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

There are multiple issues here to address.

First, it's important to understand that SwiftUI updates the view's body when it detects a change, either in a @State property, or from an ObservableObject (via @ObservedObject and @EnvironmentObject property wrappers).

In the latter case, this is done either via a @Published property, or manually with objectWillChange.send(). objectWillChange is an ObservableObjectPublisher publisher available on any ObservableObject.

This is a long way of saying that IF the change in a computed property is caused together with a change of any @Published property - for example, when another element is added from somewhere:

elements.append(Talies())

then there's no need to do anything else - SwiftUI will recompute the view that observes it, and will read the new value of the computed property cumulativeCount.

---

Of course, if the .count property of one of the Tallies objects changes, this would NOT cause a change in elements, because Tallies is a reference-type.

The best approach given your simplified example is actually to make it a value-type - a struct:

struct Tallies: Identifiable {
  let id = UUID()
  var count = 0
}

Now, a change in any of the Tallies objects would cause a change in elements, which will cause the view that "observes" it to get the now-new value of the computed property. Again, no extra work needed.

---

If you insist, however, that Tallies cannot be a value-type for whatever reason, then you'd need to listen to any changes in Tallies by subscribing to their .objectWillChange publishers:

class GroupOfTallies: Identifiable, ObservableObject {
   let id = UUID()
   @Published var elements: [Tallies] = [] {
      didSet {
         cancellables = [] // cancel the previous subscription
         elements.publisher
            .flatMap { $0.objectWillChange }
            .sink(receiveValue: self.objectWillChange.send) 
            .store(in: &cancellables)
      }
   }

   private var cancellables = Set

   var cumulativeCount: Int {
     return elements.reduce(0) { $0 + $1.count } // no changes here
   }
} 

The above will subscribe a change in the elements array (to account for additions and removals) by:

• converting the array into a Sequence publisher of each array element
• then flatMap again each array element, which is a Tallies object, into its objectWillChange publisher
• then for any output, call objectWillChange.send(), to notify of the view that observes it of its own changes.