ios - 在 UIScrollView 中调整多行 UILabel 的大小会中断滚动。为什么？

Question

我使用以下设置:

• ViewControllers View 包含一个 UIScrollView，具有顶部、前导、尾随和底部约束以匹配 VC 大小
• ScrollView 包含两个 subview :
  • 一个 UIView 来定义 ScrollView 的内容大小。它的高度与 ScrollView 相同，但宽度是它的两倍。因此只能水平滚动。
  • 一个包含一些长文本的 UILabel，带有用于设置固定大小的高度和宽度约束以及用于设置固定位置的 ScrollView 的顶部和前导约束。
• 当 ScrollView 滚动时，Label 的宽度会发生变化。

问题:如果 Label 设置为使用多行并且手动设置了 ScrollViews contenOffset 属性，则 ScrollView 停止滚动。

ViewController View
+---------------------+
|+-------------------+| 
||ScrollView         ||
||+------------------||--------------------+
|||UIView to define  || content size       |
|||                  ||                    |
|||                  ||                    |
|||  [MultiLine]     ||                    |
|||  [  Label  ]     ||                    |
|||                  ||                    |
||+------------------||--------------------+
|+-------------------+|
+---------------------+


- (void)viewDidLayoutSubviews {
    [super viewDidLayoutSubviews];

    // Setting the ContentOffset will stop scrolling 
    //[self.scrollView setContentOffset:CGPointMake(0, 0)];
}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    // Resize Label when scrolling
    self.labelWidthConstraint.constant = MAX (50, 50 + self.scrollView.contentOffset.x);
}

如果

，使用此代码调整标签大小工作正常

• 标签使用一行设置。在这种情况下，设置内容偏移不会造成任何伤害。 或
• 内容偏移没有变化(甚至没有设置为(0, 0))。在这种情况下，将标签设置为多行不会造成任何伤害

同时设置内容偏移量和使用多行不会有效。滚动条不能再滚动了。

这是为什么？知道是什么原因造成的以及如何解决吗？

Answer 1

问题似乎是，当更改标签约束时，它会触发 viewDidLayoutSubviews，然后将 UIScrollView 设置为不滚动，因为 set contentOffset 会被一遍又一遍地调用。如果您只想通过使用 bool 作为标志在初始布局上将 UIScrollView 设置为 CGPoint.zero，则可以克服此问题。显然，由于 UILabel 需要重新绘制大小更改，因此它会触发 viewDidLayoutSubviews。这是 Swift 中的示例。

import UIKit

class ViewController: UIViewController {

    lazy var scrollView : UIScrollView = {
        let sv = UIScrollView(frame: self.view.bounds)
        sv.translatesAutoresizingMaskIntoConstraints = false
        sv.delegate = self
        return sv
    }()

    lazy var contentView : UIView = {
        let v = UIView(frame: CGRect(x: 0, y: 0, width: self.view.bounds.width * 4, height: self.view.bounds.height))
        v.translatesAutoresizingMaskIntoConstraints = false
        return v
    }()

    lazy var label : UILabel = {
        let lbl = UILabel(frame: CGRect(x: 0, y: 0, width: self.view.bounds.width, height: 50))
        lbl.numberOfLines = 0
        lbl.text = "Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s"
        lbl.minimumScaleFactor = 0.5
        lbl.adjustsFontSizeToFitWidth = true
        lbl.font = UIFont.systemFont(ofSize: 22)
        lbl.translatesAutoresizingMaskIntoConstraints = false
        return lbl
    }()

    var widthConstraint : NSLayoutConstraint?
    var heightConstraint : NSLayoutConstraint?
    var startingHeight : CGFloat = 0
    var startingWidth : CGFloat = 0
    override func viewDidLoad() {
        super.viewDidLoad()
        //first scrollview
        self.view.addSubview(scrollView)
        pinToAllSides(target: scrollView)

        //now content view
        self.scrollView.addSubview(contentView)
        contentView.widthAnchor.constraint(equalTo: self.view.widthAnchor, multiplier: 2).isActive = true
        contentView.heightAnchor.constraint(equalTo: self.scrollView.heightAnchor, multiplier: 1).isActive = true
        contentView.backgroundColor = .green
        pinToAllSides(target: contentView)
        scrollView.layoutIfNeeded()

        //now the label
        self.scrollView.addSubview(label)
        label.leadingAnchor.constraint(equalTo: self.scrollView.leadingAnchor, constant: 20).isActive = true
        label.topAnchor.constraint(equalTo: self.scrollView.topAnchor, constant: 60).isActive = true
        label.backgroundColor = .red
        widthConstraint = NSLayoutConstraint(item: label, attribute: .width, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1, constant: self.view.bounds.width/2)

        heightConstraint = NSLayoutConstraint(item: label, attribute: .height, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1, constant: 300)
        if let wc = widthConstraint,
            let hc = heightConstraint{
            startingHeight = hc.constant
            startingWidth = wc.constant
            label.addConstraint(wc)
            label.addConstraint(hc)
        }

    }

    func pinToAllSides(target:UIView){
        guard let superview = target.superview else{
            return
        }
        target.leadingAnchor.constraint(equalTo: superview.leadingAnchor).isActive = true
        target.trailingAnchor.constraint(equalTo: superview.trailingAnchor).isActive = true
        target.topAnchor.constraint(equalTo: superview.topAnchor).isActive = true
        target.bottomAnchor.constraint(equalTo: superview.bottomAnchor).isActive = true
    }

    var hasHappenedOnce : Bool = false
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        if hasHappenedOnce == false{
            hasHappenedOnce = true
            self.scrollView.contentOffset = .zero
        }
    }
}

extension ViewController : UIScrollViewDelegate{

    func scrollViewDidScroll(_ scrollView: UIScrollView) {
        //hopefully it is laggy due to simulator but for the label i would ditch constraints myself
        self.widthConstraint?.constant = max(startingWidth, self.scrollView.contentOffset.x * 1.1 + startingWidth)

        let height = startingHeight - self.scrollView.contentOffset.x
        self.heightConstraint?.constant = height
        label.updateConstraints()
    }
}