Moved to a more simple navigation strategy, instead of MessagingCenter-based navigation.

Customers/App.xaml.cs

@@ -11,7 +11,7 @@ namespace Customers
 
             SubscribeToDisplayAlertMessages();
 
-            SubscribeToNavigationMessages();
+//            SubscribeToNavigationMessages();
 
             if (Device.OS == TargetPlatform.iOS)
                 navPage.BarTextColor = Color.White;
@@ -53,15 +53,15 @@ namespace Customers
         /// <summary>
         /// Subscribes to messages for navigation.
         /// </summary>
-        static void SubscribeToNavigationMessages()
-        {
-            MessagingService.Current.Subscribe(MessageKeys.PopAsync, async (service) =>
-                {
-                    var task = Application.Current?.MainPage?.Navigation?.PopAsync();
-                    if (task != null)
-                        await task;
-                });
-        }
+//        static void SubscribeToNavigationMessages()
+//        {
+//            MessagingService.Current.Subscribe(MessageKeys.PopAsync, async (service) =>
+//                {
+//                    var task = Application.Current?.MainPage?.Navigation?.PopAsync();
+//                    if (task != null)
+//                        await task;
+//                });
+//        }
     }
 }
 

Customers/Constants/MessageKeys.cs

@@ -6,9 +6,6 @@ namespace Customers
     {
         public const string DisplayAlert = "DisplayAlert";
         public const string DisplayQuestion = "DisplayQuestion";
-        public const string NavigateToDetailPage = "NavigateToDetail";
-        public const string NavigateToEditPage = "NavigateToEdit";
-        public const string PopAsync = "PopAsync";
         public const string SaveCustomer = "SaveCustomer";
         public const string DeleteCustomer = "DeleteCustomer";
         public const string CustomerLocationUpdated = "CustomerLocationUpdated";

Customers/Customers.csproj

@@ -57,6 +57,7 @@
     <Compile Include="Data\ILocalization.cs" />
     <Compile Include="Extensions\IntExtensions.cs" />
     <Compile Include="Constants\MessageKeys.cs" />
+    <Compile Include="ViewModels\NavigationAwareBaseViewModel.cs" />
   </ItemGroup>
   <Import Project="$(MSBuildExtensionsPath32)\Microsoft\Portable\$(TargetFrameworkVersion)\Microsoft.Portable.CSharp.targets" />
   <Import Project="..\packages\Xamarin.Forms.2.0.0.6490\build\portable-win+net45+wp80+win81+wpa81+MonoAndroid10+MonoTouch10+Xamarin.iOS10\Xamarin.Forms.targets" Condition="Exists('..\packages\Xamarin.Forms.2.0.0.6490\build\portable-win+net45+wp80+win81+wpa81+MonoAndroid10+MonoTouch10+Xamarin.iOS10\Xamarin.Forms.targets')" />

Customers/Pages/CustomerDetailPage.xaml.cs

@@ -23,24 +23,12 @@ namespace Customers
         {
             base.OnAppearing();
 
-            // subscribe to NavigateToEditPage messages
-            MessagingService.Current.Subscribe<CustomerDetailViewModel>(MessageKeys.NavigateToEditPage, async (service, viewmodel) =>
-                await Navigation.PushAsync(new CustomerEditPage() { BindingContext = ViewModel }));
-
             // Typically, is preferable to call into the viewmodel for OnAppearing() logic to be performed,
             // but we're not doing that in this case because we need to interact with the Xamarin.Forms.Map property on this Page.
             // In the future, the Map type and it's properties may get more binding support, so that the map setup can be omitted from code-behind.
             await SetupMap();
         }
 
-        protected override void OnDisappearing()
-        {
-            base.OnDisappearing();
-
-            // unsubscribe from NavigateToEditPage messages
-            MessagingService.Current.Unsubscribe<CustomerDetailViewModel>(MessageKeys.NavigateToEditPage);
-        }
-
         /// <summary>
         /// Sets up the map.
         /// </summary>

