SwiftUI: Can't modify passed object - swiftui

Question

I'm a beginner working on an app using the MVVM model. The basic idea is that I have Events that each contain several sessions. The events need to be able to be modified including adding and deleting sessions from the events. The main problem is that I can not modify a passed object as described below.
My model:
import SwiftUI
struct Event: Identifiable {
let id = UUID()
let trackName: String
let date: Date
var sessions: [Session]
}
struct Session: Identifiable {
let id = UUID()
let sessionNumber: Int
let time: Date
}
I then have a View Model file that publishes the array of Events :
import Foundation
class EventListViewModel: ObservableObject {
// exposed variables
#Published var events: [Event] = []
var session1 = Session(sessionNumber: 1, time: Date.now)
var session2 = Session(sessionNumber: 2, time: Date.now)
init() {
getEvents()
}
func getEvents() {
let newEvents = [
Event(trackName: "Awesome Event", date: Date.now, sessions: [session1, session2]),
Event(trackName: "Second Event", date: Date.now, sessions: [session1, session2]),
Event(trackName: "Latest Event", date: Date.now, sessions: [session1, session2]),
]
events.append(contentsOf: newEvents)
}
I then have an EventsListView that works as expected. Because "EventListViewModel" is observable, I can set it as an environment object in my View. And because "events" is published, I can access it in the View. So far so good.
import SwiftUI
struct EventsListView: View {
#EnvironmentObject var eventListViewModel: EventListViewModel
var body: some View {
NavigationView {
List{
ForEach(eventListViewModel.events) { event in
NavigationLink(destination: EventDetailView(event: event), label: {
EventCell(event: event)
})
}
.onDelete(perform: eventListViewModel.deleteEvent)
.onMove(perform: eventListViewModel.moveEvent)
}
.navigationTitle("Events")
.navigationBarItems(
leading: EditButton(),
trailing: NavigationLink("New Event", destination: AddEventView())
)
}
}
}
When you click on the event in this List, it opens another view where you can see the details of the event. That works fine. The problem is that I can read the "session" variable, but I cannot append to it. I cannot modify it. "Cannot use mutating member on immutable value: 'self' is immutable"
mport SwiftUI
struct EventDetailView: View {
//#EnvironmentObject var eventListViewModel: EventListViewModel
var event: Event
var body: some View {
//NavigationView {
VStack {
Text(event.trackName)
.font(.title)
.padding()
Text("Number of Sessions: \(event.sessions.count)")
HStack{
Text("Sessions")
.font(.headline)
.multilineTextAlignment(.leading)
.frame(maxWidth: .infinity, alignment: .leading )
Text("Add Session")
.font(.headline)
.multilineTextAlignment(.trailing)
.onTapGesture {
//eventListViewModel.addSession(event: event)
var newSession = Session(sessionNumber: event.sessions.count+1, time: Date.now)
//event.sessions.append(newSession) // <-THIS DOES NOT WORK
}
}
ForEach(event.sessions) { session in
SessionCell(session: session) // <-THIS WORKS
}
In the Event model, I set the sessions variable to a var.. I understand there is some concept where when you pass an object, it is read only. How should I set this up so that I can modify the sessions? I haven't actually gotten to this point yet, but I also need to be able to modify the Events. What's the proper way to organize this?
Thanks a lot
UPDATE------
I've implemented your suggestion, except I can't get the EventView preview to work. I've done this and it errors saying "Cannot convert value of type 'Event' to expected argument type 'Binding'". I've tried all sorts of things and nothing seems to work. I tried including as an .environmentObject modifier, combos if local variables with Binding wrappers, etc. I can't crack it.
struct EventView_Previews: PreviewProvider {
static var previews: some View {
Group {
//EventView(eventIndex: 0)
EventView( Event(name: "Event", track: "Motorsport Park", date: Date.now, notes: "", sessions: [Session(time: Date.now, bestLap: "1:32.5", setupSel: 0, tiresSel: 0, weather: "", notes: "")]) )
.environmentObject(EventListViewModel())
}
}
}

Answer 1

Try using a binding for your event, such as:
In EventsListView use this (note $):
ForEach($eventListViewModel.events) { $event in
NavigationLink(destination: EventDetailView(event: $event), label: {
...
}
and in EventDetailView, use this:
struct EventDetailView: View {
#EnvironmentObject var eventListViewModel: EventListViewModel
#Binding var event: Event // <-- here
EDIT-1: try something like this for your EventView_Previews problem
struct EventView_Previews: PreviewProvider {
#State static var event = Event(name: "Event", track: "Motorsport Park", date: Date.now, notes: "", sessions: [Session(time: Date.now)])
static var previews: some View {
Group {
EventView(event: $event)
.environmentObject(EventListViewModel())
}
}
}
This is assuming your EventView is something like this:
struct EventView: View {
#EnvironmentObject var eventListViewModel: EventListViewModel
#Binding var event: Event
// ....
}