UIImageView 用 AutoLayout 阻塞 UINavigationBar

Question

我正在尝试创建一个具有 UINavigationBar 和 UIImageView 的 UIViewController。我放入 UIImageView 的图像大小为416 × 901 pixels。我正在尝试创建一个视图，使导航栏位于顶部，然后 UIImageView 固定在屏幕边缘和工具栏下方。但是，当我运行我的应用程序时，UIImage 似乎超出了导航栏。

这是来自 Xcode 的截图：

如您所见，UINavigationBar 被 UIImage 遮挡。当我删除图像时，布局看起来像我预期的那样（我注释掉 UIImage 分配image.image = [UIImage imageNamed:@"attachment.jpg"];，您可以在下面发布的代码中看到）：

我很困惑。我猜它与 UIImageView 中缺少的参数有关，或者我需要创建一个完全不同的约束，但我不确定。我已经粘贴了下面布局的代码。我很感激我能得到的任何帮助。

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

UINavigationBar *navigation = [[UINavigationBar alloc] initWithFrame:CGRectZero];
    navigation.translatesAutoresizingMaskIntoConstraints = NO;
    UINavigationItem* navigationItem = [[UINavigationItem alloc] initWithTitle:@"Edit"];
    UIBarButtonItem* cancelBtn = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemCancel target:self action:@selector(cancel:)];
    navigationItem.leftBarButtonItem = cancelBtn;

    UIBarButtonItem *doneBtn = [[UIBarButtonItem alloc] initWithTitle:@"Save" style:UIBarButtonItemStyleDone target:self action:@selector(save:)];
    navigationItem.rightBarButtonItem = doneBtn;

    [navigation setItems:@[navigationItem]];
    navigation.delegate = self;

    UIImageView *image = [[UIImageView alloc] init];
    image.contentMode = UIViewContentModeScaleAspectFit;
    image.backgroundColor = [UIColor redColor];
    image.image = [UIImage imageNamed:@"attachment.jpg"];
    image.translatesAutoresizingMaskIntoConstraints = NO;

    UIView *content = [[UIView alloc] initWithFrame:CGRectZero];
    content.translatesAutoresizingMaskIntoConstraints = NO;
    content.backgroundColor = [UIColor yellowColor];
    [content addSubview:image];
    [content addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-0-[image]-0-|"
                                                                      options:0
                                                                      metrics:nil
                                                                        views:NSDictionaryOfVariableBindings(image)]];
    [content addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-0-[image]-0-|"
                                                                      options:0
                                                                      metrics:nil
                                                                        views:NSDictionaryOfVariableBindings(image)]];


    [self.view addSubview:navigation];
    [self.view addSubview:content];
    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-[navigation]-0-[content]-0-|"
                                                                      options:0
                                                                      metrics:nil
                                                                        views:NSDictionaryOfVariableBindings(navigation, content)]];
    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-0-[navigation]-0-|"
                                                                      options:0
                                                                      metrics:nil
                                                                        views:NSDictionaryOfVariableBindings(navigation, content)]];
    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-0-[content]-0-|"
                                                                      options:0
                                                                      metrics:nil
                                                                        views:NSDictionaryOfVariableBindings(navigation, content)]];
}


- (UIBarPosition)positionForBar:(id <UIBarPositioning>)bar {
        return UIBarPositionTopAttached;
}

- (void)cancel:(id)button {}
- (void)save:(id)button {}

@end

我刚刚注意到 Xcode 中有一些关于布局的运行时警告。这是 Xcode 向我展示的内容：

我猜导航栏的高度是模棱两可的，因为 UIImageView 的高度没有很好的定义。但是在这种情况下，我试图固定图像的高度，期望导航的高度能够很好地定义。基本上，我依靠导航来定义 UIImageView 需要多少空间。也许我需要修复两个元素之一的高度？

