我也遇到了这个问题.搜索有关左栏按钮项垂直错误定位的其他问题让我到 这个问题.其要点是，由于未知原因，如果您有一个带有图像的条形按钮项，但标题为空字符串，则会出现此问题.将标题设置为单个空格而不是空字符串:

self.backButton = [[UIBarButtonItem alloc] initWithTitle:@" " style:UIBarButtonItemStylePlain target:self action:@selector(backButtonPressed)];

我不知道它是否会为你修复它，但它对我来说主要是 - 该按钮仍然会做一个轻微的跳跃"动画，就好像它是新创建的一样(但只是第一次出现) - 但是它保持在相同的垂直位置.

编辑:传入 nil 作为标题也会删除无关的动画.似乎这只是 iOS 如何将空白字符串作为标题处理的一个特点.

I've created a UIAlertController with the preferred style of UIAlertControllerStyleAlert. The alert shows when the leftBarButtonItem is tapped. I created a UIBarButtonItem property called backButton and set the leftBarButtonItem = self.backButton. This is working as designed. I'm not using storyboards.

The problem is that the leftBarButtonItem moves down (my guess: about 20pts) when the alert shows. Why is this happening?

I know how to show/hide the button so the user can't see that the button when it has moved down. However, that sucks. Why is it happening in the first place?

I haven't found any similar issues online.

@property (strong, nonatomic) IBOutlet UIBarButtonItem *backButton;

in viewDidLoad:

self.backButton = [[UIBarButtonItem alloc] initWithTitle:@"" style:UIBarButtonItemStylePlain target:self action:@selector(backButtonPressed)];
[self.backButton setImage:[UIImage imageNamed:@"back-arrow-grey"]];
self.navigationItem.leftBarButtonItem = self.backButton;

in backButtonPressed:

{
    self.navigationItem.leftBarButtonItem = nil; //to hide backButton because it moves down
    UIAlertController *alertController = [UIAlertController alertControllerWithTitle:@"My title" preferredStyle:UIAlertControllerStyleAlert];

    UIAlertAction *actionLeave = [UIAlertAction actionWithTitle:@"Leave" style:UIAlertActionStyleDefault handler:...//which works correctly

    UIAlertAction *actionCancel = [UIAlertAction actionWithTitle:@"Go back" style:UIAlertActionStyleDefault handler:^(UIAlertAction *action) {
        self.navigationItem.leftBarButtonItem = self.backButton; //to show backButton again now that the alert is dismissed
        //other things happen here that work as designed
    }];

    [alertController addAction:actionLeave];
    [alertController addAction:actionCancel];
    [self presentViewController:alertController animated:YES completion:^{}];
}

I also encountered this issue. Searching for other issues about vertical mis-positioning of the left bar button item took me to this question. The gist of it is that this problem occurs, for unknown reasons, if you have a bar button item that has an image, but an empty string as it's title. Set the title to a single space instead of just an empty string:

self.backButton = [[UIBarButtonItem alloc] initWithTitle:@" " style:UIBarButtonItemStylePlain target:self action:@selector(backButtonPressed)];

I don't know if it will fix it for you, but it mostly did for me - the button still does a slight 'jump' animation as though it's being newly created (but only the first time it appears) - but it stays at the same vertical position.

Edit: Passing in nil as the title also removes the extraneous animation. Seems like this is just a peculiarity in how iOS handles whitespace strings as titles.