CowType.swift

//
//  ContentView.swift
//  CopyOnWrite
//
//  Created by Ray Fix on 3/27/21.
//

import SwiftUI

struct CowType {

  init() {
    self.detail = Detail(values: Array(repeating: 0, count: 1000))
  }

  private var detail: Detail

  var values: [Int] { detail.values }

  mutating func append(_ value: Int) {
    if !isKnownUniquelyReferenced(&detail) {
      print("performing expensive copy")
      detail = Detail(values: values)
    }
    else {
      print("no copy")
    }
    detail.values.append(value)
  }

  private final class Detail {
    init(values: [Int]) {
      self.values = values
    }

    deinit {
       print("calling deinit")
    }

    var values: [Int]
  }
}


final class Model: ObservableObject {
  private var state = CowType()

  var values: [Int] {
    state.values
  }

  func append(_ value: Int) {
    objectWillChange.send()
    state.append(value)
  }
}


struct ContentView: View {
  @StateObject var model = Model()
  var body: some View {
    VStack {
    Text("Hello, world! \(model.values.count)")
      .padding()
      .gesture(DragGesture().onChanged { _ in
        model.append(Int.random(in: 1...100))
      })
      Text("\(model.values.last!)")
    }
  }
}

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