在iOS 中彈窗時有時會失敗，打印下面的錯誤信息

Warning: Attempt to present on whose view is not in the window hierarchy!

調用方法是

[self presentViewController:secondView animated:YES completion:nil];

原因是因為self有可能不是頂層窗口，改用下面的方式調用

UIViewController * top= [UIApplicationsharedApplication].keyWindow.rootViewController;
[top presentViewController:secondView animated:YES completion:nil];

用上面的代碼有時還是不能解決，最終改用下面的代碼

-(UIViewController *) topMostController {
  UIViewController*topController = [UIApplicationsharedApplication].keyWindow.rootViewController;
  while(topController.presentedViewController){
    topController=topController.presentedViewController;
  }
  returntopController;
}

UIViewController * top = [self topMostController];
[top presentViewController:secondView animated:YES completion:nil];