swiftui - UIViewRepresentable 无法在模态呈现的 SwiftUI View 中工作

Question

为了定制一个UISlider ，我在 UIViewRepresentable 中使用它.它公开了一个 @Binding var value: Double这样我的 View 模型 ( ObservableObject ) View 可以观察到更改并更新 View相应地。

问题是 View 在 @Binding 时没有更新。值被改变。在下面的示例中，我有两个 slider 。一位原生Slider和一个定制SwiftUISlider .

两者都将绑定(bind)值传递给应该更新 View 的 View 模型。 native Slider会更新 View 但不会更新自定义 View 。在日志中，我可以看到 $sliderValue.sink { ... 被正确调用，但 View 未更新。

我注意到当呈现 View 有 @Environment(\.presentationMode) var presentationMode: Binding<PresentationMode> 时会发生这种情况属性(property)。如果我将其注释掉，它会按预期工作。

重现此代码的完整示例代码是

import SwiftUI
import Combine

struct ContentView: View {
    @State var isPresentingModal = false

    // comment this out
    @Environment(\.presentationMode) var presentationMode: Binding<PresentationMode>

    var body: some View {
        VStack {
            Button("Show modal") {
                isPresentingModal = true
            }
            .padding()
        }
        .sheet(isPresented: $isPresentingModal) {
            MyModalView(viewModel: TempViewModel())
        }
    }
}

class TempViewModel: ObservableObject {
    @Published var sliderText = ""
    @Published var sliderValue: Double = 0
    private var cancellable = Set<AnyCancellable>()

        init() {
            $sliderValue
                .print("view model")
                .sink { [weak self] value in
                    guard let self = self else { return }
                    print("updating view  \(value)")
                    self.sliderText = "\(value) C = \(String(format: "%.2f" ,value * 9 / 5 + 32)) F"
                }
                .store(in: &cancellable)
        }
}

struct MyModalView: View {
    @ObservedObject var viewModel: TempViewModel

    var body: some View {
        VStack(alignment: .leading) {
            Text("SwiftUI Slider")
            Slider(value: $viewModel.sliderValue, in: -100...100, step: 0.5)
                .padding(.bottom)

            Text("UIViewRepresentable Slider")
            SwiftUISlider(minValue: -100, maxValue: 100, value: $viewModel.sliderValue)
            Text(viewModel.sliderText)
        }
        .padding()
    }
}

struct SwiftUISlider: UIViewRepresentable {
    final class Coordinator: NSObject {
        var value: Binding<Double>
        init(value: Binding<Double>) {
            self.value = value
        }

        @objc func valueChanged(_ sender: UISlider) {
            let index = Int(sender.value + 0.5)
            sender.value = Float(index)
            print("value changed \(sender.value)")
            self.value.wrappedValue = Double(sender.value)
        }
    }

    var minValue: Int = 0
    var maxValue: Int = 0

    @Binding var value: Double

    func makeUIView(context: Context) -> UISlider {
        let slider = UISlider(frame: .zero)
        slider.minimumTrackTintColor = .systemRed
        slider.maximumTrackTintColor = .systemRed
        slider.maximumValue = Float(maxValue)
        slider.minimumValue = Float(minValue)

        slider.addTarget(
            context.coordinator,
            action: #selector(Coordinator.valueChanged(_:)),
            for: .valueChanged
        )

        adapt(slider, context: context)
        return slider
    }

    func updateUIView(_ uiView: UISlider, context: Context) {
        adapt(uiView, context: context)
    }

    func makeCoordinator() -> SwiftUISlider.Coordinator {
        Coordinator(value: $value)
    }

    private func adapt(_ slider: UISlider, context: Context) {
        slider.value = Float(value)
    }
}

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

Answer 1

我发现了问题。在 updateUIView的 UIViewRepresentable ，我还需要将绑定(bind)传递给 SwiftUISlider 的新实例:

func updateUIView(_ uiView: UISlider, context: Context) {
    uiView.value = Float(value)

    // the _value is the Binding<Double> of the new View struct, we pass it to the coordinator 
    context.coordinator.value = _value
}

SwiftUI.View可以随时重新创建，当它发生时 updateUIView叫做。一个新的 View 结构有一个新的 var value: Binding<Double>所以我们将它分配给我们的协调员