UIViewRepresentable 在 SwiftUI 中的列表单元格中的位置

Question

在 SwiftUI 中将自定义 UILabel 添加到 List 时，单元格重用出现错误，其中某些单元格上的标签根本不可见，而在某些单元格上，它被放置在左上角没有任何考虑单元格的填充。它总是在初始单元格上完美呈现。

使用ScrollView 时不会出现此问题。这是一个已知的错误吗？有没有好的解决方法？

GeometryReader { geometry in
    List {
        ForEach(self.testdata, id: \.self) { text in
            Group {
                AttributedLabel(attributedText: NSAttributedString(string: text), maxWidth: geometry.size.width - 40)
            }.padding(.vertical, 20)
        }
    }
}

struct AttributedLabel: UIViewRepresentable {

    let attributedText: NSAttributedString
    let maxWidth: CGFloat

    func makeUIView(context: UIViewRepresentableContext<Self>) -> UILabel {
        let label = UILabel()
        label.preferredMaxLayoutWidth = maxWidth
        label.attributedText = attributedText
        label.lineBreakMode = .byWordWrapping
        label.numberOfLines = 0
        label.backgroundColor = UIColor.red

        return label
    }

    func updateUIView(_ label: UILabel, context: UIViewRepresentableContext<Self>) {}

}

Answer 1

与 ScrollView 或 SwiftUI 错误无关。 我认为您的AttributedLabel 课程有问题。我尝试使用普通的Text，它工作正常。

List {
        ForEach(self.testdata, id: \.self) { text in
            Group {
                  Text(student.name)
                    .background(Color.red)
            }.padding(.vertical, 20)
        }
    }

Answer 2

确实似乎可以解决这个问题。

1. 第一步是让模型首先返回一个空的项目数组，然后返回实际的更新。这将强制视图更新。然后，在短暂的停顿之后，可以进行实际的更新。对于这种情况，这还不够。单独这样做仍然会导致布局问题。不知何故，该列表（可能由正在积极回收其细胞的UITableView 支持）仍然设法保持状态以某种方式引起了麻烦。所以……

2. 第二步是让视图在没有项目时提供列表以外的东西。这是使用 SwiftUI if 和 else 完成的，根据是否有任何项目使用不同的视图。随着模型的更改，按照步骤 1，每次更新都会发生这种情况。

执行步骤 (1) 和 (2) 似乎可以解决该问题。下面的示例代码还包括视图上的.animation(.none) 方法。这在我的代码中是必需的，但在下面的示例代码中似乎不需要。

此解决方法的一个缺点是您将丢失动画。很明显，如果苹果未来做出改变，它可能无法继续工作。 （不过，也许到那时这个错误已经被修复了。）

import SwiftUI

struct ContentView: View {

      @ObservedObject var model = TestData()

      var body: some View {

            VStack() {
                  GeometryReader { geometry in
                        // handle the no items case by offering up a different view
                        // this appears to be necessary to workaround the issues
                        // where table cells are re-used and the layout goes wrong
                        // Don't show the "No Data" message unless there really is no data,
                        // i.e. skip case where we're just delaying to workaround the issue.
                        if self.model.sampleList.isEmpty {
                              Text("No Data")
                                    .foregroundColor(self.model.isModelUpdating ? Color.clear : Color.secondary)
                                    .frame(width: geometry.size.width, height: geometry.size.height) // centre the text
                        }
                        else {
                              List(self.model.sampleList, id:\.self) { attributedString in
                                    AttributedLabel(attributedText: attributedString, maxWidth: geometry.size.width - 40)
                              }
                        }
                        }.animation(.none) // this MAY not be necessary for all cases
                  Spacer()
                  Button(action: { self.model.shuffle()} ) { Text("Shuffle") }.padding(20)
            }
      }
}

struct AttributedLabel: UIViewRepresentable {

      let attributedText: NSAttributedString
      let maxWidth: CGFloat

      func makeUIView(context: UIViewRepresentableContext<Self>) -> UILabel {
            let label = UILabel()
            label.preferredMaxLayoutWidth = maxWidth
            label.attributedText = attributedText
            label.lineBreakMode = .byWordWrapping
            label.numberOfLines = 0
            label.backgroundColor = UIColor.red
            return label
      }

      func updateUIView(_ label: UILabel, context: UIViewRepresentableContext<Self>) {
            // function required by protoocol - NO OP
      }

