How do I extend ButtonStyle with my generic struct conforming to ButtonStyle?

Using Swift
, ,
1

I created this generic struct that conforms to ButtonStyle (code simplified):

struct FilledButtonStyle<S: ShapeStyle>: ButtonStyle {
    let style: S

    func makeBody(configuration: Configuration) -> some View {
        configuration.label
            .background(style)
    }
}

I want to create an extension on ButtonStyle that defines a static function that returns this type like this:

extension ButtonStyle where Self == FilledButtonStyle {
    static func filled<S: ShapeStyle>(_ style: S) -> FilledButtonStyle<S> {
        FilledButtonStyle(style: style)
    }
}

I got an error on the extension line: "Reference to generic type 'FilledButtonStyle' requires arguments in <...>".

At this point I got stuck, things I have tried include:

extension ButtonStyle where Self == FilledButtonStyle<S>, S: ShapeStyle {
...
}

This gives an error "Cannot find type 'S' in scope"
I also tried using the some keyword in the FilledButtonStyle definition and in the extension, I also tried removing the Self constraint from the ButtonStyle extension and defining filled(_:) using generic parameters, this resulted in an error when using it in the call site .buttonStyle(.filled(.red)).

I'm starting to think that there is no way to extend ButtonStyle with a type generic over ShapeStyle like this.

Any idea why this is an error and if it's possible to extend button style like this?

2

When you can't constrain the extension due to generics, you can still constrain the method itself:

extension ButtonStyle {
    static func filled<S: ShapeStyle>(_ style: S) -> FilledButtonStyle<S>
    where Self == FilledButtonStyle<S>
    {
        FilledButtonStyle(style: style)
    }
}
2 Likes
3

TIL! Thank you @gwendal.roue. I appreciate your time!

1 Like