first commit

.gitignore

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+/.vs
+/bin
+/obj

App.xaml

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+<?xml version="1.0" encoding="utf-8" ?>
+<Application xmlns="http://xamarin.com/schemas/2014/forms"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="MyMobileApp.App">
+    <!--
+        Define global resources and styles here, that apply to all pages in your app.
+    -->
+    <Application.Resources>
+        <ResourceDictionary>
+            <Color x:Key="Primary">#2196F3</Color>
+            <Style TargetType="Button">
+                <Setter Property="TextColor" Value="White"></Setter>
+                <Setter Property="VisualStateManager.VisualStateGroups">
+                    <VisualStateGroupList>
+                        <VisualStateGroup x:Name="CommonStates">
+                            <VisualState x:Name="Normal">
+                                <VisualState.Setters>
+                                    <Setter Property="BackgroundColor" Value="{StaticResource Primary}" />
+                                </VisualState.Setters>
+                            </VisualState>
+                            <VisualState x:Name="Disabled">
+                                <VisualState.Setters>
+                                    <Setter Property="BackgroundColor" Value="#332196F3" />
+                                </VisualState.Setters>
+                            </VisualState>
+                        </VisualStateGroup>
+                    </VisualStateGroupList>
+                </Setter>
+            </Style>
+        </ResourceDictionary>        
+    </Application.Resources>
+</Application>

App.xaml.cs

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+using MyMobileApp.Services;
+using MyMobileApp.Views;
+using System;
+using Xamarin.Forms;
+using Xamarin.Forms.Xaml;
+
+namespace MyMobileApp
+{
+    public partial class App : Application
+    {
+
+        public App()
+        {
+            InitializeComponent();
+
+            DependencyService.Register<MockDataStore>();
+            MainPage = new SignIn();
+        }
+
+        protected override void OnStart()
+        {
+        }
+
+        protected override void OnSleep()
+        {
+        }
+
+        protected override void OnResume()
+        {
+        }
+    }
+}

