According to the SwiftUI documentation, @EnvironmentObject is a dynamic view property that uses a bindable object supplied by an ancestor view to invalidate the current view whenever the bindable object changes (source).

Essentially, you can use it to manage access to global state. [Hacking with Swift has a great article that explains the differences between @State, @ObservableObject, and @EnvironmentObject.

Particularly, it says There’s a third type of property available to use, which is @EnvironmentObject. This is a value that is made available to your views through the application itself – it’s shared data that every view can read if they want to.

It turns out, however, that while values put into the environment nicely percolate down the view hierarchy, this is not the case for modals.

Let's assume the following code:

class AppState: ObservableObject {
  @Published var counter = 0
}

struct ContentView: View {
  @EnvironmentObject var state: AppState
  @State var presentDetailsView = false
  @State var presentDetailsViewNoEnvironment = false
  
  var body: some View {
    NavigationView {
      VStack {
        Button(action: { self.state.counter += 1 }) {
          Text("Current value: \(state.counter)")
        }

        // 1

        Button(action: { self.presentDetailsView = true }) {
          Text("Present details")
        }
        Button(action: { self.presentDetailsViewNoEnvironment = true }) {
          Text("Present details w/o Environment (will crash)")
        }
      }
      .navigationBarTitle("@EnvironmentObject: Master")
      // 2
    }
  }
}

struct DetailsView: View {
  @EnvironmentObject var state: AppState
  
  var body: some View {
    Text("The value is \(state.counter)")
  }
}

If you now insert the following code at location (1), you will see that you can navigate from the master view to the details view as expected, and the details view can access the counter state via the global environment:

        NavigationLink(destination: DetailsView()) {
          Text("Navigate to details")
        }

However, adding the following code at location (2) will result in a crash:

      .sheet(isPresented: $presentDetailsViewNoEnvironment) {
        DetailsView()
      }

Fatal error: No observable object of type AppState.Type found.
A View.environmentObject(_:) for AppState.Type may be missing as an ancestor of this view.: file /BuildRoot/Library/Caches/com.apple.xbs/Sources/Monoceros_Sim/Monoceros-21.1.2/Core/EnvironmentObject.swift, line 171

Instead, in order to present the details view as a modal, you have to pass the state on, like so:

      .sheet(isPresented: $presentDetailsView) {
        DetailsView().environmentObject(self.state)
      }

Looking at the view hierarchy in the view debugger, we can see that thethis actually makes sense, as a modal screen is added to the view hierarchy in parallel to the original ContentView.

When navigating from ContentView to DetailsView using a NavigationLink, DetailsView becomes a child of ContentView:

When displaying a modal sheet using sheet(isPresented:content:), however, DetailsView will not become a child of ContentView. Instead, the hosting view will be instantiated in parallel to the originating view.