xcode 显示来自 viewDidLoad 的警报消息

Display Alert Message from viewDidLoad

提问by Shivani Arorra

I want to display a alert message from viewDidLoad()method of ViewController.minstead from viewDidAppear()method.

我想从viewDidLoad()方法ViewController.m而不是从viewDidAppear()方法显示警报消息。

Here is my code :

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];
}

and I am getting this error:

我收到此错误：

Warning: Attempt to present <UIAlertController: 0x7fbc58448960>on <ViewController: 0x7fbc585a09d0>whose view is not in the window hierarchy!

警告：试图提出<UIAlertController: 0x7fbc58448960>对<ViewController: 0x7fbc585a09d0>他们的看法是不是在窗口层次！

回答by Woodstock

OK not a bug, the issue is that in viewDidLoadthe view hierarchy is not fully set. If you use viewDidAppear, then the hierarchy is set.

好的不是错误，问题是在viewDidLoad视图层次结构中没有完全设置。如果使用viewDidAppear，则设置层次结构。

If you really want to call this alert in viewDidLoadyou can do so by wrapping your presentation call in this GCD block to cause a slight delay, waiting for the next run loop, however I suggest you don't (it's ugly).

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

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

回答by Akash

to move this call to the viewDidAppear: method.

将此调用移动到 viewDidAppear: 方法。

回答by Mayank Patel

You have to embed a navigation controller and present the controller

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

}

OR

或者

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

回答by coarist

Swift 3 iOS 10, I used operation queue to put the block of code that updates the UI onto the main thread.

在 Swift 3 iOS 10 中，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)
            })
        }
    }
}

In short we are using async. This allows the alert message to be displayed as expected (even when we are in viewDidLoad()).

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