AppShell.xaml

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+<?xml version="1.0" encoding="UTF-8"?>
+<Shell xmlns="http://xamarin.com/schemas/2014/forms" 
+       xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+       xmlns:local="clr-namespace:MyMobileApp.Views"
+       Title="MyMobileApp"
+       x:Class="MyMobileApp.AppShell">
+
+    <!--
+        The overall app visual hierarchy is defined here, along with navigation.
+    
+        https://docs.microsoft.com/xamarin/xamarin-forms/app-fundamentals/shell/
+    -->
+
+    <Shell.Resources>
+        <ResourceDictionary>
+            <Style x:Key="BaseStyle" TargetType="Element">
+                <Setter Property="Shell.BackgroundColor" Value="{StaticResource Primary}" />
+                <Setter Property="Shell.ForegroundColor" Value="White" />
+                <Setter Property="Shell.TitleColor" Value="White" />
+                <Setter Property="Shell.DisabledColor" Value="#B4FFFFFF" />
+                <Setter Property="Shell.UnselectedColor" Value="#95FFFFFF" />
+                <Setter Property="Shell.TabBarBackgroundColor" Value="{StaticResource Primary}" />
+                <Setter Property="Shell.TabBarForegroundColor" Value="White"/>
+                <Setter Property="Shell.TabBarUnselectedColor" Value="#95FFFFFF"/>
+                <Setter Property="Shell.TabBarTitleColor" Value="White"/>
+            </Style>
+            <Style TargetType="TabBar" BasedOn="{StaticResource BaseStyle}" />
+            <Style TargetType="FlyoutItem" BasedOn="{StaticResource BaseStyle}" />
+
+            <!--
+            Default Styles for all Flyout Items
+            https://docs.microsoft.com/xamarin/xamarin-forms/app-fundamentals/shell/flyout#flyoutitem-and-menuitem-style-classes
+            -->
+            <Style Class="FlyoutItemLabelStyle" TargetType="Label">
+                <Setter Property="TextColor" Value="White"></Setter>
+            </Style>
+            <Style Class="FlyoutItemLayoutStyle" TargetType="Layout" ApplyToDerivedTypes="True">
+                <Setter Property="VisualStateManager.VisualStateGroups">
+                    <VisualStateGroupList>
+                        <VisualStateGroup x:Name="CommonStates">
+                            <VisualState x:Name="Normal">
+                                <VisualState.Setters>
+                                    <Setter Property="BackgroundColor" Value="{x:OnPlatform UWP=Transparent, iOS=White}" />
+                                    <Setter TargetName="FlyoutItemLabel" Property="Label.TextColor" Value="{StaticResource Primary}" />
+                                </VisualState.Setters>
+                            </VisualState>
+                            <VisualState x:Name="Selected">
+                                <VisualState.Setters>
+                                    <Setter Property="BackgroundColor" Value="{StaticResource Primary}" />
+                                </VisualState.Setters>
+                            </VisualState>
+                        </VisualStateGroup>
+                    </VisualStateGroupList>
+                </Setter>
+            </Style>
+
+            <!--
+            Custom Style you can apply to any Flyout Item
+            -->
+            <Style Class="MenuItemLayoutStyle" TargetType="Layout" ApplyToDerivedTypes="True">
+                <Setter Property="VisualStateManager.VisualStateGroups">
+                    <VisualStateGroupList>
+                        <VisualStateGroup x:Name="CommonStates">
+                            <VisualState x:Name="Normal">
+                                <VisualState.Setters>
+                                    <Setter TargetName="FlyoutItemLabel" Property="Label.TextColor" Value="{StaticResource Primary}" />
+                                </VisualState.Setters>
+                            </VisualState>
+                        </VisualStateGroup>
+                    </VisualStateGroupList>
+                </Setter>
+            </Style>
+        </ResourceDictionary>
+    </Shell.Resources>
+
+    <!-- 
+        When the Flyout is visible this defines the content to display in the flyout.
+        FlyoutDisplayOptions="AsMultipleItems" will create a separate flyout item for each child element    
+        https://docs.microsoft.com/dotnet/api/xamarin.forms.shellgroupitem.flyoutdisplayoptions?view=xamarin-forms
+    -->
+    <FlyoutItem Title="About" Icon="icon_about.png">
+        <ShellContent Route="AboutPage" ContentTemplate="{DataTemplate local:AboutPage}" />
+    </FlyoutItem>
+    <FlyoutItem Title="Browse" Icon="icon_feed.png">
+        <ShellContent Route="ItemsPage" ContentTemplate="{DataTemplate local:ItemsPage}" />
+    </FlyoutItem>
+
+    <!-- When the Flyout is visible this will be a menu item you can tie a click behavior to  -->
+    <MenuItem Text="Logout" StyleClass="MenuItemLayoutStyle" Clicked="OnMenuItemClicked">
+    </MenuItem>
+
+    <!--
+        TabBar lets you define content that won't show up in a flyout menu. When this content is active
+        the flyout menu won't be available. This is useful for creating areas of the application where 
+        you don't want users to be able to navigate away from. If you would like to navigate to this 
+        content you can do so by calling 
+        await Shell.Current.GoToAsync("//LoginPage");
+    -->
+    <TabBar>
+        <ShellContent Route="LoginPage" ContentTemplate="{DataTemplate local:LoginPage}" />
+    </TabBar>
+
+    <!-- Optional Templates 
+    // These may be provided inline as below or as separate classes.
+
+    // This header appears at the top of the Flyout.
+    // https://docs.microsoft.com/xamarin/xamarin-forms/app-fundamentals/shell/flyout#flyout-header
+    <Shell.FlyoutHeaderTemplate>
+        <DataTemplate>
+            <Grid>ContentHere</Grid>
+        </DataTemplate>
+    </Shell.FlyoutHeaderTemplate>
+
+    // ItemTemplate is for ShellItems as displayed in a Flyout
+    // https://docs.microsoft.com/xamarin/xamarin-forms/app-fundamentals/shell/flyout#define-flyoutitem-appearance
+    <Shell.ItemTemplate>
+        <DataTemplate>
+            <ContentView>
+                Bindable Properties: Title, Icon
+            </ContentView>
+        </DataTemplate>
+    </Shell.ItemTemplate>
+
+    // MenuItemTemplate is for MenuItems as displayed in a Flyout
+    // https://docs.microsoft.com/xamarin/xamarin-forms/app-fundamentals/shell/flyout#define-menuitem-appearance
+    <Shell.MenuItemTemplate>
+        <DataTemplate>
+            <ContentView>
+                Bindable Properties: Text, Icon
+            </ContentView>
+        </DataTemplate>
+    </Shell.MenuItemTemplate>
+
+    -->
+
+</Shell>

AppShell.xaml.cs

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+using MyMobileApp.ViewModels;
+using MyMobileApp.Views;
+using System;
+using System.Collections.Generic;
+using Xamarin.Forms;
+
+namespace MyMobileApp
+{
+    public partial class AppShell : Xamarin.Forms.Shell
+    {
+        public AppShell()
+        {
+            InitializeComponent();
+            Routing.RegisterRoute(nameof(ItemDetailPage), typeof(ItemDetailPage));
+            Routing.RegisterRoute(nameof(NewItemPage), typeof(NewItemPage));
+        }
+
+        private async void OnMenuItemClicked(object sender, EventArgs e)
+        {
+            await Shell.Current.GoToAsync("//LoginPage");
+        }
+    }
+}