Customers/Pages/CustomerEditPage.xaml.cs

@@ -18,6 +18,11 @@ namespace Customers
                 Title = null; // because iOS already displays the previous page's title with the back button, we don't want to display it twice.
         }
 
+        /// <summary>
+        /// Ensures the state field has 2 characters at most.
+        /// </summary>
+        /// <param name="sender">The sender.</param>
+        /// <param name="e">The PropertyChangedEventArgs</param>
         void StateEntry_PropertyChanged (object sender, PropertyChangedEventArgs e)
         {
             if (e.PropertyName == "Text")
@@ -35,6 +40,11 @@ namespace Customers
             }
         }
 
+        /// <summary>
+        /// Ensures the zip code field has 5 characters at most.
+        /// </summary>
+        /// <param name="sender">The sender.</param>
+        /// <param name="e">The PropertyChangedEventArgs</param>
         void PostalCode_PropertyChanged (object sender, PropertyChangedEventArgs e)
         {
             if (e.PropertyName == "Text")

Customers/Pages/CustomerListPage.xaml.cs

@@ -22,18 +22,26 @@ namespace Customers
             fab.Clicked = AndroidAddButtonClicked;
         }
 
+        /// <summary>
+        /// The action to take when a list item is tapped.
+        /// </summary>
+        /// <param name="sender"> The sender.</param>
+        /// <param name="e">The ItemTappedEventArgs</param>
         void ItemTapped (object sender, ItemTappedEventArgs e)
         {
-            // send message to navigate to detail page
-            MessagingService.Current.SendMessage<CustomerDetailViewModel>(MessageKeys.NavigateToDetailPage, new CustomerDetailViewModel((Customer)e.Item));
+            Navigation.PushAsync(new CustomerDetailPage() { BindingContext = new CustomerDetailViewModel((Customer)e.Item) });
 
             ((ListView)sender).SelectedItem = null;
         }
 
+        /// <summary>
+        /// The action to take when the + button is clicked on Android.
+        /// </summary>
+        /// <param name="sender">The sender.</param>
+        /// <param name="e">The EventArgs</param>
         void AndroidAddButtonClicked (object sender, EventArgs e)
         {
-            // send message to navigate to edit page (new customer)
-            MessagingService.Current.SendMessage(MessageKeys.NavigateToEditPage);
+            Navigation.PushAsync(new CustomerEditPage() { BindingContext = new CustomerDetailViewModel(new Customer()) });
         }
 
         protected override void OnAppearing()
@@ -41,32 +49,6 @@ namespace Customers
             base.OnAppearing();
 
             ViewModel.LoadCustomersCommand.Execute(null);
-
-            // subscribe to messages to navigate to detail page
-            MessagingService.Current.Subscribe<CustomerDetailViewModel>(MessageKeys.NavigateToDetailPage, async (service, viewmodel) =>
-                await Navigation.PushAsync(new CustomerDetailPage() { BindingContext = viewmodel }));
-
-            // subscribe to messages to navigate to edit page (new customer)
-            MessagingService.Current.Subscribe(MessageKeys.NavigateToEditPage, async (service) =>
-                await Navigation.PushAsync(new CustomerEditPage() { BindingContext = new CustomerDetailViewModel(new Customer()) }));
-
-            // subscribe to messages to navigate to edit page (existing customer)
-            MessagingService.Current.Subscribe<CustomerDetailViewModel>(MessageKeys.NavigateToEditPage, async (service, viewmodel) =>
-                await Navigation.PushAsync(new CustomerEditPage() { BindingContext = viewmodel }));
-        }
-
-        protected override void OnDisappearing()
-        {
-            base.OnDisappearing();
-
-            // unsubscribe from messages to navigate to detail page
-            MessagingService.Current.Unsubscribe<CustomerDetailViewModel>(MessageKeys.NavigateToDetailPage);
-
-            // unsubscribe from messages to navigate to edit page (new customer)
-            MessagingService.Current.Unsubscribe(MessageKeys.NavigateToEditPage);
-
-            // unsubscribe from messages to navigate to edit page (existing customer)
-            MessagingService.Current.Unsubscribe<CustomerDetailViewModel>(MessageKeys.NavigateToEditPage);
         }
     }
 }

