import SwiftUI

extension View {
    /// Proposes a percentage of its received proposed size to `self`.
    ///
    /// This modifier multiplies the proposed size it receives from its parent
    /// with the given factors for width and height.
    ///
    /// If the parent proposes `nil` or `.infinity` to us in any dimension,
    /// we’ll forward these values to our child view unchanged.
    ///
    /// - Note: The size we propose to `self` will not necessarily be a percentage
    ///   of the parent view’s actual size or of the available space as not all
    ///   views propose the full available space to their children. For example,
    ///   VStack and HStack divide the available space among their subviews and
    ///   only propose a fraction to each subview.
    public func relativeProposed(width: Double = 1, height: Double = 1) -> some View {
        RelativeSizeLayout(relativeWidth: width, relativeHeight: height) {
            // Wrap content view in a container to make sure the layout only
            // receives a single subview.
            // See Chris Eidhof, SwiftUI Views are Lists (2023-01-25)
            // <https://chris.eidhof.nl/post/swiftui-views-are-lists/>
            VStack { // alternatively: `_UnaryViewAdaptor(self)`
                self
            }
        }
    }
}

/// A custom layout that proposes a percentage of its
/// received proposed size to its subview.
///
/// - Precondition: must contain exactly one subview.
fileprivate struct RelativeSizeLayout: Layout {
    var relativeWidth: Double
    var relativeHeight: Double

    func sizeThatFits(proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) -> CGSize {
        assert(subviews.count == 1, "RelativeSizeLayout expects a single subview")
        let resizedProposal = ProposedViewSize(
            width: proposal.width.map { $0 * relativeWidth },
            height: proposal.height.map { $0 * relativeHeight }
        )
        return subviews[0].sizeThatFits(resizedProposal)
    }

    func placeSubviews(in bounds: CGRect, proposal: ProposedViewSize, subviews: Subviews, cache: inout ()) {
        assert(subviews.count == 1, "RelativeSizeLayout expects a single subview")
        let resizedProposal = ProposedViewSize(
            width: proposal.width.map { $0 * relativeWidth },
            height: proposal.height.map { $0 * relativeHeight }
        )
        subviews[0].place(at: CGPoint(x: bounds.midX, y: bounds.midY), anchor: .center, proposal: resizedProposal)
    }
}