Copy-on-write types and SwiftUI

Question

Created Mar ’21

Replies 3

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I am trying to use a custom Copy-on-write type with SwiftUI. I have found that it takes the expensive path every time in the below code. Any thoughts about how I can avoid the copy and deinit for every drag event? Thanks for any insights or hints you might have.

Code Block swiftstruct CowType {
  init() {
    self.detail = Detail(values: [])
  }
  private var detail: Detail
  var values: [Int] { detail.values }
  mutating func appendValue(_ value: Int) {
    if !isKnownUniquelyReferenced(&detail) {
      print("performing expensive copy")
      detail = Detail(values: values)
    }
    detail.values.append(value)
  }
  private final class Detail {
    init(values: [Int]) {
      self.values = values
    }
    deinit {
       print("calling deinit")
    }
    var values: [Int]
  }
}
struct ContentView: View {
  @State var state = CowType()
  var body: some View {
    Text("Hello, world! \(state.values.count)")
      .padding()
      .gesture(DragGesture().onChanged { _ in
        state.appendValue(Int.random(in: 1...100))
      })
  }
}

Answered by rayfix in 668551022

As @OOPer indicated, using an object is a way around the problem:

I needed to use this to avoid the expensive copies.

Code Block swiftfinal class Container: ObservableObject {
  private var cow = CowType()
  var values: [Int] { cow.values }
  func append(_ value: Int) {
    objectWillChange.send()
    cow.append(value)
  }
}

Interestingly, @Published runs into the same problem as @State, making a copy at every iteration. e.g.,

Code Block swiftfinal class Container: ObservableObject {
  @Published var cow = CowType()
}

This makes cow make a copy every time it is mutated.

Boost

Answer 1

OOPer OP

Mar ’21

An interesting experiment.
It seems as if SwiftUI creates a copy of @State variables when executing actions, so isKnownUniquelyReferenced(_:) would never return true.

Until Apple provides us a way to implement CoW efficiently or updates the implementation of SwiftUI,
you may need to use @StateObject instead of @State where efficiency would be a good concern.

Code Block class ClassType: ObservableObject {
    init() {
        self.detail = Detail(values: [])
    }
    private var detail: Detail
    var values: [Int] { detail.values }
    func appendValue(_ value: Int) {
        objectWillChange.send()
        detail.values.append(value)
    }
    private final class Detail {
        init(values: [Int]) {
            self.values = values
        }
        deinit {
            print("calling deinit")
        }
        var values: [Int]
    }
    
}
struct ContentView: View {
    //@State var state = CowType()
    @StateObject var state = ClassType()
    var body: some View {
        Text("Hello, world! \(state.values.count)")
            .padding()
            .gesture(DragGesture().onChanged { _ in
                state.appendValue(Int.random(in: 1...100))
            })
    }
}

1

Answer 2

rayfix OP

Mar ’21

Accepted Answer

As @OOPer indicated, using an object is a way around the problem:

I needed to use this to avoid the expensive copies.

Code Block swiftfinal class Container: ObservableObject {
  private var cow = CowType()
  var values: [Int] { cow.values }
  func append(_ value: Int) {
    objectWillChange.send()
    cow.append(value)
  }
}

Interestingly, @Published runs into the same problem as @State, making a copy at every iteration. e.g.,

Code Block swiftfinal class Container: ObservableObject {
  @Published var cow = CowType()
}

This makes cow make a copy every time it is mutated.

0

Answer 3

rayfix OP

Mar ’21

UPDATE:

On the second inspection, making CowType Equatable does not fix the issue.

On the Swift.org forum, Jonathan Hise Kaldma thought that making CowType Equatable might solve the problem. It was a good thought, but didn't work out in this case.

1