Avoiding AnyView when using NSHostingView with arbitrary content. Finding a way to specify the generic parameter.

NSHostingViewGenerics.swift

import SwiftUI

struct ContentView: View {
    var body: some View {
        VStack {
            FrameworkView1 {
                Text("FrameworkView1")
            }
            .border(.red)

            FrameworkView2 {
                Text("FrameworkView2")
            }
            .border(.red)

            FrameworkView3 {
                Text("FrameworkView3")
            }
            .border(.red)
        }
        .padding()
    }
}

struct FrameworkView1<Content: View>: NSViewRepresentable {
    @ViewBuilder var content: Content

    // Problem: we want to add modifiers to the `content` view.
    // Adding modifiers changes the type of the resulting SwiftUI view.
    // We can use AnyView as the content type, but that's ugly and can make
    // SwiftUI view updates slow and/or break animations.
    typealias NSViewType = NSHostingView<AnyView>

    func makeNSView(context: Context) -> NSViewType {
        let hostingViewContent = content
            .padding()
            .background(.green)
        return NSHostingView(rootView: AnyView(hostingViewContent))
    }

    func updateNSView(_ nsView: NSViewType, context: Context) {
    }
}

// MARK: Solution 1
// Solution 1: define a protocol, then we can use `some NSHostingViewProtocol`
// as the return type.
protocol NSHostingViewProtocol: NSView {}

extension NSHostingView: NSHostingViewProtocol {}

struct FrameworkView2<Content: View>: NSViewRepresentable {
    @ViewBuilder var content: Content

    func makeNSView(context: Context) -> some NSHostingViewProtocol {
        let hostingViewContent = content
            .padding()
            .background(.green)
        print(type(of: hostingViewContent))
        return NSHostingView(rootView: hostingViewContent)
    }

    func updateNSView(_ nsView: NSViewType, context: Context) {
    }
}

// MARK: Solution 2
// Solution 2: define a wrapper view struct.
// This hides the concrete view type behind the wrapper view's `some View`.
struct FrameworkView3<Content: View>: NSViewRepresentable {
    @ViewBuilder var content: Content

    typealias NSViewType = NSHostingView<RootView>

    func makeNSView(context: Context) -> NSViewType {
        return NSHostingView(rootView: RootView(content: content))
    }

    func updateNSView(_ nsView: NSViewType, context: Context) {
    }

    struct RootView: View {
        var content: Content

        var body: some View {
            content
                .padding()
                .background(.green)
        }
    }
}

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