[SwiftUI] ObservableObject と bodyが呼び出されるタイミング

class External: ObservableObject {
  @Published var flag = false
}

struct ContentView: View {
  
  @ObservedObject var external = External()
  
  var body: some View {
    print("render")
    return VStack {
      Text("\(external.flag.description)")
      Button(action: {
        self.external.flag = true
      }) {
        Text("Hit!")
          .font(.headline)
      }
    }
  }
}