显示来自viewDidLoad的警报消息

Question

我想显示来自viewDidLoad()的ViewController.m方法的警报消息，而不是viewDidAppear()方法。

这是我的代码：

- (void)viewDidLoad {
    [super viewDidLoad];

    //A SIMPLE ALERT DIALOG
    UIAlertController *alert =   [UIAlertController
                              alertControllerWithTitle:@"My Title"
                              message:@"Enter User Credentials"
                              preferredStyle:UIAlertControllerStyleAlert];


    UIAlertAction *cancelAction = [UIAlertAction
                               actionWithTitle:NSLocalizedString(@"Cancel", @"Cancel action")
                               style:UIAlertActionStyleCancel
                               handler:^(UIAlertAction *action)
                               {
                                   NSLog(@"Cancel action");
                               }];

    UIAlertAction *okAction = [UIAlertAction
                           actionWithTitle:NSLocalizedString(@"OK", @"OK action")
                           style:UIAlertActionStyleDefault
                           handler:^(UIAlertAction *action)
                           {
                               NSLog(@"OK action");
                           }];

    [alert addAction:cancelAction];
    [alert addAction:okAction];
    [self presentViewController:alert animated:YES completion:nil];
}

我收到此错误：

警告：尝试在<UIAlertController: 0x7fbc58448960>上显示<ViewController: 0x7fbc585a09d0>，其视图不在窗口层次结构中！

Answer 1

确定不是错误，问题是在viewDidLoad中视图层次结构未完全设置。如果使用viewDidAppear，则设置层次结构。

如果你真的想在viewDidLoad中调用这个警报你可以通过将你的演示调用包装在这个GCD块中来引起轻微延迟，等待下一个运行循环，但是我建议你不要（它很难看）。

dispatch_async(dispatch_get_main_queue(), ^ {
    [self presentViewController:alert animated:YES completion:nil];
});

Answer 2

将此调用移至viewDidAppear：方法。

Answer 3

您必须嵌入导航控制器并显示控制器

- (void)viewDidLoad {
  [super viewDidLoad];

  //A SIMPLE ALERT DIALOG


   UIAlertController *alert =   [UIAlertController
                          alertControllerWithTitle:@"My Title"
                          message:@"Enter User Credentials"
                          preferredStyle:UIAlertControllerStyleAlert];


   UIAlertAction *cancelAction = [UIAlertAction
                           actionWithTitle:NSLocalizedString(@"Cancel", @"Cancel action")
                           style:UIAlertActionStyleCancel
                           handler:^(UIAlertAction *action)
                           {
                               NSLog(@"Cancel action");
                           }];

   UIAlertAction *okAction = [UIAlertAction
                       actionWithTitle:NSLocalizedString(@"OK", @"OK action")
                       style:UIAlertActionStyleDefault
                       handler:^(UIAlertAction *action)
                       {
                           NSLog(@"OK action");
                       }];

   [alert addAction:cancelAction];
   [alert addAction:okAction];

   [self.navigationController presentViewController:alert animated:NO completion:nil];
   //    [self presentViewController:cameraView animated:NO completion:nil]; //this will cause view is not in the window hierarchy error

}

要么

  [self.view addSubview:alert.view];
  [self addChildViewController:alert];
  [alert didMoveToParentViewController:self];

Answer 4

Swift 3 iOS 10，我使用操作队列将更新UI的代码块放到主线程上。

import UIKit

class ViewController2: UIViewController {

var opQueue = OperationQueue()

override func viewDidLoad() {
    super.viewDidLoad()
        let alert = UIAlertController(title: "MESSAGE", message: "HELLO WORLD!", preferredStyle: UIAlertControllerStyle.alert)
        // add an action (button, we can add more than 1 buttons)
        alert.addAction(UIAlertAction(title: "OK", style: UIAlertActionStyle.default, handler: nil))
        // show the alert
        self.opQueue.addOperation {
            // Put queue to the main thread which will update the UI
            OperationQueue.main.addOperation({
                self.present(alert, animated: true, completion: nil)
            })
        }
    }
}

简而言之，我们正在使用异步。这允许警报消息按预期显示（即使我们在viewDidLoad（）中）。