如何以编程方式设置内容拥抱和压缩

Question

当我的一个视图被隐藏时，我想动画隐藏显示。所以我使用内容拥抱优先级来制作动画，但它失败了，它在视图之间有差距。在这里，我向您展示 ui 和我的代码

这是 3 ui如上图查看代码

    scrollView.addSubview(chooseScheduleDropDown)
        chooseScheduleDropDown.translatesAutoresizingMaskIntoConstraints = false
        chooseScheduleDropDown.setContentCompressionResistancePriority(.required, for: .vertical)
        NSLayoutConstraint.activate([
            chooseScheduleDropDown.centerXAnchor.constraint(equalTo: scrollView.centerXAnchor),
            chooseScheduleDropDown.topAnchor.constraint(equalTo: scrollView.topAnchor),
            chooseScheduleDropDown.widthAnchor.constraint(equalToConstant: 285),
            chooseScheduleDropDown.heightAnchor.constraint(equalToConstant: 60)
        ])

        scrollView.addSubview(entryView)
        entryView.isHidden = true
        entryView.setContentHuggingPriority(.defaultLow, for: .vertical)
        entryView.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            entryView.centerXAnchor.constraint(equalTo: scrollView.centerXAnchor),
            entryView.topAnchor.constraint(equalTo: chooseScheduleDropDown.bottomAnchor, constant: topPadding),
            entryView.widthAnchor.constraint(equalToConstant: 285),
            entryView.heightAnchor.constraint(equalToConstant: 60)
        ])

        scrollView.addSubview(chooseDateView)
        chooseDateView.setContentHuggingPriority(.defaultLow, for: .vertical)
        chooseDateView.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            chooseDateView.centerXAnchor.constraint(equalTo: scrollView.centerXAnchor),
            chooseDateView.topAnchor.constraint(equalTo: entryView.bottomAnchor, constant: topPadding),
            chooseDateView.widthAnchor.constraint(equalToConstant: 285),
            chooseDateView.heightAnchor.constraint(equalToConstant: 60)
        ])

Answer 1

做

// declare an instance property 
var hCon:NSLayoutConstraint! 

// get only the height constraint out of the activate block
hCon = entryView.heightAnchor.constraint(equalToConstant: 60) 
hCon.isActive = true

和

一起玩

hCon.constant = 300 / 0
view.layoutIfNeeded()

Answer 2

交换意见后，你们有许多不同的任务要做。

但是，给你一个显示/隐藏 "middle" 视图和底部的方法的例子查看向上/向下移动，这里有一些尝试。它看起来像这样：

点击顶部（红色）视图将隐藏中间（绿色）视图并向上滑动底部（蓝色）视图。再次点击顶部（红色）视图将向下滑动底部（蓝色）视图并显示中间（绿色）视图。

这是通过为底部视图创建两个顶部约束来完成的。一个相对于 Top 视图的底部，另一个相对于 Middle 视图的底部，具有不同的 .priority 值。

示例代码相当简单，注释应该清楚。全部通过代码完成 - 没有 @IBOutlet 或 @IBAction 连接 - 所以只需创建一个新的视图控制器并将其自定义 class 分配给 AnimTestViewController:

class DropDownView: UIView {

}

class AnimTestViewController: UIViewController {

    let scrollView: UIScrollView = {
        let v = UIScrollView()
        return v
    }()

    let chooseScheduleDropDown: DropDownView = {
        let v = DropDownView()
        return v
    }()

    let entryView: DropDownView = {
        let v = DropDownView()
        return v
    }()

    let chooseDateView: DropDownView = {
        let v = DropDownView()
        return v
    }()

    var visibleConstraint: NSLayoutConstraint = NSLayoutConstraint()
    var hiddenConstraint: NSLayoutConstraint = NSLayoutConstraint()

    override func viewDidLoad() {
        super.viewDidLoad()

        [chooseScheduleDropDown, entryView, chooseDateView].forEach {
            v in
            v.translatesAutoresizingMaskIntoConstraints = false
            scrollView.addSubview(v)
        }

        scrollView.translatesAutoresizingMaskIntoConstraints = false

        view.addSubview(scrollView)

        let g = view.safeAreaLayoutGuide

        let topPadding: CGFloat = 20.0

        // chooseDateView top anchor when entryView is visible
        visibleConstraint = chooseDateView.topAnchor.constraint(equalTo: entryView.bottomAnchor, constant: topPadding)

        // chooseDateView top anchor when entryView is hidden
        hiddenConstraint = chooseDateView.topAnchor.constraint(equalTo: chooseScheduleDropDown.bottomAnchor, constant: topPadding)

        // we will start with entryView visible
        visibleConstraint.priority = .defaultHigh
        hiddenConstraint.priority = .defaultLow

        NSLayoutConstraint.activate([

            scrollView.topAnchor.constraint(equalTo: g.topAnchor, constant: 40.0),
            scrollView.bottomAnchor.constraint(equalTo: g.bottomAnchor, constant: -40.0),
            scrollView.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 40.0),
            scrollView.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -40.0),

            chooseScheduleDropDown.centerXAnchor.constraint(equalTo: scrollView.centerXAnchor),
            chooseScheduleDropDown.topAnchor.constraint(equalTo: scrollView.topAnchor),
            chooseScheduleDropDown.widthAnchor.constraint(equalToConstant: 285),
            chooseScheduleDropDown.heightAnchor.constraint(equalToConstant: 60),

            entryView.centerXAnchor.constraint(equalTo: scrollView.centerXAnchor),
            entryView.topAnchor.constraint(equalTo: chooseScheduleDropDown.bottomAnchor, constant: topPadding),
            entryView.widthAnchor.constraint(equalToConstant: 285),
            entryView.heightAnchor.constraint(equalToConstant: 60),

            chooseDateView.centerXAnchor.constraint(equalTo: scrollView.centerXAnchor),

            //chooseDateView.topAnchor.constraint(equalTo: entryView.bottomAnchor, constant: topPadding),
            visibleConstraint,
            hiddenConstraint,

            chooseDateView.widthAnchor.constraint(equalToConstant: 285),
            chooseDateView.heightAnchor.constraint(equalToConstant: 60),

        ])

        //entryView.isHidden = true

        chooseScheduleDropDown.backgroundColor = .red
        entryView.backgroundColor = .green
        chooseDateView.backgroundColor = .blue

        let tap = UITapGestureRecognizer(target: self, action: #selector(toggleEntryView(_:)))
        chooseScheduleDropDown.addGestureRecognizer(tap)

    }

    @objc func toggleEntryView(_ sender: UITapGestureRecognizer) -> Void {
        print("tapped")

        // if entryView IS hidden we want to
        //  un-hide entryView
        //  animate alpha to 1.0
        //  animate chooseDateView down

        // if entryView is NOT hidden we want to
        //  animate alpha to 0.0
        //  animate chooseDateView up
        //  hide entryView when animation is finished

        let animSpeed = 0.5

        if entryView.isHidden {

            entryView.isHidden = false
            hiddenConstraint.priority = .defaultLow
            visibleConstraint.priority = .defaultHigh

            UIView.animate(withDuration: animSpeed, animations: {
                self.entryView.alpha = 1.0
                self.view.layoutIfNeeded()
            }, completion: { _ in
            })

        } else {

            visibleConstraint.priority = .defaultLow
            hiddenConstraint.priority = .defaultHigh

            UIView.animate(withDuration: animSpeed, animations: {
                self.entryView.alpha = 0.0
                self.view.layoutIfNeeded()
            }, completion: { _ in
                self.entryView.isHidden = true
            })

        }

    }

}

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