NSLayoutConstraint可以将视图固定到superview的下边缘

Question

一个可重复的例子

class ViewController: UIViewController {

   var created = false

   override func viewDidLayoutSubviews() {
      super.viewDidLayoutSubviews()
      if !created {

         let scrollView = UIScrollView()
         scrollView.backgroundColor = UIColor.grayColor()
         view.addSubview(scrollView)

         let kidView = UIView()
         kidView.backgroundColor = UIColor.redColor()
         kidView.translatesAutoresizingMaskIntoConstraints = false

         scrollView.addSubview(kidView)

         scrollView.translatesAutoresizingMaskIntoConstraints = false
         kidView.translatesAutoresizingMaskIntoConstraints = false

         view.addConstraints(
            NSLayoutConstraint.constraintsWithVisualFormat("H:|[scrollView]|", options: .AlignAllLeft, metrics: nil, views: ["scrollView": scrollView])
         )
         view.addConstraints(
            NSLayoutConstraint.constraintsWithVisualFormat("V:|[scrollView]|", options: .AlignAllLeft, metrics: nil, views: ["scrollView": scrollView])
         )

         kidView.addConstraints([
            NSLayoutConstraint(item: kidView, attribute: .Width, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1, constant: 100),
            NSLayoutConstraint(item: kidView, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1, constant: 100),
         ])

         scrollView.addConstraints([
            NSLayoutConstraint(item: kidView, attribute: .CenterX, relatedBy: .Equal, toItem: scrollView, attribute: .CenterX, multiplier: 1, constant: 0),
            NSLayoutConstraint(item: kidView, attribute: .Bottom, relatedBy: .Equal, toItem: scrollView, attribute: .Bottom, multiplier: 1, constant: 0),
//            NSLayoutConstraint(item: kidView, attribute: .CenterY, relatedBy: .Equal, toItem: scrollView, attribute: .CenterY, multiplier: 1, constant: 0),
         ])

         created = true
      }
   }

}

问题

我想将我的自定义视图对齐到我的UIScrollView的底部边缘，但我似乎只能将它与顶部或垂直中心对齐。

scrollView.addConstraints([
    NSLayoutConstraint(
        item: circleContainerView, 
        attribute: .Top, 
        relatedBy: .Equal, 
        toItem: scrollView, 
        attribute: .Top, 
        multiplier: 1, 
        constant: 0),
])

scrollView.addConstraints([
    NSLayoutConstraint(
        item: circleContainerView, 
        attribute: .CenterY, 
        relatedBy: .Equal, 
        toItem: scrollView, 
        attribute: .CenterY, 
        multiplier: 1, 
        constant: 0),
])

但我想要的是拥有这样的照片。 我如何到达那里？

Answer 1

具有自动布局的ScrollView的工作方式不同，您可以通过设置translatesAutoresizingMaskIntoConstraints = true并显式设置contentSize来仅使用一个子视图。 或者你设置translatesAutoresizingMaskIntoConstraints = false并让它找出它自己的约束。

在您的情况下，您可以在滚动内添加一个不可见的视图并将其固定到顶部并将其高度设置为scrollView.bounds.size.height然后使用该不可见视图设置创建约束

像这样改变你的约束

scrollView.addConstraints([
    NSLayoutConstraint(
        item: circleContainerView, 
        attribute: .Bottom, 
        relatedBy: .Equal, 
        toItem: invisibleView, 
        attribute: .Bottom, 
        multiplier: 1, 
        constant: 0),
])

有关更多详细信息，请访问此链接 ，阅读纯自动布局方法

更新：修改后的代码

class ViewController: UIViewController {

    var created = false

    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        if !created {

            let scrollView = UIScrollView()
            scrollView.backgroundColor = UIColor.grayColor()
            view.addSubview(scrollView)

            let kidView = UIView()
            kidView.backgroundColor = UIColor.redColor()

            scrollView.addSubview(kidView)

            scrollView.translatesAutoresizingMaskIntoConstraints = false
            kidView.translatesAutoresizingMaskIntoConstraints = false

            scrollView.frame = view.bounds;
            scrollView.contentSize = view.bounds.size

            // Add an invisible view

            let inV = UIView()
            inV.backgroundColor = UIColor.clearColor()
            inV.translatesAutoresizingMaskIntoConstraints = false
            scrollView.addSubview(inV)


            view.addConstraints(
                NSLayoutConstraint.constraintsWithVisualFormat("H:|[scrollView]|", options: .AlignAllLeft, metrics: nil, views: ["scrollView": scrollView])
            )
            view.addConstraints(
                NSLayoutConstraint.constraintsWithVisualFormat("V:|[scrollView]|", options: .AlignAllLeft, metrics: nil, views: ["scrollView": scrollView])
            )

            var constraint = NSLayoutConstraint(item: inV, attribute: .Top, relatedBy: .Equal, toItem: scrollView, attribute: .Top, multiplier: 1, constant: 0)

            view.addConstraint(constraint)

            constraint = NSLayoutConstraint(item: inV, attribute: .Left, relatedBy: .Equal, toItem: scrollView, attribute: .Left, multiplier: 1, constant: 0)
            view.addConstraint(constraint)

            inV.addConstraints([
                NSLayoutConstraint(item: inV, attribute: .Width, relatedBy: .Equal, toItem: nil, attribute: .Width, multiplier: 1, constant: 10),
                NSLayoutConstraint(item: inV, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .Height, multiplier: 1, constant: self.view.bounds.size.height),
                ])

            //

            kidView.addConstraints([
                NSLayoutConstraint(item: kidView, attribute: .Width, relatedBy: .Equal, toItem: nil, attribute: .Width, multiplier: 1, constant: 100),
                NSLayoutConstraint(item: kidView, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .Height, multiplier: 1, constant: 100),
                ])

            view.addConstraints([
                NSLayoutConstraint(item: kidView, attribute: .CenterX, relatedBy: .Equal, toItem: scrollView, attribute: .CenterX, multiplier: 1, constant: 0),
                NSLayoutConstraint(item: kidView, attribute: .Bottom, relatedBy: .Equal, toItem: inV, attribute: .Bottom, multiplier: 1, constant: 0),
                ])

            created = true
        }
    }

}