ios - 如何以编程方式更改锚定样式约束？

Question

我进行了搜索，甚至想出了潜在的解决方案。遗憾的是，他们都没有说话。我正在尝试通过增加 View 的leadingAnchor-Constraint来为 View 的x位置设置动画。

我像这样创建约束:

let margins = self.layoutMarginsGuide
    horizontalConstraint = underline.leadingAnchor.constraintEqualToAnchor(margins.leadingAnchor)

并像这样编辑约束常量:

func updateUnderlinePosition(offset:CGFloat, pageCount:Int) {

    //Underline just over width of one Screen:
    var underlineOffset = offset / CGFloat(pageCount)

    //Add left margin:
    let margins = self.layoutMarginsGuide
    underlineOffset += 15

    //reposition underlineview
    horizontalConstraint.constant += underlineOffset
    self.layoutIfNeeded()
}

为什么我会收到错误消息，告诉我存在冲突的约束？

"<NSLayoutConstraint:0x7fca2b8c72c0 UIView:0x7fca29d1c830.leading == UILayoutGuide:0x7fca2b8c4970'UIViewLayoutMarginsGuide'.leading>",
"<NSLayoutConstraint:0x7fca29c2ccd0 UIView:0x7fca29d1c830.leading == UILayoutGuide:0x7fca2b8c4970'UIViewLayoutMarginsGuide'.leading + 16.25>"

编辑:

约束是这样创建的:

override func layoutSubviews() {
    super.layoutSubviews()

    //underline.frame = CGRectMake(15, self.frame.height-10, 40, 2)
    underline.translatesAutoresizingMaskIntoConstraints = false
    underline.backgroundColor = UIColor.whiteColor()
    underline.layer.cornerRadius = 1.0
    self.addSubview(underline)
    print(self.backItem)
    let margins = self.layoutMarginsGuide
    horizontalConstraint = underline.leadingAnchor.constraintEqualToAnchor(margins.leadingAnchor)
    let verticalConstraint = underline.centerYAnchor.constraintEqualToAnchor(margins.bottomAnchor)
    let widthConstraint = underline.widthAnchor.constraintEqualToAnchor(nil, constant: 40)
    let heightConstraint = underline.heightAnchor.constraintEqualToAnchor(nil, constant: 2)
    NSLayoutConstraint.activateConstraints([horizontalConstraint, verticalConstraint, widthConstraint, heightConstraint])
}

Answer 1

检查layoutSubviews是否被调用两次