}

class TestData : ObservableObject {

      @Published var sampleList = [NSAttributedString]()
      @Published var isModelUpdating = false
      private var allSamples = [NSAttributedString]()


      func shuffle() {

            let filtered = allSamples.filter{ _ in Bool.random() }
            let shuffled = filtered.shuffled()

            // empty the sampleList - this will trigger the View that is
            // observing the model to update and handle the no items case
            self.sampleList = [NSAttributedString]()
            self.isModelUpdating = true

            // after a short delay update the sampleList - this will trigger
            // the view that is observing the model to update
            DispatchQueue.main.asyncAfter(deadline: .now() + 0.01) {
                  self.sampleList = shuffled
                  self.isModelUpdating = false
            }
      }

      init() {
            generateSamples()
            shuffle()
      }

      func generateSamples() {

            DispatchQueue.main.async {
                  var samples = [NSAttributedString]()

                  samples.append("The <em>quick</em> brown fox <strong>boldly</strong> jumped over the <em>lazy</em> dog.".fromHTML)
                  samples.append("<h1>SwiftUI</h1><p>At the time of writing, still very much a <em>work in progress</em>. Normal and <em>italic</em>. And <strong>strong</strong> too.</p><p>At the time of writing, still very much a <em>work in progress</em>. Normal and <em>italic</em>. And <strong>strong</strong> too.</p><p>At the time of writing, still very much a <em>work in progress</em>. Normal and <em>italic</em>. And <strong>strong</strong> too.</p><p>At the time of writing, still very much a <em>work in progress</em>. Normal and <em>italic</em>. And <strong>strong</strong> too.</p><p>At the time of writing, still very much a <em>work in progress</em>. Normal and <em>italic</em>. And <strong>strong</strong> too.</p>".fromHTML)
                  samples.append("<h1>Test Cells</h1><p>Include cells that have different heights to demonstrate what is going on. Make some of them really quite long. If they are all showing the list is going to need to scroll at least on smaller devices.</p><p>Include cells that have different heights to demonstrate what is going on. Make some of them really quite long. If they are all showing the list is going to need to scroll at least on smaller devices.</p><p>Include cells that have different heights to demonstrate what is going on. Make some of them really quite long. If they are all showing the list is going to need to scroll at least on smaller devices.</p> ".fromHTML)
                  samples.append("<h3>List of the day</h3><p>And he said:<ul><li>Expect the unexpected</li><li>The sheep is not a creature of the air</li><li>Chance favours the prepared observer</li></ul>And now, maybe, some commentary on that quote.".fromHTML)
                  samples.append("Something that is quite short but that is more than just one line long on a phone maybe. This might do it.".fromHTML)

                  self.allSamples = samples
            }
      }
}

extension String {
      var fromHTML : NSAttributedString {
            do {
                  return try NSAttributedString(data: Data(self.utf8), options: [
                        .documentType: NSAttributedString.DocumentType.html,
                        .characterEncoding: String.Encoding.utf8.rawValue
                  ], documentAttributes: nil)
            }
            catch {
                  return NSAttributedString(string: self)
            }
      }
}

struct ContentView_Previews: PreviewProvider {
      static var previews: some View {
            ContentView()
      }
}

Answer 3

我遇到了类似的问题，并通过使用getTextFrame(text)向 UIViewRepresentable 添加框架来解决它，

GeometryReader { geometry in
    List {
        ForEach(self.testdata, id: \.self) { text in
            Group {
                AttributedLabel(attributedText: NSAttributedString(string: text), maxWidth: geometry.size.width - 40)
                // add this frame
                .frame(width: getTextFrame(text).width height: getTextFrame(text).height)
            }.padding(.vertical, 20)
        }
    }
}

func getTextFrame(for text: String, maxWidth: CGFloat? = nil, maxHeight: CGFloat? = nil) -> CGSize {
        let attributes: [NSAttributedString.Key: Any] = [
            .font: UIFont.preferredFont(forTextStyle: .body)
        ]
        let attributedText = NSAttributedString(string: text, attributes: attributes)
        let width = maxWidth != nil ? min(maxWidth!, CGFloat.greatestFiniteMagnitude) : CGFloat.greatestFiniteMagnitude
        let height = maxHeight != nil ? min(maxHeight!, CGFloat.greatestFiniteMagnitude) : CGFloat.greatestFiniteMagnitude
        let constraintBox = CGSize(width: width, height: height)
        let rect = attributedText.boundingRect(with: constraintBox, options: [.usesLineFragmentOrigin, .usesFontLeading], context: nil).integral
        return rect.size
    }