iOS – 默认情况下,选择器/处理程序是否在主队列上运行？

假设,我正在从主踏板上展示一个UIAlertController,比如myAlert. myAlert有一个动作,defaultAction.我想知道默认情况下defaultActions处理程序是否在主队列上运行.换句话说,我想知道在下面的代码中是否在dostuff中有一些与UI相关的操作,我是否需要用主队列包装这些UI任务,或者保证在OS中在主队列中运行？

UIAlertController* myAlert = [UIAlertController alertControllerWithtitle:@"My Alert"
                               message:@"This is an alert."
                               preferredStyle:UIAlertControllerStyleAlert];

UIAlertAction* defaultAction = [UIAlertAction actionWithtitle:@"OK" style:UIAlertActionStyleDefault
   handler:^(UIAlertAction * action) {
      dostuff() // should I wrap dostuff in main queue,if Dostuff has ui operations?
   }
];

[alert addAction:defaultAction];
[self presentViewController:alert animated:YES completion:nil];