Swift - 以编程方式更改约束

Question

我正在尝试通过以编程方式操作约束来“显示”和“隐藏”集合视图。

我的应用是用代码编写的，没有使用故事板或@IBOutlets。

第一次按下按钮时，集合视图按预期正确显示。

我第二次按下按钮时，集合视图只是停留在原地，并没有“隐藏”。

openMenu 代码中的打印语句确认正在调用每个约束块。即：我收到“打开”和“关闭”的控制台消息。

创建集合视图没有问题，只是以编程方式设置约束不会关闭菜单。

我的代码如下...

override func viewDidLoad() {
    super.viewDidLoad()

    navigationController?.isNavigationBarHidden = false

    view.backgroundColor = .white

    view.addSubview(bgImageView)

    view.addSubview(myListCV)
}

lazy var myListCV: UICollectionView = {

    let myListLayout = UICollectionViewFlowLayout()
    myListLayout.itemSize = CGSize(width: 200, height: 40)
    myListLayout.minimumLineSpacing = 1
    myListLayout.sectionHeadersPinToVisibleBounds = true

    let myListView = UICollectionView(frame: .zero, collectionViewLayout: myListLayout)
    myListView.translatesAutoresizingMaskIntoConstraints = false
    myListView.delegate = self
    myListView.dataSource = self

    myListView.bounces = false
    myListView.alwaysBounceVertical = false
    myListView.showsVerticalScrollIndicator = true
    myListView.backgroundColor = UIColor(r: 203, g: 203, b: 203)

    return myListView
}()

var menuShowing = false

func openMenu() {

    if (menuShowing) {
        print("closed")
        myListCV.leftAnchor.constraint(equalTo: view.rightAnchor).isActive = true
        myListCV.topAnchor.constraint(equalTo: view.topAnchor, constant: 64).isActive = true
        myListCV.rightAnchor.constraint(equalTo: view.rightAnchor).isActive = true
        myListCV.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true

    } else {
        print("open")
        myListCV.leftAnchor.constraint(equalTo: view.rightAnchor, constant: -200).isActive = true
        myListCV.topAnchor.constraint(equalTo: view.topAnchor, constant: 64).isActive = true
        myListCV.rightAnchor.constraint(equalTo: view.rightAnchor).isActive = true
        myListCV.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
    }

    menuShowing = !menuShowing
}

Answer 1

上述代码的问题在于，每次用户打开或关闭该部分时，您都会不断设置约束，因此根据用户执行此操作的次数，您最终会得到数百个不存在的约束需要。

您应该做的是设置默认状态的约束，我假设在这种情况下关闭，并将您希望更改的约束存储在属性中。然后你可以简单地调整这个约束的常数来显示/隐藏你的菜单。

例如

private var myListCVLeftConstraint: NSLayoutConstraint?

override func viewDidLoad() {
    super.viewDidLoad()

    navigationController?.isNavigationBarHidden = false

    view.backgroundColor = .white

    view.addSubview(bgImageView)

    self.configMyListCV()
}

lazy var myListCV: UICollectionView = {

    let myListLayout = UICollectionViewFlowLayout()
    myListLayout.itemSize = CGSize(width: 200, height: 40)
    myListLayout.minimumLineSpacing = 1
    myListLayout.sectionHeadersPinToVisibleBounds = true

    let myListView = UICollectionView(frame: .zero, collectionViewLayout: myListLayout)
    myListView.translatesAutoresizingMaskIntoConstraints = false
    myListView.delegate = self
    myListView.dataSource = self

    myListView.bounces = false
    myListView.alwaysBounceVertical = false
    myListView.showsVerticalScrollIndicator = true
    myListView.backgroundColor = UIColor(r: 203, g: 203, b: 203)

    return myListView
}()

var menuShowing = false

private func configMyListCV() -> Void {

    view.addSubview(myListCV)

    self.myListCV.topAnchor.constraint(equalTo: view.topAnchor, constant: 64).isActive = true
    self.myListCV.rightAnchor.constraint(equalTo: view.rightAnchor).isActive = true
    self.myListCV.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true

    self.myListCVLeftConstraint = myListCV.leftAnchor.constraint(equalTo: view.rightAnchor)
    self.myListCVLeftConstraint.isActive = true
}

func openMenu() {

    if (menuShowing) {

        self.myListCVLeftConstraint?.constant = 0.0

    } else {

        self.myListCVLeftConstraint?.constant = -200.0

    }

    menuShowing = !menuShowing
}

这对于这个简单的用户案例很有效；但是，如果您将来做更深入的事情，我建议您在视图上设置多个约束，并根据需要简单地禁用/启用所需的约束。