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

debugcn 发表于 Dev

安德烈·赫福德

我使用以下设置：

ViewControllers 视图包含UIScrollViewTop、Leading、Trailing 和 Bottom 约束以匹配 VC 的大小
ScrollView 包含两个子视图：
- AUIView定义滚动视图的内容大小。它的高度与 ScrollView 相同，但宽度是其两倍。因此只能水平滚动。
- 一个UILabel有些长文本具有高度和宽度约束到一个固定的大小和顶部和领先的约束设置为滚动型设置一个固定的位置。
当 ScrollView 滚动时，Label 的宽度会发生变化。

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

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);
}

使用此代码调整标签大小工作正常，如果

标签设置使用一行。在这种情况下，设置内容偏移量不会造成任何伤害。或者
内容偏移量未更改（甚至未设置为(0, 0)）。在这种情况下，将标签设置为多行不会造成任何伤害

设置内容偏移并采用多线同时DOES NOT工作。无法再滚动滚动条。

为什么是这样？知道什么可能导致这种情况以及如何解决吗？

阿吉布森007

问题似乎是，当标签约束更改时，它会触发 viewDidLayoutSubviews，然后将 UIScrollView 设置为不滚动，因为 set contentOffset 然后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，您可以克服这个问题。显然，由于 UILabel 需要在大小更改时重绘它会触发 viewDidLayoutSubviews。这是 Swift 中的一个示例。

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()
    }
}

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