iphone - NSLayoutConstraints 代码使 View 居中并保持其纵横比

Question

我希望我的 subview 是一个以其父 View 顶部为中心的 16:9 矩形。换句话说，我希望它:

1. 与其父 View 一样宽，但不超过 400 像素(UI 可以旋转为横向)，
2. 当它比父 View 窄时水平居中，
3. 将其顶部固定到其父 View 的顶部，并且
4. 更改其高度以保持 16:9 的纵横比。

这段代码几乎做到了，除了我很难使水平约束起作用并且没有超过或低于约束...

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    
    UIView *contentView = [[UIView alloc] init];
    contentView.backgroundColor = [UIColor redColor];
    [self.view addSubview:contentView];
    
    contentView.translatesAutoresizingMaskIntoConstraints = NO;
    
    NSDictionary *views = NSDictionaryOfVariableBindings(contentView);
    NSMutableArray *constraints = [NSMutableArray array];

    // this layout string is more like 'wishful coding'.  I don't see why it wouldn't work
    // but clearly this one is the problem
    [constraints addObjectsFromArray:[NSLayoutConstraint
                                      constraintsWithVisualFormat:@"H:|-(>=0)-[contentView(<=400)-(>=0)-]"
                                      options:0 metrics:0 views:views]];
    
    // this centering constraint below almost does the job, but doesn't give me a way
    // to specify width, changing the one above to just @"H:[contentView(<=400)]"
    // doesn't work either
    [constraints addObject:
     [NSLayoutConstraint constraintWithItem:contentView
                                  attribute:NSLayoutAttributeCenterY
                                  relatedBy:NSLayoutRelationEqual
                                     toItem:self.view
                                  attribute:NSLayoutAttributeCenterY
                                 multiplier:1.f constant:0.f]];

    // 9:16 works fine, I think
    [constraints addObject:
     [NSLayoutConstraint constraintWithItem:contentView
                                  attribute:NSLayoutAttributeHeight
                                  relatedBy:NSLayoutRelationEqual
                                     toItem:contentView attribute:NSLayoutAttributeWidth
                                 multiplier:9.0/16.0 constant:0.0]];
    
    // pin the tops works fine, I think
    [constraints addObjectsFromArray:
     [NSLayoutConstraint constraintsWithVisualFormat:@"V:|-0-[contentView]"
                                             options:0 metrics:0 views:views]];

    [self.view addConstraints:constraints];
}

Answer 1

如果你想使红色框水平居中，那么你想等同于 View 的 CenterX，而不是 CenterY。因此该约束应如下所示:

[NSLayoutConstraint constraintWithItem:contentView
                             attribute:NSLayoutAttributeCenterX
                             relatedBy:NSLayoutRelationEqual
                                toItem:self.view
                             attribute:NSLayoutAttributeCenterX
                            multiplier:1.f constant:0.f]];

然后在第一个约束上，你的约束是不明确的，因为有不止一种方法可以满足 >=0每边的边距和<=400在宽度上。更好的方法是准确地说出您在问题中所说的内容，即您需要宽度为 <=400 并且您希望边距为 0如果可能的话。

所以像这样:

[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-(0@900)-[contentView(<=400)]-(0@900)-|"
                                        options:0 metrics:0 views:views]];

我相信这会得到您想要的？