动画约束相对于另一个约束

Question

我设置了一个动画，以在打开另一个开关/标签时隐藏一个开关/标签。同时，刚刚打开的开关向上移动。这里的简单解释非常有效。

但是，当我在开关/标签关闭后尝试向下移动时，它不会移动。另一个开关会很好地重新出现，但不会触发顶部约束更改。

我对这种类型的设置和动画都是以编程方式进行的相对较新，在花了一个小时之后，我被难住了。是因为我正在制作相对于另一个顶部约束的动画吗？如果它第一次工作，这有什么关系？即使隐藏开关的alpha设置为零，它的框架仍然存在，对吗？还是我愚蠢地做了一些简单的事情？

// Works Perfectly!

func hideVeg() {

    self.view.layoutIfNeeded()

    UIView.animate(withDuration: 1, delay: 0, options: [.curveEaseIn], animations: {
        self.vegetarianSwitch.alpha = 0
        self.vegetarianLabel.alpha = 0
        self.veganSwitch.topAnchor.constraint(equalTo: self.vegetarianSwitch.bottomAnchor, constant: -30).isActive = true
        self.view.layoutIfNeeded()
    })
}

// Showing the label and switch works, but the topAnchor constraint never changes!

func showVeg() {

    self.view.layoutIfNeeded()

    UIView.animate(withDuration: 1, delay: 0, options: [.curveEaseIn], animations: {
        self.vegetarianSwitch.alpha = 1
        self.vegetarianLabel.alpha = 1

        // This is the constraint that doesn't change.
        // This is exactly what it was set to before the other hideVeg() runs.
        self.veganSwitch.topAnchor.constraint(equalTo: self.vegetarianSwitch.bottomAnchor, constant: 40).isActive = true
        self.view.layoutIfNeeded()
    })
}

Answer 1

常量需要随着动画而改变，而不是创建一个全新的约束。旧的约束仍然存在，导致了这个问题。

var veganTopConstraint = NSLayoutConstraint()

// Top Constraint set up this way so it can be animated later.
veganTopConstraint = veganSwitch.topAnchor.constraint(equalTo: vegetarianSwitch.bottomAnchor, constant: 40)
veganTopConstraint.isActive = true


func hideVeg() {

    UIView.animate(withDuration: 1, delay: 0, options: [.curveEaseIn], animations: {
        self.vegetarianSwitch.alpha = 0
        self.vegetarianLabel.alpha = 0

        self.veganTopConstraint.constant = -30

        self.view.layoutIfNeeded()
    })
}

func showVeg() {

    self.view.layoutIfNeeded()

    UIView.animate(withDuration: 1, delay: 0, options: [.curveEaseIn], animations: {
        self.vegetarianSwitch.alpha = 1
        self.vegetarianLabel.alpha = 1

        self.veganTopConstraint.constant = 40

        self.view.layoutIfNeeded()
    })
}

Answer 2

这里的问题是，您不是在修改约束，而是在每个动画中实际创建新的约束。相反，您只需创建一次约束（可以在代码中或在Interface Builder中进行，然后拖动并导出）。然后您只需更改。动画块中现有约束的常量字段。