how can I pass a fill style like .fill.secondary as a parameter to a function?

To pass a fill style like .fill.secondary as a parameter to a function in SwiftUI, you can define a function that accepts a ShapeStyle parameter. Here's an example of how you might do this:

import SwiftUI

struct ContentView: View {
    var body: some View {
        VStack {
            customShape(fillStyle: .fill.secondary)
        }
    }
    
    func customShape(fillStyle: ShapeStyle) -> some View {
        Circle()
            .fill(fillStyle)
            .frame(width: 100, height: 100)
    }
}

In this example, the customShape function takes a ShapeStyle parameter called fillStyle. When calling this function, you can pass .fill.secondary or any other fill style you want to use.

For more detailed information on creating custom visual effects with SwiftUI, you might find the session Create custom visual effects with SwiftUI particularly useful. This session covers various aspects of custom visual effects, including color treatments and view transitions.