ImageView.swift

/// - WARNING: Don't use it, this an example how _not_ to do it.
public struct ImageView: View {
    @ObservedObject private var viewModel: FetchImage
    private let image: (PlatformImage) -> AnyView
    private let placeholder: () -> AnyView
    private let error: (Error) -> AnyView

    public init<ImageView: View, PlaceholderView: View, ErrorView: View>(
        viewModel: FetchImage,
        @ViewBuilder image: @escaping (PlatformImage) -> ImageView,
        @ViewBuilder placeholder: @escaping () -> PlaceholderView,
        @ViewBuilder error: @escaping (Error) -> ErrorView
    ) {
        self.viewModel = viewModel
        self.image = { AnyView(image($0)) }
        self.error = { AnyView(error($0)) }
        self.placeholder = { AnyView(placeholder()) }
    }

    public init<ImageView: View>(
        viewModel: FetchImage,
        @ViewBuilder image: @escaping (PlatformImage) -> ImageView
    ) {
        self.init(
            viewModel: viewModel,
            image: image,
            placeholder: { Rectangle().foregroundColor(.clear) },
            error: { _ in Rectangle().foregroundColor(.clear) }
        )
    }

    public init(viewModel: FetchImage) {
        self.init(
            viewModel: viewModel,
            image: ImageView.makeImage(with:),
            placeholder: { Rectangle().foregroundColor(.clear) },
            error: { _ in Rectangle().foregroundColor(.clear) }
        )
    }

    public var body: some View {
        if let image = viewModel.image {
            return self.image(image)
        } else if let error = viewModel.error {
            return AnyView(self.error(error)
                .onAppear(perform: viewModel.fetch)
                .onDisappear(perform: viewModel.cancel))
        } else {
            return AnyView(placeholder()
                .onAppear(perform: viewModel.fetch)
                .onDisappear(perform: viewModel.cancel))
        }
    }

    public static func makeImage(with image: PlatformImage) -> SwiftUI.Image {
        #if os(macOS)
        return Image(nsImage: image)
        #else
        return Image(uiImage: image)
        #endif
    }
}