How to set an Environment Object in preview

You add it using .environmentObject(_:) modifier:

struct ContentView_Previews: PreviewProvider {

    static var previews: some View {
        ContentView()
            .environmentObject(YourObject())
    }
}

This userData property gets its value automatically, as long as the environmentObject(_:) modifier has been applied to a parent.

     struct UserList: View {

        @EnvironmentObject var userData: UserData

        var body: some View {
            NavigationView {
                List {
                    Toggle(isOn: $userData.showFavoritesOnly) {
                        Text("Users Fav only")
                    }

                    ForEach(landmarkData) { landmark in
                        if !self.userData.showFavoritesOnly || landmark.isFavorite {
                            NavigationLink(destination: LandmarkDetail(landmark: landmark)) {
                                UserRow(landmark: landmark)
                            }
                        }
                    }
                }
                .navigationBarTitle(Text("Users"))
            }
        }
    }

    struct UserList_Previews: PreviewProvider {
        static var previews: some View {
            UserList()
            .environmentObject(UserData())
        }
    }