In case it helps anyone, my issue was extremely silly. Totally my fault of course. A notification was triggering a method that was calling the modal. But I wasn't removing the notification correctly, so at some point, I would have more than one notification, so the modal would get called multiple times. Of course, after you call the modal once, the viewcontroller that calls it it's not longer in the view hierarchy, that's why we see this issue. My situation caused a bunch of other issue too, as you would expect.

总之，无论你做什么，都要确保模态不会被调用超过一次。

你从哪里调用这个方法?我有一个问题，我试图在viewDidLoad方法中呈现一个模态视图控制器。我的解决方案是将这个调用移动到viewDidAppear:方法。

我的假设是，视图控制器的视图在它被加载时(当viewDidLoad消息被发送时)不在窗口的视图层次结构中，但在它被呈现后(当viewDidAppear:消息被发送时)它在窗口层次结构中。

谨慎

如果你确实调用了viewDidAppear:中的presentViewController:animated:completion:，你可能会遇到这样一个问题，即每当视图控制器的视图出现时，模态视图控制器总是被呈现(这是有意义的!)，因此模态视图控制器被呈现永远不会消失……

也许这不是呈现模态视图控制器的最佳位置，或者可能需要保留一些额外的状态让呈现的视图控制器决定是否应该立即呈现模态视图控制器。

你只能有1个rootViewController，它是最近出现的一个。所以不要尝试让一个视图控制器呈现另一个视图控制器当它已经呈现了一个没有被解散的视图控制器。

在做了一些自己的测试之后，我得出了一个结论。

如果你有一个rootViewController你想要显示所有东西，那么你就会遇到这个问题。

下面是我的rootController代码(打开是从根节点显示视图控制器的快捷方式)。

func open(controller:UIViewController)
{
    if (Context.ROOTWINDOW.rootViewController == nil)
    {
        Context.ROOTWINDOW.rootViewController = ROOT_VIEW_CONTROLLER
        Context.ROOTWINDOW.makeKeyAndVisible()
    }

    ROOT_VIEW_CONTROLLER.presentViewController(controller, animated: true, completion: {})
}

如果我在一行中调用open两次(不管时间流逝)，这将在第一个开放上工作，但不是在第二个开放上。第二次打开尝试将导致上述错误。

然而，如果我关闭最近呈现的视图然后调用open，当我再次调用open(在另一个视图控制器上)时，它工作得很好。

func close(controller:UIViewController)
{
    ROOT_VIEW_CONTROLLER.dismissViewControllerAnimated(true, completion: nil)
}

我的结论是，只有MOST-RECENT-CALL的rootViewController在视图层次结构上(即使您没有解散它或删除视图)。我试着玩所有的加载器调用(viewDidLoad, viewDidAppear，并做延迟调度调用)，我发现我能让它工作的唯一方法是从最顶层的视图控制器调用present。

斯威夫特5

我在viewDidLayoutSubviews中调用present，因为在viewDidAppear中呈现会在模态加载之前导致视图控制器的瞬间显示，这看起来像一个丑陋的故障

确保检查窗口是否存在并只执行一次代码

var alreadyPresentedVCOnDisplay = false

override func viewDidLayoutSubviews() {
        
    super.viewDidLayoutSubviews()
    
    // we call present in viewDidLayoutSubviews as
    // presenting in viewDidAppear causes a split second showing 
    // of the view controller before the modal is loaded
    
    guard let _ = view?.window else {
        // window must be assigned
        return
    }
    
    if !alreadyPresentedVCOnDisplay {
        alreadyPresentedVCOnDisplay = true
        present(...)
    }
    
}

我在Swift 4.2上也有类似的问题，但我的观点没有从视图周期中呈现出来。我发现我有多个segue要同时呈现。所以我使用dispatchAsyncAfter。

func updateView() {

 DispatchQueue.main.asyncAfter(deadline: .now() + 0.1) { [weak self] in

// for programmatically presenting view controller 
// present(viewController, animated: true, completion: nil)

//For Story board segue. you will also have to setup prepare segue for this to work. 
 self?.performSegue(withIdentifier: "Identifier", sender: nil)
  }
}

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"];