如何以編程方式更改錨定樣式約束？

Question

我進行了搜索，甚至提出了潛在的解決方案。 可悲的是，他們都沒有話。我試圖通過增加View的leadAnchor-Constraint來使其動畫化。

我這樣創建約束：

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

然后編輯約束常量，如下所示：

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()
}

為什么我會錯誤地告訴我約束存在沖突？

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

編輯：

約束是這樣創建的：

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

檢查layoutSubviews是否被調用兩次

Answer 2

問題是您要在layoutSubviews中添加約束，可以多次調用該約束。 您一次實例化underline ，但是可以多次調用layoutSubviews ，引用underline的同一實例，從而在此過程中丟失了對原始horizontalConstraint引用。

坦白說，添加子視圖和約束並不真正屬於layoutSubviews 。 您應該在實例化underline視圖的地方設置約束，可以在適當的視圖init方法或awakeFromNIB進行約束。 這取決於如何實例化視圖。