sample.swift

## sample.swift
import Foundation

// A lens is a getter and a setter combined
struct Lens<Whole, Part> {
    let get: (Whole) -> Part
    let set: (inout Whole, Part) -> ()
}

// We can create a lens from a key path
extension Lens {
    init(_ keyPath: WritableKeyPath<Whole, Part>) {
        get = { $0[keyPath: keyPath]}
        set = { w, p in w[keyPath: keyPath] = p}
    }
}

final class Disposable {
    var dispose: () -> ()
    init(_ dispose: @escaping () -> ()) {
        self.dispose = dispose
    }

    deinit { dispose() }
}

// A mutable, observable variable. The type `A` should have value semantics (e.g. a struct or enum)
final class Var<A> {
    private var _get: () -> A
    private var _set: (A) -> ()
    typealias AddObserver = (@escaping (A) -> ()) -> Disposable
    let addObserver: AddObserver

    var storage: A {
        get {
            return _get()
        }
        set {
            _set(newValue)
        }
    }

    init(_ value: A) {
        var freshTokens = (0...).makeIterator()
        var observers: [Int: (A) -> ()] = [:]
        var x = value {
            didSet {
                observers.values.forEach { $0(x) }
            }
        }
        addObserver = {
            let token = freshTokens.next()!
            observers[token] = $0
            return Disposable {
                observers[token] = nil
            }
        }
        _get = { x }
        _set = { x = $0 }
    }

    private init(get: @escaping () -> A, set: @escaping (A) -> (), addObserver: @escaping AddObserver) {
        self._get = get
        self._set = set
        self.addObserver = addObserver
    }

    // If we have a keypath, we can create a variable for that keypath. The variable will be observable and *mutable*. It is a reference, i.e. mutating it will mutate the "parent":
    subscript<Part>(_ kp: WritableKeyPath<A, Part>) -> Var<Part> {
    return self[Lens(kp)]
    }


    // Keypaths are just a special case of lenses
    subscript<Part>(_ lens: Lens<A, Part>) -> Var<Part> {
    return Var<Part>(get: {
    lens.get(self.storage)
    }, set: {
    lens.set(&self.storage, $0)
    }, addObserver: { o in
    self.addObserver { newValue in
    o(lens.get(newValue))
    }
    })
    }

}

// If the variable is a mutable collection, we can even project out its elements as `Var`s:
extension Var where A: MutableCollection {
    subscript(_ index: A.Index) -> Var<A.Element> {
        let lens = Lens<A, A.Element>(get: { $0[index] }, set: { w, p in w[index] = p })
        return self[lens]
    }
}

// We could create a Store that automatically serializes an A, given that A is Codable. By exposing its value as a `Var`, we can mutate the value, and it'll get persisted on disk:
final class Store<A: Codable> {
    let value: Var<A>
    let disposeBag: Any?

    init(url: URL, defaultValue: A) {
        if let data = try? Data(contentsOf: url),
            let decoded = try? JSONDecoder().decode(A.self, from: data) {
            self.value = Var(decoded)
        } else {
            self.value = Var(defaultValue)
        }
        let encoder = JSONEncoder()
        disposeBag = self.value.addObserver { newValue in
            let data = try! encoder.encode(newValue)
            try! data.write(to: url)
        }
    }
}

// Given a `Var<[A]>`, we can create a generic table view controller which observes *and* mutates an A.

import UIKit
final class GenericTableViewController<A>: UITableViewController {
    let items: Var<[A]>
    let configure: (UITableViewCell, A) -> ()
    var didSelect: ((Var<A>) -> ())?
    var disposeBag: Any?

    init(items: Var<[A]>, configure: @escaping (UITableViewCell, A) -> ()) {
        self.items = items
        self.configure = configure
        super.init(style: .plain)
        disposeBag = items.addObserver { [weak self] _ in self?.tableView.reloadData() }
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError("Not implemented")
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        tableView.register(UITableViewCell.self, forCellReuseIdentifier: "Cell")
    }

