本文记录的是，当在UIViewController的根view.window上添加子视图maskView，且在该子视图maskView上的某一操作会触发UIAlertController的弹出的情况下，如何保证UIAlertController是显示在整个视图的最上方。如不做任何处理，UIAlertController会处在子视图maskView的视图层级之下，如下图所示:

错误显示的UIAlertController
错误显示的UIAlertController

此时，主要代码如下:

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- (IBAction)showPopView:(id)sender {
    
    UIView *maskView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, SCREEN_SIZE.width, SCREEN_SIZE.height)];
    maskView.backgroundColor = [[UIColor blackColor] colorWithAlphaComponent:0.6];
    
    CGFloat leftMargin = 50;
    CGFloat topMargin = 80;
    CGRect popViewFrame = CGRectMake(leftMargin, topMargin, SCREEN_SIZE.width - 2 * leftMargin, 200);
    UINib *viewNib = [UINib nibWithNibName:@"PopView" bundle:nil];
    NSArray *views = [viewNib instantiateWithOwner:self options:nil];
    for (id view in views) {
        if ([view isKindOfClass:[PopView class]]) {
            _popView = view;
        }
    }
    _popView.frame = popViewFrame;
    [_popView.showAlertBtn addTarget:self action:@selector(showAlertBtnPressed:) forControlEvents:UIControlEventTouchUpInside];
    [maskView addSubview:_popView];
    
    [self.view.window addSubview:maskView];
}

- (void)showAlertBtnPressed:(id)sender {
    if ([UIAlertController class]) {
        UIAlertController *alertController = [UIAlertController alertControllerWithTitle:@"我要在最上面" message:@"谁也别惹我" preferredStyle:UIAlertControllerStyleAlert];
        UIAlertAction *confirmAction = [UIAlertAction actionWithTitle:@"确定" style:UIAlertActionStyleDefault handler:nil];
        [alertController addAction:confirmAction];
        [self presentViewController:alertController animated:YES completion:nil];
    } else {
        UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"我要在最上面" message:@"谁也别惹我" delegate:self cancelButtonTitle:@"确定" otherButtonTitles:nil];
        [alertView show];
    }
}

要想正确显示UIAlertController，需要用到UIWindow的创建及切换。首先创建一个UIWindow对象，如下:

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@property (strong, nonatomic) UIWindow *alertWindow;

- (UIWindow *)alertWindow {
    if (!_alertWindow) {
        _alertWindow = [[UIWindow alloc] initWithFrame:[UIScreen mainScreen].bounds];
        UIViewController *viewController = [[UIViewController alloc] init];
        _alertWindow.rootViewController = viewController;
    }
    
    return _alertWindow;
}

然后在presentViewController的时候进行UIWindow的切换，如下所示:

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[self.alertWindow makeKeyAndVisible];
[self.alertWindow.rootViewController presentViewController:alertController animated:YES completion:nil];

最后，在用户进行交互操作让UIAlertController消失的时候将UIWindow切换回系统的window，如下所示:

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AppDelegate *appDelegate = [UIApplication sharedApplication].delegate;
[appDelegate.window makeKeyAndVisible];

整个显示UIAlertController代码改变后如下:

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- (void)showAlertBtnPressed:(id)sender {
    if ([UIAlertController class]) {
        UIAlertController *alertController = [UIAlertController alertControllerWithTitle:@"我要在最上面" message:@"谁也别惹我" preferredStyle:UIAlertControllerStyleAlert];
        UIAlertAction *confirmAction = [UIAlertAction actionWithTitle:@"确定" style:UIAlertActionStyleDefault handler:^(UIAlertAction *action) {
            AppDelegate *appDelegate = [UIApplication sharedApplication].delegate;
            [appDelegate.window makeKeyAndVisible];
        }];
        [alertController addAction:confirmAction];
        
        [self.alertWindow makeKeyAndVisible];
        [self.alertWindow.rootViewController presentViewController:alertController animated:YES completion:nil];
    } else {
        UIAlertView *alertView = [[UIAlertView alloc] initWithTitle:@"我要在最上面" message:@"谁也别惹我" delegate:self cancelButtonTitle:@"确定" otherButtonTitles:nil];
        [alertView show];
    }
}

效果图如下:

正确显示的UIAlertController
正确显示的UIAlertController

另外，在 iOS8.0 以下使用UIAlertView时不会出现任何问题，不用做任何更改.

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