动画约束时无法同时满足约束答案

【问题标题】：Unable to simultaneously satisfy constraints when animating constraint动画约束时无法同时满足约束
【发布时间】：2018-10-19 18:32:06
【问题描述】：

这是我的代码：

    class AvailableTrainScene: UIView {
          let seeMoreButton: UIButton = {
        let v = UIButton()
        v.translatesAutoresizingMaskIntoConstraints = false
        v.backgroundColor = .blue
        v.layer.cornerRadius = 20
        v.setTitle("Voir Plus", for: .normal)
        return v
    }()

    var seeMoreBottom: NSLayoutConstraint?

    override init(frame: CGRect) {
        super.init(frame: frame)
        addSubview(seeMoreButton)
    }

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }

    override func layoutSubviews() {
        NSLayoutConstraint.activate([
            seeMoreButton.heightAnchor.constraint(equalToConstant: 40),
            seeMoreButton.widthAnchor.constraint(equalTo: widthAnchor, multiplier: 0.8),
            seeMoreButton.centerXAnchor.constraint(equalTo: centerXAnchor)
            ])
            seeMoreBottom = seeMoreButton.bottomAnchor.constraint(equalTo: bottomAnchor, constant: -8)
        seeMoreBottom?.isActive = false
    }

    func hideSeeMore(_ hide: Bool) {
        if hide {
            seeMoreBottom?.constant = 40
        }else{
            seeMoreBottom?.constant = -8
        }
        UIView.animate(withDuration: 0.4, delay: 0, usingSpringWithDamping: 0.6, initialSpringVelocity: 4, options: .curveEaseOut, animations: {
            self.layoutIfNeeded()
        }, completion: nil)
    }
}

使用函数 hideSeeMore 制作动画时出现此错误：

LayoutConstraints] Unable to simultaneously satisfy constraints.
    Probably at least one of the constraints in the following list is one you don't want. 
    Try this: 
        (1) look at each constraint and try to figure out which you don't expect; 
        (2) find the code that added the unwanted constraint or constraints and fix it. 
(
    "<NSLayoutConstraint:0x28060dc70 UIButton:0x105534920'Voir Plus'.bottom == DzTrain_2_0.AvailableTrainScene:0x1055300d0.bottom - 8   (active)>",
    "<NSLayoutConstraint:0x2806894a0 UIButton:0x105534920'Voir Plus'.bottom == DzTrain_2_0.AvailableTrainScene:0x1055300d0.bottom + 40   (active)>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x2806894a0 UIButton:0x105534920'Voir Plus'.bottom == DzTrain_2_0.AvailableTrainScene:0x1055300d0.bottom + 40   (active)>

Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKitCore/UIView.h> may also be helpful.

我尝试添加两个约束，一个要隐藏，另一个要显示（优先级不同），然后激活/停用它们，但什么也没发生？有人以前有过这个吗？

【问题讨论】：

标签： ios swift animation autolayout

【解决方案1】：

加入约束

override init(frame: CGRect) {
    super.init(frame: frame)
    addSubview(seeMoreButton)
    NSLayoutConstraint.activate([
        seeMoreButton.heightAnchor.constraint(equalToConstant: 40),
        seeMoreButton.widthAnchor.constraint(equalTo: widthAnchor, multiplier: 0.8),
        seeMoreButton.centerXAnchor.constraint(equalTo: centerXAnchor)
   ])
   seeMoreBottom = seeMoreButton.bottomAnchor.constraint(equalTo: bottomAnchor, constant: -8)
   seeMoreBottom?.isActive = false
}

这样

override func layoutSubviews()

被任何人调用

self.layoutIfNeeded()

这会导致您的编辑不断与此处的新覆盖发生冲突

seeMoreBottom = seeMoreButton.bottomAnchor.constraint(equalTo: bottomAnchor, constant: -8)
seeMoreBottom?.isActive = false

【讨论】：

就是这样，谢谢Sh_Khan。我认为 override func layoutSubviews() 毕竟只用于框架调整。