cancel button", nil)];
              dispatch_async(dispatch_get_main_queue(), ^{
                 [[MYAppDelegate theDelegate] exitAndLock];
              });
  
              break;
  
           case LAErrorUserFallback:
              msg = [NSString stringWithFormat:NSLocalizedString(@"User pressed \"Enter Password\"", nil)];
              //Here I want to fallback to iOS build-in passcode authenticate view, and get the auth result.
              break;
  
           default:
              msg = [NSString stringWithFormat:NSLocalizedString(@"Touch ID is not configured", nil)];
              // ...
              break;
        }
        NSLog(@"%@",authenticationError.localizedDescription);
  
     }
  
  
  }];
  </code></pre>