有没有办法在 xib 文件中的视图和顶部布局指南之间添加约束？

Question

在 iOS 7 中，我们现在可以在视图和顶部布局指南之间添加一个约束，我认为这对于解决 iOS7 中的状态栏偏移问题非常有用（尤其是当视图中没有导航栏时）。

在故事板文件中，我可以轻松添加这种约束。只需按住控制键，然后将视图拖到容器中，它将显示“顶部空间到顶部布局指南”选项。

但是当我在一个 xib 文件中做同样的操作时，这个选项就消失了。

那么，有没有办法在 xib 文件中添加这种约束？还是我必须用代码添加它们？

Answer 1

Cao Huu Loc's answer with swift 4

extension NSLayoutConstraint {

    func constraintReplacing(firstItemWith newFirstItem: UILayoutSupport, withAttribute newFirstAttribute: NSLayoutAttribute) -> NSLayoutConstraint {
        return NSLayoutConstraint(item: newFirstItem, attribute: newFirstAttribute, relatedBy: relation, toItem: secondItem, attribute: secondAttribute, multiplier: multiplier, constant: constant)
    }

    func constraintReplacing(secondItemWith newSecondItem: UILayoutSupport, withAttribute newSecondAttribute: NSLayoutAttribute) -> NSLayoutConstraint {
        return NSLayoutConstraint(item: firstItem as Any, attribute: firstAttribute, relatedBy: relation, toItem: newSecondItem, attribute: newSecondAttribute, multiplier: multiplier, constant: constant)
    }
}

class YourViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        for constraint in view.constraints {
            if constraint.firstItem === view && constraint.firstAttribute == .top {
                let newConstraint = constraint.constraintReplacing(firstItemWith: topLayoutGuide, withAttribute: .bottom)
                view.removeConstraint(constraint)
                view.addConstraint(newConstraint)
            }
            if constraint.secondItem === view && constraint.secondAttribute == .top {
                let newConstraint = constraint.constraintReplacing(secondItemWith: topLayoutGuide, withAttribute: .bottom)
                view.removeConstraint(constraint)
                view.addConstraint(newConstraint)
            }
        }
    }
}

Answer 2

从 iOS 9 开始，您还可以使用 topLayoutGuide 的 anchors 更简单：

• 斯威夫特 3

view.topAnchor.constraint(equalTo: self.topLayoutGuide.bottomAnchor).isActive = true

• ObjC

[controller.view.topAnchor constraintEqualToAnchor:controller.topLayoutGuide.bottomAnchor].active = YES;

Answer 3

当然，您不仅可以通过编程方式在视图和top layout guide 或bottom layout guide 之间添加约束，还可以在KVConstraintExtensionsMaster 库的帮助下删除和访问视图和top and bottom layout guide 之间的约束.

// create containerView
UIView *containerView = [UIView prepareNewViewForAutoLayout];
[containerView setBackgroundColor:[UIColor brownColor]];
[self.view addSubview:containerView];

// To add Top and Bottom Layout Guide constraint of containerView
[self applyTopLayoutGuideConstraintToView:containerView withPadding:0];
[self applyBottomLayoutGuideConstraintToView:containerView withPadding:50];

// To access top Layout Guide Constraint and update it's constant value.
NSLayoutConstraint *topLayoutGuideConstraint = [self accessAppliedTopLayoutGuideConstraintFromView:containerView];
topLayoutGuideConstraint.constant = 50;

// To access bottom Layout Guide Constraint and update it's constant value with animation
NSLayoutConstraint *bottomLayoutGuideConstraint = [self accessAppliedBottomLayoutGuideConstraintFromView:containerView];
bottomLayoutGuideConstraint.constant = 80;
[self.view updateModifyConstraintsWithAnimation:NULL]; // call this for animation

// To remove Top and Bottom Layout Guide constraint of containerView
[self removeAppliedTopLayoutGuideConstraintFromView:containerView];
[self removeAppliedBottomLayoutGuideConstraintFromView:containerView ];

Answer 4

这是我的替代解决方案。

找到一个 UIView 作为枢轴，将其顶部布局约束设置为容器顶部的固定垂直空间。

Control-拖动这个布局约束为一个IBOutlet，比如

@property (weak, nonatomic) IBOutlet NSLayoutConstraint *topLayoutConstraint;

