Viewmodels are now maintained throughout the app lifecycle

Aurora/Design/Components/NavigationMenu/NavigationItem.cs

@@ -17,5 +17,7 @@ namespace Aurora.Design.Components.NavigationMenu
         public string Group { get; set; }
 
         public Type TargetType { get; set; }
+
+        public Type TargetViewModelType { get; set; }
     }
 }

Aurora/Design/Views/Artists/ArtistsViewModel.cs

@@ -1,7 +1,7 @@
 using System;
 namespace Aurora.Design.Views.Artists
 {
-    public class ArtistsViewModel
+    public class ArtistsViewModel : BaseViewModel
     {
         public ArtistsViewModel()
         {

Aurora/Design/Views/BaseViewModel.cs

@@ -1,6 +1,7 @@
 using System;
 using System.ComponentModel;
 using System.Runtime.CompilerServices;
+using System.Threading.Tasks;
 
 namespace Aurora.Design.Views
 {
@@ -10,6 +11,13 @@ namespace Aurora.Design.Views
         {
         }
 
+        #region Lifecycle
+        public virtual void OnActive() { }
+
+        public virtual void OnInactive() { }
+
+        #endregion
+
         #region INotifyPropertyChanged Implementation
         public bool SetProperty<T>(ref T storage, T value, [CallerMemberName] string propertyName = null)
         {

Aurora/Design/Views/MainView/MainView.xaml.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.ObjectModel;
+using System.Collections.Generic;
 using System.Linq;
 using Aurora.Design.Components.NavigationMenu;
 using Aurora.Design.Views.MainView;
@@ -11,10 +12,14 @@ namespace Aurora.Design.Views.Main
     [XamlCompilation(XamlCompilationOptions.Compile)]
     public partial class MainView : MasterDetailPage
     {
+        private Dictionary<int, BaseViewModel> _viewModels;
+        private BaseViewModel _lastViewModel;
+
         public MainView()
         {
             InitializeComponent();
             BindingContext = new MainViewModel();
+            _viewModels = new Dictionary<int, BaseViewModel>();
             MasterPage.ListView.ItemSelected += ListView_ItemSelected;
 
             Appearing += OnAppearing;
@@ -32,6 +37,33 @@ namespace Aurora.Design.Views.Main
 
             var view = (View)Activator.CreateInstance(item.TargetType);
 
+            //Check if we have an instantiated viewModel
+            BaseViewModel vm = new BaseViewModel();
+            if (_viewModels.ContainsKey(item.Id))
+            {
+                _viewModels.TryGetValue(item.Id, out vm);
+            }
+            else
+            {
+                if (item.TargetViewModelType.BaseType != typeof(BaseViewModel))
+                {
+                    throw new InvalidOperationException("TargetViewModel field must be of type BaseViewModel");
+                }
+
+                //Instantiate new view model
+                vm = (BaseViewModel)Activator.CreateInstance(item.TargetViewModelType);
+                _viewModels.Add(item.Id, vm);
+            }
+            //Activate viewmodel
+            vm.OnActive();
+
+            //Deactivate last viewModel
+            _lastViewModel.OnInactive();
+
+            //Assign viewModel
+            _lastViewModel = vm;
+            view.BindingContext = vm;
+
             ContentPresenter viewContent = (ContentPresenter)ContentPage.Content.FindByName("ViewContent");
             viewContent.Content = view;
 
@@ -47,7 +79,26 @@ namespace Aurora.Design.Views.Main
         {
             //Set initial view from first item in list
             ObservableCollection<NavigationGroupItem> screenList = (ObservableCollection<NavigationGroupItem>)MasterPage.ListView.ItemsSource;
-            var view = (View)Activator.CreateInstance(screenList.FirstOrDefault().FirstOrDefault().TargetType);
+
+            //Assign viewModel
+            NavigationItem firstNavItem = screenList.FirstOrDefault().FirstOrDefault();
+            var view = (View)Activator.CreateInstance(firstNavItem.TargetType);
+
+            BaseViewModel vm = new BaseViewModel();
+            if (_viewModels.ContainsKey(firstNavItem.Id))
+            {
+                _viewModels.TryGetValue(firstNavItem.Id, out vm);
+            }
+            else
+            {
+                //Instantiate new view model
+                vm = (BaseViewModel)Activator.CreateInstance(firstNavItem.TargetViewModelType);
+                _viewModels.Add(firstNavItem.Id, vm);
+            }
+
+            view.BindingContext = vm;
+            _lastViewModel = vm;
+            vm.OnActive();
 
