“Generic parameter could not be inferred” in SwiftUI UIViewRepresentable

问题

I've got the following code, which makes it possible to use the UIKit's UIScrollView in my SwiftUI code. It can be pasted in a new SwiftUI project.

struct LegacyScrollView<Content: View>: UIViewRepresentable {
    enum Action {
        case idle
        case offset(x: CGFloat, y: CGFloat, animated: Bool)
    }

    @Binding var action: Action
    let content: Content

    func makeCoordinator() -> Coordinator {
        Coordinator(self)
    }

    func makeUIView(context: Context) -> UIScrollView {
        let hosting = UIHostingController(rootView: self.content)
        hosting.view.translatesAutoresizingMaskIntoConstraints = false

        let uiScrollView = UIScrollView()
        uiScrollView.addSubview(hosting.view)

        let constraints = [
            hosting.view.leadingAnchor.constraint(equalTo: uiScrollView.leadingAnchor),
            hosting.view.trailingAnchor.constraint(equalTo: uiScrollView.trailingAnchor),
            hosting.view.topAnchor.constraint(equalTo: uiScrollView.contentLayoutGuide.topAnchor),
            hosting.view.bottomAnchor.constraint(equalTo: uiScrollView.contentLayoutGuide.bottomAnchor),
            hosting.view.widthAnchor.constraint(equalTo: uiScrollView.widthAnchor)
        ]
        uiScrollView.addConstraints(constraints)

        return uiScrollView
    }

    func updateUIView(_ uiView: UIScrollView, context: Context) {
        switch self.action {
        case .offset(let x, let y, let animated):
            uiView.setContentOffset(CGPoint(x: x, y: y), animated: animated)
            DispatchQueue.main.async {
                self.action = .idle
            }
        default:
            break
        }
    }

    class Coordinator: NSObject {
        let legacyScrollView: LegacyScrollView

        init(_ legacyScrollView: LegacyScrollView) {
            self.legacyScrollView = legacyScrollView
        }
    }

    init(@ViewBuilder content: () -> Content) {
        self._action = Binding.constant(Action.idle)
        self.content = content()
    }

    init(action: Binding<Action>, @ViewBuilder content: () -> Content) {
        self._action = action
        self.content = content()
    }
}

struct ContentView: View {
    @State private var action = LegacyScrollView.Action.idle

    var body: some View {
        VStack(spacing: 0) {
            LegacyScrollView(action: self.$action) {
                ForEach(0 ..< 40) { _ in
                    Text("Hello, World!")
                }
            }
            .padding(20)
            .background(Color.gray)
            Spacer()
            Button("Set offset") {
                self.action = LegacyScrollView.Action.offset(x: 0, y: 200, animated: true)
            }.padding()
        }
    }
}

The code above will give Generic parameter 'Content' could not be inferred on the first line of the ContentView. I've tried to change the line to:

@State private var action = LegacyScrollView<AnyView>.Action.idle

but that will give another error. It works when I place the enum Action outside the struct LegacyScrollView. But in my opinion, that's a rather inelegant scoping of this enum. How can I solve the error message?

回答1:

Here is possible approach that allows usage of provided ContentView as-is.

Just change the direction of... instead of making entire type generic, which is actually not needed in this case, just make a generic only initialisation, like below.

Also it actually makes clear that Action is Content-independent, that is really correct.

Tested & works with Xcode 11.2 / iOS 13.2 (w/o no changes in ContentView)

struct LegacyScrollView: UIViewRepresentable {
    enum Action {
        case idle
        case offset(x: CGFloat, y: CGFloat, animated: Bool)
    }

    @Binding var action: Action
    private let uiScrollView: UIScrollView

    init<Content: View>(content: Content) {
        let hosting = UIHostingController(rootView: content)
        hosting.view.translatesAutoresizingMaskIntoConstraints = false

        uiScrollView = UIScrollView()
        uiScrollView.addSubview(hosting.view)

        let constraints = [
            hosting.view.leadingAnchor.constraint(equalTo: uiScrollView.leadingAnchor),
            hosting.view.trailingAnchor.constraint(equalTo: uiScrollView.trailingAnchor),
            hosting.view.topAnchor.constraint(equalTo: uiScrollView.contentLayoutGuide.topAnchor),
            hosting.view.bottomAnchor.constraint(equalTo: uiScrollView.contentLayoutGuide.bottomAnchor),
            hosting.view.widthAnchor.constraint(equalTo: uiScrollView.widthAnchor)
        ]
        uiScrollView.addConstraints(constraints)

        self._action = Binding.constant(Action.idle)
    }

    init<Content: View>(@ViewBuilder content: () -> Content) {
        self.init(content: content())
    }

    init<Content: View>(action: Binding<Action>, @ViewBuilder content: () -> Content) {
        self.init(content: content())
        self._action = action
    }

    func makeCoordinator() -> Coordinator {
        Coordinator(self)
    }

    func makeUIView(context: Context) -> UIScrollView {
        return uiScrollView
    }

    func updateUIView(_ uiView: UIScrollView, context: Context) {
        switch self.action {
        case .offset(let x, let y, let animated):
            uiView.setContentOffset(CGPoint(x: x, y: y), animated: animated)
            DispatchQueue.main.async {
                self.action = .idle
            }
        default:
            break
        }
    }

    class Coordinator: NSObject {
        let legacyScrollView: LegacyScrollView

        init(_ legacyScrollView: LegacyScrollView) {
            self.legacyScrollView = legacyScrollView
        }
    }

}

回答2:

I disagree with your assertion that the enum should be nested inside the class for the following reasons:

The enum is intended to be used both inside and outside of the class, with a generic type being required in order to use it.
The enum does not make use of, and therefore has no dependency on, the generic Content type.
With a good enough name, the intended use of the enum would be obvious.

If you really want to nest the enum definition, I would suggest the following:

Drop the generic type requirement on the class definition,
Convert your content member to be of AnyView type,
Make your init functions generic and store the return values of the given view builders into type-erased views, like so:

init<Content: View>(@ViewBuilder content: () -> Content) {
    self._action = Binding.constant(Action.idle)
    self.content = AnyView(content())
}

init<Content: View>(action: Binding<Action>, @ViewBuilder content: () -> Content) {
    self._action = action
    self.content = AnyView(content())
}

Of course, with this approach, you will:

Lose the type information of the underlying content view.
Possibly incur a greater runtime cost with type-erased views.

So it depends what you value more in this case... Ahhh, tradeoffs...

来源：https://stackoverflow.com/questions/60165059/generic-parameter-could-not-be-inferred-in-swiftui-uiviewrepresentable

标签

swift

generics

swiftui