    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return items.storage.count
    }

    override func tableView(_ tableView: UITableView, canEditRowAt indexPath: IndexPath) -> Bool {
        return true
    }

    override func tableView(_ tableView: UITableView, commit editingStyle: UITableViewCellEditingStyle, forRowAt indexPath: IndexPath) {
        guard editingStyle == .delete else { return }
        items.storage.remove(at: indexPath.row)
    }

    override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCell(withIdentifier: "Cell")!
        configure(cell, items.storage[indexPath.row])
        return cell
    }

    override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
        didSelect?(items[indexPath.row])
    }
}


// For example, an address book:
struct Address: Codable {
    var street: String
}

struct Person: Codable {
    var name: String
    var address: [Address]
}

// Finally, let's put everything together. Here's a  sample app which displays a list of people, can add new people. You can also tap on a person and see their addresses. Both table views are mutable, and because they work with `Var`s, they automatically communicate back! The entire state will be persisted automatically whenever you make a change.
import PlaygroundSupport

final class AddressViewController: UIViewController, UITextFieldDelegate {
    let address: Var<Address>
    let street = UITextField()
    var disposeBag: Any?

    init(address: Var<Address>) {
        self.address = address
        super.init(nibName: nil, bundle: nil)
        disposeBag = address.addObserver { [weak self] newValue in
            self?.configure(for: newValue)
        }
    }

    required init?(coder aDecoder: NSCoder) {
        fatalError()
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        let stackView = UIStackView()
        stackView.axis = .vertical
        stackView.backgroundColor = .white
        street.borderStyle = .roundedRect
        street.placeholder = "Street"
        street.delegate = self
        stackView.addArrangedSubview(street)
        view.addSubview(stackView)

        self.configure(for: address.storage)
    }

    override func viewDidAppear(_ animated: Bool) {
        view.subviews[0].frame = view.bounds // hack, too lazy for autolayout
    }

    func configure(for: Address) {
        guard address.storage.street != street.text else { return }
        street.text = address.storage.street
    }

    func textFieldDidEndEditing(_ textField: UITextField) {
        guard let value = street.text else { return }
        print("Text field did end editing")
        address.storage.street = value
    }

    func textFieldShouldReturn(_ textField: UITextField) -> Bool {
        return true
    }

    deinit {
        print("Deiniting address vc: \(address.storage)")
    }
}

final class Coordinator: NSObject {
    var root: UINavigationController!
    let store: Store<[Person]>

    override init() {
        let storeURL = playgroundSharedDataDirectory.appendingPathComponent("test.json")
        store = Store<[Person]>(url: storeURL, defaultValue: [])
        super.init()


        let people = GenericTableViewController<Person>(items: store.value) { cell, person in
            cell.textLabel?.text = person.name
            cell.accessoryType = .disclosureIndicator
        }
        people.navigationItem.rightBarButtonItem = UIBarButtonItem(barButtonSystemItem: .add, target: self, action: #selector(addPerson(sender:)))
        people.didSelect = { [weak self] (person: Var<Person>) in
            self?.showPerson(person)
        }
        root = UINavigationController(rootViewController: people)

    }

    func showPerson(_ person: Var<Person>) {
        let vc = GenericTableViewController<Address>(items: person[\.address]) { cell, address in
            cell.textLabel?.text = address.street
        }
        vc.didSelect = { [weak self] address in
            self?.showAddress(address)
        }

        root.pushViewController(vc, animated: true)
    }

    func showAddress(_ address: Var<Address>) {
        let vc = AddressViewController(address: address)
        root.pushViewController(vc, animated: true)
    }

    @objc func addPerson(sender: Any) {
        store.value.storage.append(Person(name: "Sample Person", address: [
            Address(street: "Sample Street"),
            Address(street: "Sample Street 2"),
            Address(street: "Sample Street 3")
            ]))
    }
}

print(playgroundSharedDataDirectory)
let c = Coordinator()
PlaygroundPage.current.liveView = c.root