Answer 1

添加自己的导航栏有点奇怪，因为它需要在设备旋转时调整大小。

像这样尝试（我不喜欢 VFL - 有时似乎有它自己的问题）：

@interface ViewController ()
@property (strong, nonatomic) NSLayoutConstraint *navBarHeightCon;
@end

@implementation ViewController

- (void)viewWillLayoutSubviews {
    [super viewWillLayoutSubviews];
    _navBarHeightCon.constant = (self.view.bounds.size.height > self.view.bounds.size.width)? 44 : 32;
}

- (void)viewDidLoad {
    [super viewDidLoad];

    UINavigationBar *navigation = [[UINavigationBar alloc] initWithFrame:CGRectZero];
    navigation.translatesAutoresizingMaskIntoConstraints = NO;
    UINavigationItem* navigationItem = [[UINavigationItem alloc] initWithTitle:@"Edit"];
    UIBarButtonItem* cancelBtn = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemCancel target:self action:@selector(cancel:)];
    navigationItem.leftBarButtonItem = cancelBtn;

    UIBarButtonItem *doneBtn = [[UIBarButtonItem alloc] initWithTitle:@"Save" style:UIBarButtonItemStyleDone target:self action:@selector(save:)];
    navigationItem.rightBarButtonItem = doneBtn;

    [navigation setItems:@[navigationItem]];
    navigation.delegate = self;

    UIImageView *image = [[UIImageView alloc] init];
    image.contentMode = UIViewContentModeScaleAspectFit;
    image.backgroundColor = [UIColor redColor];

    image.image = [UIImage imageNamed:@"attachment.jpg"];
    image.translatesAutoresizingMaskIntoConstraints = NO;

    UIView *content = [[UIView alloc] initWithFrame:CGRectZero];
    content.translatesAutoresizingMaskIntoConstraints = NO;
    content.backgroundColor = [UIColor yellowColor];

    [content addSubview:image];

    [self.view addSubview:navigation];
    [self.view addSubview:content];

    UILayoutGuide *g = self.view.safeAreaLayoutGuide;

    _navBarHeightCon = [navigation.heightAnchor constraintEqualToConstant:44.0];

    [NSLayoutConstraint activateConstraints:@[

        // constrain content view top to navigation bar bottom
        [content.topAnchor constraintEqualToAnchor:navigation.bottomAnchor],

        // constrain content view bottom / leading / trailing to safe area
        [content.bottomAnchor constraintEqualToAnchor:g.bottomAnchor],
        [content.leadingAnchor constraintEqualToAnchor:g.leadingAnchor],
        [content.trailingAnchor constraintEqualToAnchor:g.trailingAnchor],

        // constrain image top / bottom / leading / trailing to content view
        [image.topAnchor constraintEqualToAnchor:content.topAnchor],
        [image.bottomAnchor constraintEqualToAnchor:content.bottomAnchor],
        [image.leadingAnchor constraintEqualToAnchor:content.leadingAnchor],
        [image.trailingAnchor constraintEqualToAnchor:content.trailingAnchor],

        // constrain navigation bar top / leading / trailing to safe area
        [navigation.topAnchor constraintEqualToAnchor:g.topAnchor],
        [navigation.leadingAnchor constraintEqualToAnchor:g.leadingAnchor],
        [navigation.trailingAnchor constraintEqualToAnchor:g.trailingAnchor],

        // height constraint will be set in viewWillLayoutSubviews
        _navBarHeightCon,

    ]];

}

- (UIBarPosition)positionForBar:(id <UIBarPositioning>)bar {
    return UIBarPositionTopAttached;
}

- (void)cancel:(id)button {}
- (void)save:(id)button {}

@end

Answer 2

经过（过多）调查后，here 发布的解决方案对我有用。

具体

解决方法是将SuperView的Hugging Priority提高到High（750以上），将UIImageView的抗压优先级降低到Low（250以下）。这将使约束覆盖图像的固有大小。