Layout Protocol: новые возможности SwiftUI

    
    public protocol Layout : Animatable {

	associatedtype Cache = Void

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

	func placeSubviews(
		in bounds: CGRect, 
		proposal: ProposedViewSize,
		subviews: Self.Subviews, 
		cache: inout Self.Cache
	)
	//...
}

public func replacingUnspecifiedDimensions(
	by size: CGSize = CGSize(width: 10, height: 10)
) -> CGSize

struct CircleLayout: Layout {
	
	let radius: CGFloat
	
	func sizeThatFits(
		proposal: ProposedViewSize,
		subviews: Subviews,
		cache: inout ()
	) -> CGSize {
		let maxSize = getMaxSize(for: subviews)
		
		return CGSize(
			width: radius * 2 + maxSize.width,
			height: radius * 2 + maxSize.height
		)
	}
	//...

	private func getMaxSize(for subviews: Subviews) -> CGSize {
		subviews.reduce(.zero) { currentMaxSize, subview in
			let currentSize = subview.sizeThatFits(.unspecified)
			return CGSize(
				width: max(currentSize.width, currentMaxSize.width),
				height: max(currentSize.height, currentMaxSize.height)
			)
		}
	}
}

struct CircleLayout: Layout {
	//...
	func placeSubviews(
		in bounds: CGRect,
		proposal: ProposedViewSize,
		subviews: Subviews,
		cache: inout ()
	) {
		let maxSize = getMaxSize(for: subviews)
		let proposal = ProposedViewSize(maxSize)
		for (index, subview) in subviews.enumerated() {
			let center = CGPoint(x: bounds.midX, y: bounds.midY)
			let origin = getPosition(
				forSubviewIndex: index,
				radius: radius,
				center: center,
				subviewsCount: subviews.count
			)
			subview.place(at: origin, proposal: proposal)
		}
	}

	//...

	private func getPosition(
		forSubviewIndex index: Int,
		radius: CGFloat,
		center: CGPoint,
		subviewsCount: Int
	) -> CGPoint {
		let theta = (CGFloat.pi * 2 / CGFloat(subviewsCount)) * CGFloat(index)
		return CGPoint(
			x: center.x + radius * cos(theta),
			y: center.y + radius * sin(theta)
		)
	}
}

public protocol Layout : Animatable {
	static var layoutProperties: LayoutProperties { get }
	associatedtype Cache = Void
	typealias Subviews = LayoutSubviews
	func makeCache(subviews: Self.Subviews) -> Self.Cache
	func updateCache(_ cache: inout Self.Cache, subviews: Self.Subviews)
	func spacing(subviews: Self.Subviews, cache: inout Self.Cache) -> ViewSpacing
	func sizeThatFits(proposal: ProposedViewSize, subviews: Self.Subviews, cache: inout Self.Cache) -> CGSize
	func placeSubviews(in bounds: CGRect, proposal: ProposedViewSize, subviews: Self.Subviews, cache: inout Self.Cache)
	func explicitAlignment(of guide: HorizontalAlignment, in bounds: CGRect, proposal: ProposedViewSize, subviews: Self.Subviews, cache: inout Self.Cache) -> CGFloat?
	func explicitAlignment(of guide: VerticalAlignment, in bounds: CGRect, proposal: ProposedViewSize, subviews: Self.Subviews, cache: inout Self.Cache) -> CGFloat?
}

enum LayoutKind: Int, CaseIterable {
		case vertical
		case horizontal
		case z
		case grid
		case circle
	}
	@State var layoutKind: LayoutKind = .vertical
       // Между бетами стандартные классы Layout-ов могут меняться
       // В будущем, в идеале можно будет использовать стандартные VStack и HStack
	var layout: AnyLayout {
		switch layoutKind {
		case .vertical:
			return AnyLayout(_VStackLayout())
		case .horizontal:
			return AnyLayout(_HStackLayout())
		case .z:
			return AnyLayout(_ZStackLayout())
		case .grid:
			return AnyLayout(_GridLayout())
		case .circle:
			return AnyLayout(CircleLayout(radius: 100))
		}
	}