             ContentPresenter viewContent = (ContentPresenter)ContentPage.Content.FindByName("ViewContent");
             viewContent.Content = view;

Aurora/Design/Views/MainView/MainViewModel.cs

@@ -32,12 +32,12 @@ namespace Aurora.Design.Views.MainView
         {
             _pages = new ObservableCollection<NavigationItem>(new[]
             {
-                new NavigationItem { Id = 4, Title = "Party", Group="Social", TargetType = typeof(PartyView)},
+                new NavigationItem { Id = 0, Title = "Songs", Group="Library", TargetType = typeof(SongsView), TargetViewModelType = typeof(SongsViewModel) },
-                new NavigationItem { Id = 5, Title = "Profile", Group="Social", TargetType = typeof(ProfileView)},
+                new NavigationItem { Id = 1, Title = "Artists", Group="Library", TargetType = typeof(ArtistsView), TargetViewModelType = typeof(ArtistsViewModel)},
-                new NavigationItem { Id = 0, Title = "Songs", Group="Library", TargetType = typeof(SongsView) },
+                new NavigationItem { Id = 2, Title = "Albums", Group="Library", TargetType = typeof(AlbumsView), TargetViewModelType = typeof(AlbumsViewModel)},
-                new NavigationItem { Id = 1, Title = "Artists", Group="Library", TargetType = typeof(ArtistsView)},
+                new NavigationItem { Id = 3, Title = "Stations", Group="Library", TargetType = typeof(StationsView), TargetViewModelType = typeof(StationsViewModel)},
-                new NavigationItem { Id = 2, Title = "Albums", Group="Library", TargetType = typeof(AlbumsView)},
+                new NavigationItem { Id = 4, Title = "Party", Group="Social", TargetType = typeof(PartyView), TargetViewModelType = typeof(PartyViewModel)},
-                new NavigationItem { Id = 3, Title = "Stations", Group="Library", TargetType = typeof(StationsView)},
+                new NavigationItem { Id = 5, Title = "Profile", Group="Social", TargetType = typeof(ProfileView), TargetViewModelType = typeof(ProfileViewModel)},
             });
 
         }

Aurora/Design/Views/Party/PartyView.xaml.cs

@@ -8,7 +8,6 @@ namespace Aurora.Design.Views.Party
         public PartyView()
         {
             InitializeComponent();
-            BindingContext = new PartyViewModel();
         }
     }
 }

Aurora/Design/Views/Profile/ProfileView.xaml.cs

@@ -9,7 +9,6 @@ namespace Aurora.Design.Views.Profile
         public ProfileView()
         {
             InitializeComponent();
-            BindingContext = new ProfileViewModel();
         }
     }
 }

Aurora/Design/Views/Songs/SongsView.xaml

@@ -5,10 +5,6 @@
      xmlns:songs="clr-namespace:Aurora.Design.Views.Songs"
      xmlns:dg="clr-namespace:Xamarin.Forms.DataGrid;assembly=Xamarin.Forms.DataGrid"
      x:Class="Aurora.Design.Views.Songs.SongsView">
-    <ContentPage.BindingContext>
-        <songs:SongsViewModel
-             x:Name="songsViewModel"/>
-    </ContentPage.BindingContext>
     <ContentPage.Content>
         <dg:DataGrid
              ItemsSource="{Binding SongsList}"

Aurora/Design/Views/Stations/StatiosnViewModel.cs

@@ -1,7 +1,7 @@
 using System;
 namespace Aurora.Design.Views.Stations
 {
-    public class StationsViewModel
+    public class StationsViewModel : BaseViewModel
     {
         public StationsViewModel()
         {