How do i work in Swift 5 with protocol function parameters that use protocols with associated types (i.e. .pickerStyle())

Question

I am using SwiftUI and would like to set the pickerStyle of a View depending on the number of items in the Picker. With a few items, SegmentedPickerStyle() is ideal, with more WheelPickerStyle() is better.

}.pickerStyle(productsObserver.product.productFamilies?.count ?? 0 < 5 ? SegmentedPickerStyle() : WheelPickerStyle())

The function signature reads: func pickerStyle<S>(_ style: S) -> some View where S : PickerStyle which i have learned uses a generic in the functiona signature because PickerStyle uses an associated type.

It shouldn't be so difficult a problem and probably isn't - Protocols should work like this easy = , but i can't see it. Any help is highly appreciated!

Answer 1

pickerStyle is a generic method that accepts a concrete type (at compile-time) that conforms PickerStyle. So, it cannot be either SegmentedPickerStyle or WheelPickerStyle (determined at run-time) - it has to be one or the other.

So, one suggestion would be to create a view modifier and apply the picker style conditionally. The critical difference here is that it returns a conditional view of type _ConditionalContent<TrueContent, FalseContent>.

struct PickerStyleOption<P1: PickerStyle, P2: PickerStyle>: ViewModifier {
    let predicate: () -> Bool
    let style1: P1
    let style2: P2
    
    @ViewBuilder
    func body(content: Content) -> some View {
        if predicate() {
            content
                .pickerStyle(style1)
        } else {
            content
                .pickerStyle(style2)
        }
    }
}

For convenience, you could create an extension:

extension View {
    func pickerStyleOption<P1: PickerStyle, P2: PickerStyle>(
            _ condition: @autoclosure @escaping () -> Bool,
            then style1: P1, 
            else style2: P2) -> some View {

        self.modifier(
            PickerStyleOption(predicate: condition, style1: style1, style2: style2)
        )
    }
}

And use it like so:

Picker(...) {
   ...
}
.pickerStyleOption((productsObserver.product.productFamilies?.count ?? 0) < 5, 
   then: SegmentedPickerStyle(), else: WheelPickerStyle())