Published computed properties in SwiftUI model objects

Question

Suppose I have a data model in my SwiftUI app that looks like the following:

class Tallies: Identifiable, ObservableObject {
  let id = UUID()
  @Published va         


        

Answer 1

This is similar to the last option of @New Devs answer, but a little shorter, essentially just passing the objectWillChange notification to the parent object:

import Combine

class Tallies: Identifiable, ObservableObject {
  let id = UUID()
  @Published var count = 0

  func increase() {
    count += 1
  }
}

class GroupOfTallies: Identifiable, ObservableObject {
  let id = UUID()
  var sinks: [AnyCancellable] = []
  
  @Published var elements: [Tallies] = [] {
    didSet {
      sinks = elements.map {
        $0.objectWillChange.sink( receiveValue: objectWillChange.send)
      }
    }
  }

  var cumulativeCount: Int {
    return elements.reduce(0) { $0 + $1.count }
  }
}

SwiftUI Demo:

struct ContentView: View {
  @ObservedObject
  var group: GroupOfTallies

  init() {
    let group = GroupOfTallies()
    group.elements.append(contentsOf: [Tallies(), Tallies()])
    self.group = group
  }

  var body: some View {
    VStack(spacing: 50) {
      Text( "\(group.cumulativeCount)")
      Button( action: group.elements.first!.increase) {
        Text( "Increase first")
      }
      Button( action: group.elements.last!.increase) {
        Text( "Increase last")
      }
    }
  }
}