Builder.swift

@dynamicMemberLookup
public struct Builder<Base> {

    public var build: () -> Base

    public init(_ build: @escaping () -> Base) {
        self.build = build
    }

    public init(_ base: Base) {
        self.build = { base }
    }

    public subscript<Value>(dynamicMember keyPath: ReferenceWritableKeyPath<Base, Value>) -> (Value) -> Builder<Base> {
        { [build = build] value in
            Builder {
                let object = build()
                object[keyPath: keyPath] = value
                return object
            }
        }
    }

    public subscript<Value>(dynamicMember keyPath: WritableKeyPath<Base, Value>) -> (Value) -> Builder<Base> {
        { [build = build] value in
            Builder {
                var object = build()
                object[keyPath: keyPath] = value
                return object
            }
        }
    }

    public func `do`(_ handler: @escaping (Base) throws -> Void) rethrows -> Builder<Base> {
        let object = build()
        try handler(object)
        return Builder(object)
    }
}

public protocol Buildable {
    associatedtype Base
    var builder: Builder<Base> { get }
}

public extension Buildable where Self: AnyObject {
    var builder: Builder<Self> {
        Builder(self)
    }
}

public extension Buildable where Self: Any {
    var builder: Builder<Self> {
        Builder(self)
    }
}

extension NSObject: Buildable {}