动画时如何以编程方式禁用约束

Question

我有一个固定宽度固定在左侧的视图。我希望这个视图通过将其锚定到右侧（从而移除左锚）来设置动画（从左到右移动）。我该怎么做呢？

我尝试设置优先级，但我不确定语法。我也尝试禁用约束，但没有奏效。

fileprivate func sparkle() {
    let sparkleView = UIView()
    sparkleView.backgroundColor = UIColor.yellow
    sparkleView.alpha = 0.5
    addSubview(sparkleView)
    sparkleView.translatesAutoresizingMaskIntoConstraints = false
    sparkleView.topAnchor.constraint(equalTo: topAnchor).isActive = true
    sparkleView.bottomAnchor.constraint(equalTo: bottomAnchor).isActive = true
    sparkleView.widthAnchor.constraint(equalToConstant: frame.width / 5).isActive = true
    sparkleView.leftAnchor.constraint(equalTo: leftAnchor).priority = UILayoutPriorityDefaultLow // Doesn't work
    sparkleView.leftAnchor.constraint(equalTo: leftAnchor).isActive = true
    layoutIfNeeded()
    sparkleView.leftAnchor.constraint(equalTo: leftAnchor).isActive = false // Doesn't work
    sparkleView.rightAnchor.constraint(equalTo: self.rightAnchor).isActive = true
    UIView.animate(withDuration: 2, animations: {
        self.layoutIfNeeded()
    }) { (_) in
        sparkleView.removeFromSuperview()
    }
}

目前，由于我们分配了左右锚点，因此宽度限制被打破。

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x6000020b8eb0 UIView:0x7fc846d551d0.width == 15.35   (active)>

-- 编辑--

我已经尝试创建一个实例变量来保存约束。

let leftCons = sparkleView.leftAnchor.constraint(equalTo: leftAnchor)
leftCons.isActive = true

然后我尝试修改优先级

leftCons.priority = UILayoutPriorityDefaultLow

但是，这会导致错误

'Mutating a priority from required to not on an installed constraint (or vice-versa) is not supported.  You passed priority 250 and the existing priority was 1000.'

相反，我只需将其设置为非活动状态（来自答案）

leftCons.active = false

-- 如果有兴趣，请更正代码..--

fileprivate func sparkle() {
    let sparkleView = UIView()
    sparkleView.backgroundColor = UIColor.yellow
    sparkleView.alpha = 0.5
    addSubview(sparkleView)
    sparkleView.translatesAutoresizingMaskIntoConstraints = false
    sparkleView.topAnchor.constraint(equalTo: topAnchor).isActive = true
    sparkleView.bottomAnchor.constraint(equalTo: bottomAnchor).isActive = true
    sparkleView.widthAnchor.constraint(equalToConstant: frame.width / 5).isActive = true
    let leftCons = sparkleView.leftAnchor.constraint(equalTo: leftAnchor)
    leftCons.isActive = true
    layoutIfNeeded()
    leftCons.isActive = false
    sparkleView.rightAnchor.constraint(equalTo: self.rightAnchor).isActive = true
    UIView.animate(withDuration: 0.5, animations: {
        self.layoutIfNeeded()
    }) { (_) in
        sparkleView.removeFromSuperview()
    }
}

Answer 1

你需要创建一个变量

var leftCon:NSLayoutConstraint!

然后改变它，你的问题是每一行都创建一个新的约束

sparkleView.widthAnchor.constraint(equalToConstant: frame.width / 5).isActive = true
sparkleView.leftAnchor.constraint(equalTo: leftAnchor).isActive = true
sparkleView.rightAnchor.constraint(equalTo: self.rightAnchor).isActive = true

注意：以上 3 个约束会产生冲突，因为不能同时将视图固定在左右两侧，而宽度约束正是您看到冲突中断的原因 => NSLayoutConstraint:0x6000020b8eb0 UIView:0x7fc846d551d0.width == 15.35（活动）

不管活动的分配，所以那些

sparkleView.leftAnchor.constraint(equalTo: leftAnchor).priority = UILayoutPriorityDefaultLow // Doesn't work 
sparkleView.leftAnchor.constraint(equalTo: leftAnchor).isActive = false // Doesn't work

在运行中并且不改变创建的约束，所以是

leftCon = sparkleView.leftAnchor.constraint(equalTo: leftAnchor)
leftCon.isActive = true

---

关于这次崩溃

'不支持在已安装的约束上将优先级从需要更改为不设置（反之亦然）

激活前需要设置优先级

leftCon.priority = UILayoutPriorityDefaultLow
leftCon.isActive = true