最后，只需像下面这样覆盖 UIViewController 的 viewWillLayoutSubviews 方法

- (void)viewWillLayoutSubviews
{
    [super viewWillLayoutSubviews];

    self.topLayoutConstraint.constant = [self.topLayoutGuide length] + YOUR_TOP_CONSTRSINT;
}

所有其他视图的顶部约束都基于此枢轴视图，全部完成:)

Answer 5

直到现在，即使使用 XCode 6，我也无法将控件与 .xib 文件的顶部布局指南对齐。相反，我正在使用另一种方式。

首先，在界面生成器上，我仍然将控件对齐到 viewcontroler 视图的上边框。

然后，在 viewDidLoad 方法中，我替换了一些约束，以便它们与顶部布局指南而不是主视图对齐：

- (void)viewDidLoad
{
    [super viewDidLoad];

    NSArray *constraints = self.view.constraints;
    for (NSLayoutConstraint *constraint in constraints) {
        if ( (constraint.firstItem == self.view) && (constraint.firstAttribute == NSLayoutAttributeTop) ) {
            NSLayoutConstraint *newConstraint = [self constraint:constraint replaceFirstItemBy:self.topLayoutGuide attribute:NSLayoutAttributeBottom];
            [self.view removeConstraint:constraint];
            [self.view addConstraint:newConstraint];
        } else if ( (constraint.secondItem == self.view) && (constraint.secondAttribute == NSLayoutAttributeTop) ) {
            NSLayoutConstraint *newConstraint = [self constraint:constraint replaceSecondItemBy:self.topLayoutGuide attribute:NSLayoutAttributeBottom];
            [self.view removeConstraint:constraint];
            [self.view addConstraint:newConstraint];
        }
    }
}

- (NSLayoutConstraint*)constraint:(NSLayoutConstraint*)constraint replaceFirstItemBy:(id)newItem attribute:(NSLayoutAttribute)newAttribute {
    UILayoutPriority priority = constraint.priority;
    NSLayoutRelation relation = constraint.relation;
    id secondItem = constraint.secondItem;
    NSLayoutAttribute secondAttribute = constraint.secondAttribute;
    CGFloat multiplier = constraint.multiplier;
    CGFloat constant = constraint.constant;

    NSLayoutConstraint *newConstraint = [NSLayoutConstraint constraintWithItem:newItem attribute:newAttribute relatedBy:relation toItem:secondItem attribute:secondAttribute multiplier:multiplier constant:constant];
    newConstraint.priority = priority;
    return newConstraint;
}

- (NSLayoutConstraint*)constraint:(NSLayoutConstraint*)constraint replaceSecondItemBy:(id)newItem attribute:(NSLayoutAttribute)newAttribute {
    UILayoutPriority priority = constraint.priority;
    id firstItem = constraint.firstItem;
    NSLayoutAttribute firstAttribute = constraint.firstAttribute;
    NSLayoutRelation relation = constraint.relation;
    CGFloat multiplier = constraint.multiplier;
    CGFloat constant = constraint.constant;

    NSLayoutConstraint *newConstraint = [NSLayoutConstraint constraintWithItem:firstItem attribute:firstAttribute relatedBy:relation toItem:newItem attribute:newAttribute multiplier:multiplier constant:constant];
    newConstraint.priority = priority;
    return newConstraint;
}

认为这不是最好的方法，因为我们替换了我们在界面生成器上定义的对象。但这可能是我们可以考虑的另一种方式。

Answer 6

我认为它们没有出现在 XIB 中的原因是在这种情况下不了解视图控制器层次结构，而在故事板中，IB 可以从视图控制器层次结构中分辨出指南的位置在。即如果包含在一个UINavigationController中，它可以确定顶部布局指南在导航工具栏的下方。

Answer 7

您应该参考以下示例，这肯定会帮助您解决问题。我是从http://developer.apple.com 那里得到的。

[button setTranslatesAutoresizingMaskIntoConstraints: NO];
id topGuide = myViewController.topLayoutGuide;
NSDictionary *viewsDictionary = NSDictionaryOfVariableBindings (button, topGuide);

[myViewController.view addConstraints:
    [NSLayoutConstraint constraintsWithVisualFormat: @"V:[topGuide]-20-[button]"
    options: 0
    metrics: nil
    views: viewsDictionary]
];