Enum.swift

import AppKit

struct Icon {}
struct Canvas {}

protocol ToolController {
    var icon: Icon { get }
    var name: String { get }

    func apply(/*...*/)
}

@dynamicMemberLookup
enum Tool: CaseIterable {
    case pen
    case brush
    case fill
    case text

    private static var toolCache = DefaultDictionary<Tool, ToolController> { tool in
        switch tool {
        case .pen:   return PenToolController()
        case .brush: return BrushToolController()
        case .fill:  return FillToolController()
        case .text:  return TextToolController()
        }
    }

    subscript<T>(dynamicMember keyPath: KeyPath<ToolController, T>) -> T {
        Self.toolCache[self][keyPath: keyPath]
    }

    /*
    private static let toolCache = [Tool: ToolController](uniqueKeysWithValues: Tool.allCases.map { tool in
        switch tool {
        case .pen:   return (tool, PenToolController())
        case .brush: return (tool, BrushToolController())
        case .fill:  return (tool, FillToolController())
        case .text:  return (tool, TextToolController())
        }
    })

    subscript<T>(dynamicMember keyPath: KeyPath<ToolController, T>) -> T {
        Self.toolCache[self]![keyPath: keyPath]
    }
    */
}

struct DefaultDictionary<Key: Hashable, Value> {
    private var storage: [Key: Value] = [:]

    let getDefault: (Key) -> Value
    init(getDefault: @escaping (Key) -> Value) {
        self.getDefault = getDefault
    }

    subscript(_ key: Key) -> Value {
        mutating get {
            if let value = storage[key] {
                return value
            }

            let value = getDefault(key)
            storage[key] = value
            return value
        }
    }
}

class PenToolController: ToolController {
    var icon: Icon { Icon() }
    var name: String { "Pen" }

    func apply() {}
}

class BrushToolController: ToolController {
    var icon: Icon { Icon() }
    var name: String { "Brush" }

    func apply() {}
}

class FillToolController: ToolController {
    var icon: Icon { Icon() }
    var name: String { "Fill" }

    func apply() {}
}

class TextToolController: ToolController {
    var icon: Icon { Icon() }
    var name: String { "Text" }

    func apply() {}
}


print(Tool.pen.name)