Implementation of builder pattern using Swift's powerful keypaths

BuilderPattern.swift

/// Simple protocol which types who want to implement builder pattern conform to for free – gives conforming types two simple functions for building
protocol Buildable {}

extension Buildable {
    /// Returns a new instance whose property defined by the keypath is set to `value`.
    /// - Parameters:
    ///   - keyPath: `WriteableKeyPath` to the property which shall be modified
    ///   - value: The value which will be set to the property
    ///
    /// This function is used for types with value semantics.
    func build<Value>(keyPath: WritableKeyPath<Self, Value>, value: Value) -> Self {
        var newSelf = self
        newSelf[keyPath: keyPath] = value
        return newSelf
    }
    
    /// Returns a new instance whose property defined by the keypath is set to `value`.
    /// - Parameters:
    ///   - keyPath: `ReferenceWriteableKeyPath` to the property which shall be modified
    ///   - value: The value which will be set to the property
    ///
    /// This function is used for types with reference semantics.
    func build<Value>(keyPath: ReferenceWritableKeyPath<Self, Value>, value: Value) -> Self {
        let newSelf = self
        newSelf[keyPath: keyPath] = value
        return newSelf
    }
}

struct Person {
    var name: String?
    var age: Int?
    var city: String?
    var country: String?
    var planet: String?
}

// Where the builder functions are defined
extension Person: Buildable {
    func called(_ name: String) -> Self {
        build(keyPath: \.name, value: name)
    }
    
    func aged(_ age: Int) -> Self {
        build(keyPath: \.age, value: age)
    }
    
    func living(in city: String, country: String, planet: String) -> Self {
        build(keyPath: \.city, value: city)
            .build(keyPath: \.country, value: country)
            .build(keyPath: \.planet, value: planet) // highly composable
    }
}

let person = Person()
    .called("Chris")
    .aged(25)
    .living(in: "San Francisco", country: "United States", planet: "Earth")


import UIKit

extension UIView: Buildable {
    func backgroundColor(_ color: UIColor) -> Self {
        build(keyPath: \.backgroundColor, value: color)
    }
    
    func cornerRadius(_ radius: CGFloat) -> Self {
        build(keyPath: \.layer.cornerRadius, value: radius)
    }
}

// Works great with inheritance
extension UILabel {
    func textColor(_ color: UIColor) -> Self {
        build(keyPath: \.textColor, value: color)
    }
}

let label = UILabel()
    .backgroundColor(.blue)
    .cornerRadius(25)
    .textColor(.white)
    
// Thanks to fermoya for the original implementation: https://github.com/fermoya/SwiftUIPager/blob/develop/Sources/SwiftUIPager/Helpers/Buildable.swift