SwiftUI using @EnvironmentObject with picker causes ScrollView contentOffset binding warning

Question

I have a watchOS app which has two screens. When I navigate to the second screen I get the following warning in the console:

ScrollView contentOffset binding has been read; this will cause grossly inefficient view performance as the ScrollView's content will be updated whenever its contentOffset changes. Read the contentOffset binding in a view that is not parented between the creator of the binding and the ScrollView to avoid this.

This seems to be related to using the @EnvironmentObject for the picker selection in the second screen. The warning does not happen if I remove the @EnvironmentObject and replace it with @State for the picker selection. (But then the updates are not reflected on the first screen).

Why is this happening? What can I do to stop this warning?

Here is my code:

First screen:

import SwiftUI

struct ContentView: View {
  @EnvironmentObject var itemManager: ItemManager

  var body: some View {
    List {
      ForEach(itemManager.items.indices) { index in
        NavigationLink(destination: ItemView(index: index)) {
          VStack(alignment: HorizontalAlignment.leading) {
            HStack {
              Text(self.itemManager.items[index].name)
              Spacer()
              Text("x")
              Text(String(self.itemManager.items[index].quantity))
            }
            Text(self.itemManager.items[index].type.rawValue).font(.footnote)
          }
        }
      }
    }
  }
}

struct ContentView_Previews: PreviewProvider {
  static var previews: some View {
    ContentView().environmentObject(ItemManager())
  }
}

Second screen:

import SwiftUI

struct ItemView: View {

  @EnvironmentObject var itemManager: ItemManager
  var index: Int

  var body: some View {

    VStack {
      Text("Update")
      Form {
        Section {
          Picker(selection: $itemManager.items[index].type, label: Text("Food Type")) {
            ForEach(FoodType.allCases.sorted()) { type in
              Text(type.rawValue).tag(type)
            }
          }
        }
      }
    }.navigationBarTitle(Text("Item"))
  }
}

struct ItemView_Previews: PreviewProvider {
  static var previews: some View {

    return ItemView(index: 0).environmentObject(ItemManager())
  }
}

Model:

import Foundation

enum FoodType: String, CaseIterable, Identifiable, Comparable{
  case fruit
  case vegetable
  case poultry
  case bakery
  var id: FoodType{self}

  static func < (lhs: FoodType, rhs: FoodType) -> Bool {
    lhs.rawValue < rhs.rawValue
  }
}

struct Item {
  var type: FoodType
  var quantity: Int
  var name: String
}

class ItemManager: ObservableObject {

  @Published var items: [Item] =
    [
      Item(type: FoodType.fruit, quantity: 1, name: "apple"),
      Item(type: FoodType.bakery, quantity: 1, name: "french bread"),
      Item(type: FoodType.vegetable, quantity: 6, name: "carrots")
  ]
}

HostingController:

import WatchKit
import Foundation
import SwiftUI

class HostingController: WKHostingController<AnyView> {
    override var body: AnyView {
      return AnyView(ContentView()
      .environmentObject(ItemManager()))
    }
}

Monica Granbois · Accepted Answer · 2019-12-31 17:30:39Z

I ended up using the @State it the second screen (ItemView) and setting the @EnvironmentObject itemManager when the view disappears.

First Screen:

import SwiftUI

struct ContentView: View {
  @EnvironmentObject var itemManager: ItemManager

  var body: some View {
    List {
      ForEach(itemManager.items, id: \.self) { item in
        NavigationLink(destination: ItemView(selection: item.type, food: item)) {
          VStack(alignment: HorizontalAlignment.leading) {
            HStack {
              Text(item.name)
              Spacer()
              Text("x")
              Text(String(item.quantity))
            }
            Text(item.type.rawValue).font(.footnote)
          }
        }
      }
    }
  }
}

struct ContentView_Previews: PreviewProvider {
  static var previews: some View {
    ContentView().environmentObject(ItemManager())
  }
}

Note: I got the var itemIndex: Int strategry from the Landmark apple tutorial

Second Screen:

import SwiftUI

struct ItemView: View {

  @EnvironmentObject var itemManager: ItemManager
  @State var selection : FoodType
  var food: Item
  var itemIndex: Int { 
    itemManager.items.firstIndex(where: { $0.id == food.id })!
  }

  var body: some View {

    VStack {
      Text("Update")
      Form {
        Section {
          Picker(selection: $selection, label: Text("Food Type")) {
            ForEach(FoodType.allCases.sorted()) { type in
              Text(type.rawValue).tag(type)
            }
          }
        }
      }
    }.navigationBarTitle(Text("Item"))
    .onDisappear {
      self.itemManager.items[self.itemIndex].type = self.selection
    }
  }
}

struct ItemView_Previews: PreviewProvider {
  static var previews: some View {
    let food = Item(id: UUID(), type: FoodType.fruit, quantity: 1, name: "apple")
    return ItemView(selection: FoodType.fruit, food: food)
  }
}

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