在主窗口中，可能总是会出现与显示警报不兼容的过渡。为了允许在应用程序生命周期的任何时候显示警报，您应该有一个单独的窗口来完成这项工作。

/// independant window for alerts
@interface AlertWindow: UIWindow

+ (void)presentAlertWithTitle:(NSString *)title message:(NSString *)message;

@end

@implementation AlertWindow

+ (AlertWindow *)sharedInstance
{
    static AlertWindow *sharedInstance;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        sharedInstance = [[AlertWindow alloc] initWithFrame:UIScreen.mainScreen.bounds];
    });
    return sharedInstance;
}

+ (void)presentAlertWithTitle:(NSString *)title message:(NSString *)message
{
    // Using a separate window to solve "Warning: Attempt to present <UIAlertController> on <UIViewController> whose view is not in the window hierarchy!"
    UIWindow *shared = AlertWindow.sharedInstance;
    shared.userInteractionEnabled = YES;
    UIViewController *root = shared.rootViewController;
    UIAlertController *alert = [UIAlertController alertControllerWithTitle:title message:message preferredStyle:UIAlertControllerStyleAlert];
    alert.modalInPopover = true;
    [alert addAction:[UIAlertAction actionWithTitle:@"OK" style:UIAlertActionStyleCancel handler:^(UIAlertAction *action) {
        shared.userInteractionEnabled = NO;
        [root dismissViewControllerAnimated:YES completion:nil];
    }]];
    [root presentViewController:alert animated:YES completion:nil];
}

- (instancetype)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];

    self.userInteractionEnabled = NO;
    self.windowLevel = CGFLOAT_MAX;
    self.backgroundColor = UIColor.clearColor;
    self.hidden = NO;
    self.rootViewController = UIViewController.new;

    [NSNotificationCenter.defaultCenter addObserver:self
                                           selector:@selector(bringWindowToTop:)
                                               name:UIWindowDidBecomeVisibleNotification
                                             object:nil];

    return self;
}

/// Bring AlertWindow to top when another window is being shown.
- (void)bringWindowToTop:(NSNotification *)notification {
    if (![notification.object isKindOfClass:[AlertWindow class]]) {
        self.hidden = YES;
        self.hidden = NO;
    }
}

@end

在设计上总是成功的基本用法:

[AlertWindow presentAlertWithTitle:@"My title" message:@"My message"];

我已经结束了这样一个代码，最终工作给我(Swift)，考虑到你想要显示一些viewController从几乎任何地方。当没有可用的rootViewController时，这段代码显然会崩溃，这是开放式结局。它也不包括通常需要切换到UI线程使用

dispatch_sync(dispatch_get_main_queue(), {
    guard !NSBundle.mainBundle().bundlePath.hasSuffix(".appex") else {
       return; // skip operation when embedded to App Extension
    }

    if let delegate = UIApplication.sharedApplication().delegate {
        delegate.window!!.rootViewController?.presentViewController(viewController, animated: true, completion: { () -> Void in
            // optional completion code
        })
    }
}

如果其他解决方案因为某些原因看起来不太好，你仍然可以使用这个不错的老方法，用延迟0表示，就像这样:

dispatch_after(0, dispatch_get_main_queue(), ^{
    finishViewController *finished = [self.storyboard instantiateViewControllerWithIdentifier:@"finishViewController"];
    [self presentViewController:finished animated:NO completion:NULL];    
});

虽然我没有看到任何文件保证你的VC会在调度块计划执行的时间上处于视图层次结构上，但我观察到它会工作得很好。

使用延迟，例如0.2秒也是一种选择。最好的事情-这样你就不需要在viewDidAppear中混乱布尔变量:

Swift 5 -后台线程

如果一个警报控制器在后台线程上执行，那么“Attempt to present…”“其视图不在窗口层次结构中”的错误可能会发生。

所以这个:

present(alert, animated: true, completion: nil)
    

是这样固定的:

DispatchQueue.main.async { [weak self] in
    self?.present(alert, animated: true, completion: nil)
}

它工作得很好，试试这个。链接

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

可能和我一样，你有一个错误的根viewController

我想在非uiviewcontroller上下文中显示一个ViewController，

所以我不能使用这样的代码:

[self presentViewController:]

我得到一个UIViewController

[[[[UIApplication sharedApplication] delegate] window] rootViewController]

由于某种原因(逻辑错误)，rootViewController是其他的东西比预期的(一个正常的UIViewController)。然后我纠正了这个错误，用UINavigationController替换了rootViewController，问题就解决了。