辺の比がフィボナッチ数の正方形になるように配置するLayoutプロトコル

FibonacciLayout.swift

import SwiftUI

struct FibonacciLayout: Layout {
    struct Cache {
        var fib: [Int128]
    }

    func makeCache(subviews: Subviews) -> Cache {
        switch subviews.count {
        case ...2:
            return Cache(fib: Array(repeating: 1, count: subviews.count))
        default:
            var fib: [Int128] = [1, 1]
            for i in 2..<subviews.count {
                fib.append(fib[i - 1] + fib[i - 2])
            }
            return Cache(fib: fib)
        }
    }

    func updateCache(_ cache: inout Cache, subviews: Subviews) {
        guard subviews.count - cache.fib.count > 0 else {
            return
        }
        for i in 0..<(subviews.count - cache.fib.count) {
            let c = cache.fib.count - 1
            cache.fib.append(cache.fib[i + c] + cache.fib[i - 1 + c])
        }
    }

    func sizeThatFits(proposal: ProposedViewSize, subviews: Subviews, cache: inout Cache) -> CGSize {
        proposal.replacingUnspecifiedDimensions()
    }

    func placeSubviews(in bounds: CGRect, proposal: ProposedViewSize, subviews: Subviews, cache: inout Cache) {
        var cache = cache
        var subviews = subviews
        var bounds = bounds
        var count = 0
        while !subviews.isEmpty {
            switch subviews.count {
            case 1:
                subviews[0].place(at: bounds.origin, proposal: ProposedViewSize(bounds.size))
            default:
                let i = subviews.count-1
                if count % 2 == 0 {
                    var size = bounds.size
                    size.height *= (CGFloat(cache.fib[i]) / CGFloat(cache.fib[i] + cache.fib[i-1]))
                    if size.width <= size.height {
                        size.height = size.width
                    } else {
                        size.width = size.height
                    }
                    if count == 0 {
                        let h = size.height * (1 + CGFloat(cache.fib[i-1]) / CGFloat(cache.fib[i]))
                        bounds.origin.y += (bounds.size.height - h)/2
                    }
                    subviews.first?.place(at: bounds.origin, proposal: ProposedViewSize(size))

                    bounds.origin.y += size.height
                    bounds.size.height -= size.height
                    bounds.size.width = size.width
                } else {
                    var size = bounds.size
                    size.width *= (CGFloat(cache.fib[i]) / CGFloat(cache.fib[i] + cache.fib[i-1]))
                    if size.height <= size.width {
                        size.width = size.height
                    } else {
                        size.height = size.width
                    }
                    subviews.first?.place(at: bounds.origin, proposal: ProposedViewSize(size))

                    bounds.origin.x += size.width
                    bounds.size.width -= size.width
                    bounds.size.height = size.height
                }
            }
            cache.fib.removeLast()
            subviews = subviews[1..<subviews.count]
            count += 1
        }
    }
}

struct ContentView: View {
    var numberOfItems = 3

    var body: some View {
        FibonacciLayout {
            ForEach(0..<numberOfItems, id: \.self) { offset in
                color(at: offset)
            }
        }
        .padding()
    }

    func color(at index: Int) -> Color {
        let hue = (Double(index) * .pi * (8.0 / 7.0)).truncatingRemainder(dividingBy: 1.0)
        return Color(hue: hue, saturation: 0.6, brightness: 1.0)
    }
}

#Preview {
    @Previewable @State var numberOfItems: Int = 3
    ContentView(numberOfItems: numberOfItems)
        .frame(maxHeight: .infinity)
        .overlay {
            VStack {
                Stepper("Count", value: $numberOfItems, in: 3...10)
                Button("Reset") {
                    numberOfItems = 3
                }
                .frame(maxWidth: .infinity, alignment: .trailing)
            }
            .frame(maxHeight: .infinity, alignment: .bottom)
            .padding()
        }
        .animation(.default, value: numberOfItems)
}