Customers/ViewModels/CustomerDetailViewModel.cs

@@ -11,7 +11,7 @@ using FormsToolkit;
 
 namespace Customers
 {
-    public class CustomerDetailViewModel : BaseViewModel
+    public class CustomerDetailViewModel : NavigationAwareBaseViewModel
     {
         bool _IsNewCustomer;
 
@@ -78,11 +78,11 @@ namespace Customers
             get
             {
                 return _SaveCustomerCommand ??
-                (_SaveCustomerCommand = new Command(ExecuteSaveCustomerCommand));
+                    (_SaveCustomerCommand = new Command(() => ExecuteSaveCustomerCommand()));
             }
         }
 
-        void ExecuteSaveCustomerCommand()
+        async Task ExecuteSaveCustomerCommand()
         {
             if (String.IsNullOrWhiteSpace(Customer.LastName) || String.IsNullOrWhiteSpace(Customer.FirstName))
             {
@@ -104,11 +104,9 @@ namespace Customers
             }
             else
             {
-                // send a message that we want the given customer to be saved
                 MessagingService.Current.SendMessage<Customer>(MessageKeys.SaveCustomer, this.Customer);
 
-                // perform a pop in order to navigate back to the customer list
-                MessagingService.Current.SendMessage(MessageKeys.PopAsync);
+                await PopAsync();
             }
         }
 
@@ -143,13 +141,13 @@ namespace Customers
             get
             {
                 return _EditCustomerCommand ??
-                (_EditCustomerCommand = new Command(ExecuteEditCustomerCommand));
+                    (_EditCustomerCommand = new Command(async () => await ExecuteEditCustomerCommand()));
             }
         }
 
-        void ExecuteEditCustomerCommand()
+        async Task ExecuteEditCustomerCommand()
         {
-            MessagingService.Current.SendMessage<CustomerDetailViewModel>(MessageKeys.NavigateToEditPage, this);
+            await PushAsync(new CustomerEditPage() { BindingContext = this });
         }
 
 
@@ -163,7 +161,7 @@ namespace Customers
             get
             {
                 return _DeleteCustomerCommand ??
-                (_DeleteCustomerCommand = new Command(ExecuteDeleteCustomerCommand));
+                    (_DeleteCustomerCommand = new Command(ExecuteDeleteCustomerCommand));
             }
         }
 
@@ -175,16 +173,16 @@ namespace Customers
                     Question = null,
                     Positive = "Delete",
                     Negative = "Cancel",
-                    OnCompleted = new Action<bool>(result =>
+                    OnCompleted = new Action<bool>(async result =>
                         {
                             if (result)
                             {
+                                await PopAsync(false);
+
+                                await PopAsync();
+
                                 // send a message that we want the given customer to be deleted
                                 MessagingService.Current.SendMessage<Customer>(MessageKeys.DeleteCustomer, this.Customer);
-
-                                MessagingService.Current.SendMessage(MessageKeys.PopAsync);
-
-                                MessagingService.Current.SendMessage(MessageKeys.PopAsync);
                             }
                         })
                 });

Customers/ViewModels/CustomerListViewModel.cs

@@ -9,7 +9,7 @@ using FormsToolkit;
 
 namespace Customers
 {
-    public class CustomerListViewModel : BaseViewModel
+    public class CustomerListViewModel : NavigationAwareBaseViewModel
     {
         public CustomerListViewModel()
         {
@@ -98,14 +98,13 @@ namespace Customers
             get
             {
                 return _NewCustomerCommand ??
-                (_NewCustomerCommand = new Command(ExecuteNewCustomerCommand));
+                    (_NewCustomerCommand = new Command(async () => await ExecuteNewCustomerCommand()));
             }
         }
 
