在 SwiftUI 中，如何在调用 updateUIView 时从 UIViewRepresentable 访问属性？

Question

我在尝试从 SwiftUI 中的 UIKit 组件访问

count

等属性时遇到问题，编译器发出紫色警告：

在视图更新期间修改状态，这将导致未定义的行为。

尽管将状态修改包含在

DispatchQueue.main.asyncAfter

内可以避免出现警告，但它被认为是次优解决方案。直接将 uiView 暴露给 SwiftUI 进行属性访问是另一种解决方法，但它也不太理想。

那么还有其他方法吗？

是否有更合适的方法来实现这一目标而不诉诸这些解决方法？

请注意，下面的示例经过简化以说明问题。虽然我知道

text.count

可以在 SwiftUI 中直接获取，但在某些情况下需要通过 UIKit 组件访问此类属性。

struct ContentView: View {
    @State private var text = ""
    @State private var count = 0
    var body: some View {
        VStack {
            UIKitTextField(text: $text, count: $count)
            Text("Word count: \(count)")
        }
    }
}
struct UIKitTextField: UIViewRepresentable {
    @Binding var text: String
    @Binding var count: Int
    func makeUIView(context: Context) -> UITextField {
        let textField = UITextField(frame: .zero)
        textField.delegate = context.coordinator
        return textField
    }
    func updateUIView(_ uiView: UITextField, context: Context) {
        uiView.text = text
        count = uiView.text?.count ?? 0
//        DispatchQueue.main.asyncAfter(deadline: .now() + 0.1) {
//            count = uiView.text?.count ?? 0
//        }
    }
    func makeCoordinator() -> Coordinator {
        Coordinator(self)
    }
    class Coordinator: NSObject, UITextFieldDelegate {
        var parent: UIKitTextField
        init(_ textField: UIKitTextField) {
            self.parent = textField
        }
        func textFieldDidChangeSelection(_ textField: UITextField) {
            parent.text = textField.text ?? ""
        }
    }
}

---

更新另一个示例

struct UIKitImageView: UIViewRepresentable {
    let asset: Asset?
    let hdrOpen: Bool
    @Binding var isImageHDR: Bool
    func makeUIView(context: Context) -> UIImageView {
        let view = UIImageView()
        update(view)
        view.setContentCompressionResistancePriority(.defaultLow, for: .horizontal)
        view.setContentCompressionResistancePriority(.defaultLow, for: .vertical)
        view.setContentHuggingPriority(.required, for: .horizontal)
        view.setContentHuggingPriority(.required, for: .vertical)
        return view
    }
    func updateUIView(_ view: UIImageView, context: Context) {
        update(view)
    }
    func update(_ view: UIImageView) {
        if let url = asset?.file {
            view.image = ImageReader.image(contentsOf: url)
        } else if let data = asset?.data {
            view.image = ImageReader.image(data: data)
        } else {
            view.image = UIImage()
        }
        if hdrOpen {
            view.preferredImageDynamicRange = .high
        } else {
            view.preferredImageDynamicRange = .standard
        }
        isImageHDR = view.image!.isHighDynamicRange
    }
}

Answer 1

您在可表示的实现中犯了一个错误，请像这样：

    func updateUIView(_ uiView: UITextField, context: Context) {
        context.coordinator.textDidChange = nil
        uiView.text = text
        //count = uiView.text?.count ?? 0 make this a computed property in the parent View
        context.coordinator.textDidChange = { newText in
            text = newText
        }
    }
    
    func makeCoordinator() -> Coordinator {
        Coordinator() // can't pass self because this is a struct not an object 
    }

由于文本状态是你的事实来源，你必须通过任何其他方式的计算属性访问它的长度，并且它会不一致。

struct ContentView: View {
    @State private var text = ""
    var count: Int {
        text.count
    }
    var body: some View {
        VStack {
            UIKitTextField(text: $text)
            Text("Word count: \(count)")
        }
    }
}

第二个示例的固定代码：

   let url: URL

   func makeUIView(context: Context) -> UIImageView {
        let view = UIImageView()
        //update(view) updateUIView is already called after makeUIView so this is unnecessary
        view.setContentCompressionResistancePriority(.defaultLow, for: .horizontal)
        view.setContentCompressionResistancePriority(.defaultLow, for: .vertical)
        view.setContentHuggingPriority(.required, for: .horizontal)
        view.setContentHuggingPriority(.required, for: .vertical)
        return view
    }
    func updateUIView(_ view: UIImageView, context: Context) {
        // need to check if url has changed
        if url != context.coordinator.url {
            update(view)
        }
    }