Apply a background to all cells in a GridRow, with rounded corners.

FilledBackgroundModifier.swift

/// Apply a background to all cells in a GridRow, with rounded corners.
///
/// Example:
/// ```swift
/// Grid(/* ... */) {
///     ForEach(...) { element in
///         GridRow(alignment: .center) {
///             Text(element.title)
///             .modifier(FilledBackgroundModifier(
///                 selected: element == selectedElement, // some @State property
///                 position: .start
///             ))
///         }
///     }
/// }
/// ```
struct FilledBackgroundModifier: ViewModifier {
    var selected: Bool
    var position: CellPosition = .middle

    /// .start/.end used to determine if corners are drawn.
    enum CellPosition {
        case start, middle, end
    }

    func body(content: Content) -> some View {
        content
            .padding(.vertical, 10)
            .padding(.trailing, 20)
            .padding(.leading, position == .start ? 10 : 0)
            .frame(
                minWidth: 100,
                maxWidth: .infinity,
                maxHeight: .infinity,
                alignment: .leading
            )
            .background(
                shapeWithPositionalCorners
                    .fill(selected ? .blue : .clear)
            )
    }

    private var shapeWithPositionalCorners: UnevenRoundedRectangle {
        var radii = RectangleCornerRadii()
        let radius = 8

        switch position {
        case .start:
            radii.topLeading = radius
            radii.bottomLeading = radius
        case .end:
            radii.topTrailing = radius
            radii.bottomTrailing = radius
        default: break
        }
        return UnevenRoundedRectangle(cornerRadii: radii)
    }
}