-        void ExecuteNewCustomerCommand()
+        async Task ExecuteNewCustomerCommand()
         {
-            // send message to navigate to edit page (new customer)
-            MessagingService.Current.SendMessage(MessageKeys.NavigateToEditPage, new CustomerDetailViewModel(new Customer()));
+            await PushAsync(new CustomerEditPage() { BindingContext = new CustomerDetailViewModel(new Customer()) });
         }
 
         Command _DialNumberCommand;

Customers/ViewModels/NavigationAwareBaseViewModel.cs

@@ -0,0 +1,113 @@
+using System;
+using MvvmHelpers;
+using Xamarin.Forms;
+using System.Threading.Tasks;
+using System.Collections.Generic;
+
+namespace Customers
+{
+    /// <summary>
+    /// Implements the INavigation interface on top of BaseViewModel.
+    /// </summary>
+    public abstract class NavigationAwareBaseViewModel : BaseViewModel, INavigation
+    {
+        readonly INavigation _Navigation;
+
+        protected NavigationAwareBaseViewModel()
+        {
+            // If Navigation is available on Application.Current.MainPage, get it.
+            _Navigation = Application.Current?.MainPage?.Navigation;
+        }
+
+        #region INavigation implementation
+
+        public void RemovePage(Page page)
+        {
+            _Navigation?.RemovePage(page);
+        }
+
+        public void InsertPageBefore(Page page, Page before)
+        {
+            _Navigation?.InsertPageBefore(page, before);
+        }
+
+        public async Task PushAsync(Page page)
+        {
+            var task = _Navigation?.PushAsync(page);
+            if (task != null)
+                await task;
+        }
+
+        public async Task<Page> PopAsync()
+        {
+            var task = _Navigation?.PopAsync();
+            return task != null ? await task : await Task.FromResult(null as Page);
+        }
+
+        public async Task PopToRootAsync()
+        {
+            var task = _Navigation?.PopToRootAsync();
+            if (task != null)
+                await task;
+        }
+
+        public async Task PushModalAsync(Page page)
+        {
+            var task = _Navigation?.PushModalAsync(page);
+            if (task != null)
+                await task;
+        }
+
+        public async Task<Page> PopModalAsync()
+        {
+            var task = _Navigation?.PopModalAsync();
+            return (task != null) ? await task : await Task.FromResult(null as Page);
+        }
+
+        public async Task PushAsync(Page page, bool animated)
+        {
+            var task = _Navigation?.PushAsync(page, animated);
+            if (task != null)
+                await task;
+        }
+
+        public async Task<Page> PopAsync(bool animated)
+        {
+            var task = _Navigation?.PopAsync(animated);
+            return (task != null) ? await task : await Task.FromResult(null as Page);
+        }
+
+        public async Task PopToRootAsync(bool animated)
+        {
+            var task = _Navigation?.PopToRootAsync(animated);
+            if (task != null)
+                await task;
+        }
+
+        public async Task PushModalAsync(Page page, bool animated)
+        {
+            var task = _Navigation?.PushModalAsync(page, animated);
+            if (task != null)
+                await task;
+        }
+
+        public async Task<Page> PopModalAsync(bool animated)
+        {
+            var task = _Navigation?.PopModalAsync(animated);
+            return (task != null) ? await task : await Task.FromResult(null as Page);
+        }
+
+        public IReadOnlyList<Page> NavigationStack
+        {
+            get { return _Navigation?.NavigationStack; }
+        }
+
+        public IReadOnlyList<Page> ModalStack
+        {
+            get { return _Navigation?.ModalStack; }
+        }
+
+        #endregion
+    }
+}
+