Cached sort

sort3.swift

import SwiftUI

@Observable
class Counter: Identifiable {
    var count = 0
}

@Observable
class Model {
    var counters: [Counter] = [.init(), .init(), .init()]
}

@Observable
class SortedModel {
    @ObservationIgnored
    var model: Model! {
        didSet {
            if oldValue !== model {
                _counters = nil
            }
        }
    }
    var ascending = false
    
    var _counters: [Counter]?
    var counters: [Counter] {
        if _counters == nil {
            _counters = withObservationTracking {
                // tracks self.ascending and model.counters.counts
                let sort = SortDescriptor(\Counter.count, order: ascending ? .forward : .reverse)
                return model.counters.sorted(using: sort)
            } onChange: { [weak self] in
                self?._counters = nil
            }
        }
        return _counters!
    }
}

struct ContentView: View {
    @State var model: Model?
    @State var callBody = 0
    
    var body: some View {
        NavigationStack {
            if let model {
                Button("Call body \(callBody)") {
                    callBody += 1
                }
                Button("Add Counter") {
                    model.counters.append(.init())
                }
                Button("New Model") {
                    self.model = Model()
                }
                HStack {
                    CounterView(x: callBody, model: model)
                    CounterView(x: callBody, model: model)
                }
            }
        }
        .onAppear {
            model = Model()
        }
    }
}



struct CounterView: View {
    let x: Int
    let model: Model
    @State var sortedModel: SortedModel?
    
    var body: some View {
        List {
            if let sortedModel {
                let _ = sortedModel.model = model
                
                Button(sortedModel.ascending ? "Ascending" : "Descending") {
                    withAnimation {
                        sortedModel.ascending.toggle()
                    }
                }
                ForEach(sortedModel.counters) { c in
                    Button("\(c.count, format: .number)") {
                        withAnimation {
                            c.count += 1
                        }
                    }
                }
            }
        }
        .onAppear {
            sortedModel = SortedModel()
        }
    }
}