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

The simplest & fastest is to use value-type model.

Here is a simple demo. Tested & worked with Xcode 12 / iOS 14

struct TestTallies: View {
    @StateObject private var group = GroupOfTallies()     // SwiftUI 2.0
//    @ObservedObject private var group = GroupOfTallies()     // SwiftUI 1.0

    var body: some View {
        VStack {
            Text("Cumulative: \(group.cumulativeCount)")
            Divider()
            Button("Add") { group.elements.append(Tallies(count: 1)) }
            Button("Update") { group.elements[0].count = 5 }
        }
    }
}

struct Tallies: Identifiable {       // << make struct !!
  let id = UUID()
  var count = 0
}

class GroupOfTallies: Identifiable, ObservableObject {
  let id = UUID()
  @Published var elements: [Tallies] = []

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