SwiftUI ViewModifier for contextMenu with generics

I want to add the iOS16 API for 'contextMenu forSelectionType: menu:', but I also need to support iOS15, so I need a conditional ViewModifier for this. I'm struggling with the correct syntax for it.

From what I can work out, this is what my ViewModifier should look like:

struct CompatibilityListContextMenuModifier<V>: ViewModifier where V: View {

    let menu: (Set<Person.ID>) -> V
    
    func body(content: Content) -> some View {
      if #available(iOS 16.0, macOS 13.0, *) {
          content
              .contextMenu(forSelectionType: Person.ID.self, menu: menu)
        } else {
            content
        }
    }
}

But how do I use this in my List:

List (selection: $contactsListOptions.multiSelectedContacts){
   // ...
}
.modifier(CompatibilityListContextMenuModifier(menu: items in {
            if items.isEmpty { // Empty area menu.
                Button("New Item") { }

            } else if items.count == 1 { // Single item menu.
                Button("Copy") { }
                Button("Delete", role: .destructive) { }
            }
            else {
                Button("MultipleSelection Items") { }
            }
        }))

This gives me a syntax error:

Cannot find 'items' in scope

If I try to pass in the selection binding:

.modifier(CompatibilityListContextMenuModifier(menu: contactsListOptions.multiSelectedContacts in {
        if contactsListOptions.multiSelectedContacts.isEmpty { // Empty area menu.
            Button("New Item") { }

        } else if contactsListOptions.multiSelectedContacts.count == 1 { // Single item menu.
            Button("Copy") { }
            Button("Delete", role: .destructive) { }
        }
        else {
            Text("MultipleSelection Items")
        }
    }))

I get the error:

Cannot convert value of type 'Set<Person.ID>' (aka 'Set') to expected argument type '(Set<Person.ID>) -> V' (aka '(Set) -> V')

and

Generic parameter 'V' could not be inferred

What's the right way to conditionally compile this API?

The syntax for creating the modifier should be:

CompatibilityListContextMenuModifier { items in ... }

The closure parameter should go inside the { ... }.

The closure should also be a @ViewBuilder, otherwise you cannot use if statements like that in the closure.

// I also abstracted away Person.ID as the Selection type parameter
struct CompatibilityListContextMenuModifier<Menu, Selection>: ViewModifier
    where Menu: View, Selection: Hashable {

    @ViewBuilder
    let menu: (Set<Selection>) -> Menu
    
    func body(content: Content) -> some View {
      if #available(iOS 16.0, macOS 13.0, *) {
          content
              .contextMenu(forSelectionType: Selection.self, menu: menu)
        } else {
            content
        }
    }
}

Now you can use it like this:

.modifier(CompatibilityListContextMenuModifier<_, Person.ID> { items in
    if items.isEmpty {
        Button("New Item") { }

    } else if items.count == 1 {
        Button("Copy") { }
        Button("Delete", role: .destructive) { }
    }
    else {
        Button("MultipleSelection Items") { }
    }
})

I'd also suggest that you write your own View extension to write this more conveniently:

extension View {
    func compatibilityContextMenu<Menu: View, Selection: Hashable>(
        forSelectionType type: Selection.Type, 
        @ViewBuilder menu: @escaping (Set<Selection>) -> Menu
    ) -> some View {
        modifier(CompatibilityListContextMenuModifier(menu: menu))
    }
}

.compatibilityContextMenu(forSelectionType: Person.ID.self) { items in
    // ...
}