视图控制器不会以编程方式返回到以前的视图控制器

Question

场景是当应用程序第一次加载（控制器 A 加载）时，它会检查 NSDefaultuser 是否存在。如果它不存在，则将其发送到（控制器 B）。在控制器 B 中，用户输入他/她的用户名，然后单击确定。现在，NSDefault 正在保存用户名，它还应该将其重定向回主视图控制器。代码如下：

if([Username.text  isEqual: @""]) {
    [self showOkAlert:@"Username required"];
}
else if([Password.text isEqual:@""]) {
    [self showOkAlert:@"Password required"];
}
else  {
    //suppose that login was successfully.
    [self SaveCredentials];


    NSString * storyboardName = @"MainStoryboard";
    UIStoryboard *storyboard = [UIStoryboard storyboardWithName:storyboardName bundle: nil];
    UIViewController * vc = [storyboard instantiateViewControllerWithIdentifier:@"LatestNews"];
    [self presentViewController:vc animated:YES completion:nil];

}

控制器 A：

- (void)LoadCredentials {
//load username
NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
NSString *username = [defaults objectForKey:@"User"];

if(!username) {

    [self showAlert:@"user does not exist"];

    [self dismissViewControllerAnimated:YES completion:nil];

    NSString * storyboardName = @"MainStoryboard";
    UIStoryboard *storyboard = [UIStoryboard storyboardWithName:storyboardName bundle: nil];
    UIViewController * vc = [storyboard instantiateViewControllerWithIdentifier:@"Login"];
    [self presentViewController:vc animated:YES completion:nil];

}else{
    [self showAlert:@"user exists"]; 
    // for testing purposes I want it to reset when user logged in. Just so I know it resets. Later this method will be called from logout button.
    [self resetUsernameWhenLogout];       
}
}
-(void)resetUsernameWhenLogout {
//reset username when logout
NSDictionary *defaultsDictionary = [[NSUserDefaults standardUserDefaults]     dictionaryRepresentation];
for (NSString *key in [defaultsDictionary allKeys]) {
    [[NSUserDefaults standardUserDefaults] removeObjectForKey:key];
}
[[NSUserDefaults standardUserDefaults] synchronize];
}

- (void)viewDidLoad
{
    [self LoadCredentials];
}

Answer 1

这里有几种可能性；首先，iOS8 似乎比 iOS7 更早地实例化 VC，因此 viewDidLoad 中的导航树不完整，您应该只使用 viewDidLoad 进行附加初始化。尝试移动您的代码 [self LoadCredentials]； } 查看WillAppear。

其次，如果你有一个导航控制器，你应该通过导航控制器将新的 VC 添加到导航树中。如果是这种情况，请尝试以下代码。

  if([Username.text  isEqual: @""]) {
      [self showOkAlert:@"Username required"];
  }
  else if([Password.text isEqual:@""]) {
      [self showOkAlert:@"Password required"];
  }
  else  {
      //suppose that login was successfully.
      [self SaveCredentials];

     //This does not work - I receive a thread error not sure how to debug it 
      UIStoryboard *storyBoard = self.storyboard;
      UIViewController *targetViewController = [storyBoard instantiateViewControllerWithIdentifier:@"LatestNews"];
      UINavigationController *navController = self.navigationController;
    if (navController) {
        [navController pushViewController:targetViewController animated:YES];
    }

  }