设置活动时自动布局约束中断

Question

我有一组AL约束来定位一个子vc，它有两个位置，展开和折叠。

我发现，当我添加折叠约束时，带有常量的上锚到下锚约束，当第一次创建vc时，当我激活它时，似乎有额外的间距。似乎是因为当时没有实际的高度。

当我在viewDidLayoutSubviews中添加约束时，额外的间距消失了，约束行为正常。除了现在在动画中的约束之间切换时，切换到展开约束和约束打断时，无法停用折叠的约束。可能是因为在整个过渡动画中调用viewDidLayoutSubviews。

下面是vc设置的摘要。

var foregroundExpandedConstraint: NSLayoutConstraint!
var foregroundCollapsedConstraint: NSLayoutConstraint!

var foregroundViewController: UIViewController? {
    didSet {

        setupforegroundViewController(foregroundViewController: foregroundViewController!)
    }
}

func setupforegroundViewController(foregroundViewController: UIViewController) {

    addChildViewController(foregroundViewController)
    foregroundViewController.didMove(toParentViewController: self)

    guard let foregroundView = foregroundViewController.view else { return }
    foregroundView.translatesAutoresizingMaskIntoConstraints = false
    view.addSubview(foregroundView)

    foregroundExpandedConstraint = foregroundView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 15)

    let height =  view.safeAreaLayoutGuide.layoutFrame.height - 50 - 15
    let cellHeight = ((height) / 6)        
    foregroundCollapsedConstraint = NSLayoutConstraint(item: foregroundView, attribute: .top, relatedBy: .equal, toItem: view.safeAreaLayoutGuide, attribute: .bottom, multiplier: 1, constant: (-cellHeight) * 2 - 50)

    let foregroundViewControllerViewConstraints = [
        foregroundView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
        foregroundView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
        foregroundView.heightAnchor.constraint(equalTo: view.safeAreaLayoutGuide.heightAnchor, constant: -50 - 15),
        foregroundExpandedConstraint!
        ]


    NSLayoutConstraint.activate(foregroundViewControllerViewConstraints)
}

这里的动画是使用UIViewProperty tyAnimator预先形成的。

func animateTransitionIfNeeded(state: ForegroundState, duration: TimeInterval) {

    let containerFrameAnimator = UIViewPropertyAnimator(duration: duration, dampingRatio: 1) {
        [unowned self] in

        switch state {
        case .expanded:
            self.foregroundCollapsedConstraint?.isActive = false
            self.foregroundExpandedConstraint?.isActive = true
            self.view.layoutIfNeeded()
        case .collapsed:
            self.foregroundExpandedConstraint?.isActive = false
            self.foregroundCollapsedConstraint?.isActive = true
            self.view.layoutIfNeeded()
        }
    }

    containerFrameAnimator.addCompletion {  [weak self] (position) in

        if position == .start {
            switch state {
            case .collapsed:
                self?.foregroundCollapsedConstraint?.isActive = false
                self?.foregroundExpandedConstraint?.isActive = true
                self?.foregroundIsExpanded = true
                self?.view.layoutIfNeeded()
            case .expanded:
                self?.foregroundExpandedConstraint?.isActive = false
                self?.foregroundCollapsedConstraint?.isActive = true
                self?.foregroundIsExpanded = false
                self?.view.layoutIfNeeded()
            }
        } else if position == .end {
            switch state {
            case .collapsed:
                self?.foregroundExpandedConstraint?.isActive = false
                self?.foregroundCollapsedConstraint?.isActive = true
                self?.foregroundIsExpanded = false
            case .expanded:
                self?.foregroundExpandedConstraint?.isActive = false
                self?.foregroundCollapsedConstraint?.isActive = true
                self?.foregroundIsExpanded = true
            }
        }
        self?.runningAnimations.removeAll()
    }

再次重申，当我使用以下代码时，在vc添加到视图层次结构时设置约束，它不会正确布局。检查约束我看到它们在调用view Dod layout子视图后发生更改。除折叠的约束外，每个约束都会相应更改。

当我在视图did layout子视图中添加折叠的约束时，它的行为正常，但我无法继续停用它，约束会中断。

override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()

    let height =  view.safeAreaLayoutGuide.layoutFrame.height - 50 - 15
    let cellHeight = ((height) / 6)

    if let v = foregroundViewController?.view {
        foregroundCollapsedConstraint = NSLayoutConstraint(item: v, attribute: .top, relatedBy: .equal, toItem: view.safeAreaLayoutGuide, attribute: .bottom, multiplier: 1, constant: (-cellHeight) * 2 - 50)
    }
}

编辑：我创建了一份回购协议，演示了该问题：https://github.com/louiss98/UIViewPropertyAnimator-Layout-Test

有什么建议吗？

Answer 1

通过更改常量而不是创建新约束，可以消除“断开”约束。

在viewdilayoutsubviews（）中，

改变：

override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()

    let height =  view.safeAreaLayoutGuide.layoutFrame.height - 50 - 15
    let cellHeight = ((height) / 6)

    foregroundCollapsedConstraint = NSLayoutConstraint(item: testViewController.view, attribute: .top, relatedBy: .equal, toItem: view.safeAreaLayoutGuide, attribute: .bottom, multiplier: 1, constant: (-cellHeight) * 2 - 50)
}

收件人：

override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()

    let height =  view.safeAreaLayoutGuide.layoutFrame.height - 50 - 15
    let cellHeight = ((height) / 6)

    foregroundCollapsedConstraint.constant = (-cellHeight) * 2 - 50
}