刚开始使用Xcode 4.5，我在控制台得到了这个错误:
警告:试图在< ViewController: 0x1ec3e000>上显示< finishViewController: 0x1e56e0a0 >，其视图不在窗口层次结构中!
视图仍在显示，应用程序中的一切都在正常工作。这是iOS 6的新功能吗?
这是我用来在视图之间更改的代码:
UIStoryboard *storyboard = self.storyboard;
finishViewController *finished =
[storyboard instantiateViewControllerWithIdentifier:@"finishViewController"];
[self presentViewController:finished animated:NO completion:NULL];
当前回答
使用故事板，我注意到我有一个按钮连接到两个@IBActions，这两个@IBActions函数分别呈现一个ViewController。在我移除一个连接后，问题就解决了。
其他回答
在我的情况下，我不能把我的类重写。所以，这是我得到的:
let viewController = self // I had viewController passed in as a function,
// but otherwise you can do this
// Present the view controller
let currentViewController = UIApplication.shared.keyWindow?.rootViewController
currentViewController?.dismiss(animated: true, completion: nil)
if viewController.presentedViewController == nil {
currentViewController?.present(alert, animated: true, completion: nil)
} else {
viewController.present(alert, animated: true, completion: nil)
}
这适用于呈现任何视图控制器，如果你有可用的导航控制器。 self.navigationController ?。present(MyViewController, animated: true, completion: nil) 此外，我可以能够呈现警报和邮件控制器也。
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"];
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(...)
}
}
我在更新Xcode后发现了这个bug，我相信是Swift 5。当我在展开视图控制器后通过编程方式直接启动segue时，问题就发生了。
解决方案到来的同时修复了一个相关的错误，即用户现在可以通过向下滑动页面来unwind segue。这打破了我程序的逻辑。
通过将所有视图控制器上的演示模式从自动更改为全屏，可以解决这个问题。
你可以在界面构建器的属性面板中完成。或者查看这个答案，了解如何以编程方式完成。
