ShapeBuilder

ShapeBuilder.swift

import SwiftUI

struct ContentView: View {
    var value: Int = 0

    var body: some View {
        Text("Hello")
            .padding()
            .background(.orange)
            .clipShape(shape)
    }

    @ShapeBuilder
    var shape: some Shape {
        switch value {
        case 0: Circle()
        case 1: Rectangle()
        default: Ellipse()
        }
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

@resultBuilder
public enum ShapeBuilder {
    public static func buildBlock<S: Shape>(_ builder: S) -> some Shape {
        builder
    }

    static func buildOptional<S: Shape>(_ component: S?) -> EitherShape<S, EmptyShape> {
        component.flatMap(EitherShape.first) ?? EitherShape.second(EmptyShape())
    }

    static func buildEither<First: Shape, Second: Shape>(first component: First) -> EitherShape<First, Second> {
        .first(component)
    }

    static func buildEither<First: Shape, Second: Shape>(second component: Second) -> EitherShape<First, Second> {
        .second(component)
    }
}

public enum EitherShape<First: Shape, Second: Shape>: Shape {
    case first(First)
    case second(Second)

    public func path(in rect: CGRect) -> Path {
        switch self {
        case let .first(first):
            return first.path(in: rect)
        case let .second(second):
            return second.path(in: rect)
        }
    }
}

public struct EmptyShape: Shape {
    public init() {}

    public func path(in rect: CGRect) -> Path {
        Path()
    }
}