AssemblyInfo.cs

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+using Xamarin.Forms.Xaml;
+
+[assembly: XamlCompilation(XamlCompilationOptions.Compile)]

GettingStarted.txt

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+Welcome to Xamarin.Forms! Here are some tips to get started building your app.
+
+Building Your App UI
+--------------------
+
+XAML Hot Reload quickly applies UI changes as you make them to your running app.
+This is the most productive way to preview and iteratively create your UI.
+
+Try it out:
+
+1. Run the app by clicking the Start Debugging (play) button in the above toolbar. 
+2. Open <MainProject>\Views\AboutPage.xaml.
+   Don't stop the app - keep it running while making changes.
+3. Change something! Hint: change the Accent color on line 14 from "#96d1ff" to "Pink".
+4. Watch the About screen update on the device or emulator, with the logo background now pink.
+
+Keep going and try more changes!
+
+QuickStart Guide
+----------------
+
+Learn more of the fundamentals for building apps with Xamarin here: https://aka.ms/xamarin-quickstart
+
+Your App Shell
+--------------
+
+This template uses Shell, an app container that reduces the complexity of your apps by providing fundamental features including:
+
+- A single place to describe the app's visual hierarchy.
+- Common navigation such as a flyout menu and tabs.
+- A URI-based navigation scheme that permits navigation to any page in the application.
+- An integrated search handler.
+
+Open AppShell.xaml to begin exploring. To learn more about Shell visit: https://docs.microsoft.com/xamarin/xamarin-forms/app-fundamentals/shell/introduction

Models/Item.cs

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+using System;
+
+namespace MyMobileApp.Models
+{
+    public class Item
+    {
+        public string Id { get; set; }
+        public string Text { get; set; }
+        public string Description { get; set; }
+    }
+}

MyMobileApp.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>netstandard2.0</TargetFramework>
+    <ProduceReferenceAssembly>true</ProduceReferenceAssembly>
+    <ProduceReferenceAssemblyInOutDir>true</ProduceReferenceAssemblyInOutDir>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Xamarin.Forms" Version="5.0.0.2196" />  
+    <PackageReference Include="Xamarin.Essentials" Version="1.7.0" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <EmbeddedResource Update="SecondPage.xaml">
+      <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
+    </EmbeddedResource>
+    <EmbeddedResource Update="SignIn.xaml">
+      <Generator>MSBuild:UpdateDesignTimeXaml</Generator>
+    </EmbeddedResource>
+  </ItemGroup>
+</Project>

SecondPage.xaml

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+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="MyMobileApp.SecondPage">
+    <ContentPage.Content>
+        <StackLayout>
+            <Button Text="Выйти"
+                    VerticalOptions="Center"
+                    HorizontalOptions="Center"
+                    FontSize="18"
+                    TextTransform="None"
+                    CornerRadius="15"
+                    WidthRequest="350"
+                    BackgroundColor="#48B2E7"
+                    Clicked="Button_Clicked" />
+        </StackLayout>
+    </ContentPage.Content>
+</ContentPage>

SecondPage.xaml.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+using Xamarin.Forms;
+using Xamarin.Forms.Xaml;
+
+namespace MyMobileApp
+{
+	[XamlCompilation(XamlCompilationOptions.Compile)]
+	public partial class SecondPage : ContentPage
+	{
+		public SecondPage ()
+		{
+			InitializeComponent ();
+		}
+
+        private void Button_Clicked(object sender, EventArgs e)
+        {
+            
+        }
+    }
+}

Services/IDataStore.cs

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+using System;
+using System.Collections.Generic;
+using System.Threading.Tasks;
+
+namespace MyMobileApp.Services
+{
+    public interface IDataStore<T>
+    {
+        Task<bool> AddItemAsync(T item);
+        Task<bool> UpdateItemAsync(T item);
+        Task<bool> DeleteItemAsync(string id);
+        Task<T> GetItemAsync(string id);
+        Task<IEnumerable<T>> GetItemsAsync(bool forceRefresh = false);
+    }
+}

Services/MockDataStore.cs

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+using MyMobileApp.Models;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace MyMobileApp.Services
+{
+    public class MockDataStore : IDataStore<Item>
+    {
+        readonly List<Item> items;
+
+        public MockDataStore()
+        {
+            items = new List<Item>()
+            {
+                new Item { Id = Guid.NewGuid().ToString(), Text = "First item", Description="This is an item description." },
+                new Item { Id = Guid.NewGuid().ToString(), Text = "Second item", Description="This is an item description." },
+                new Item { Id = Guid.NewGuid().ToString(), Text = "Third item", Description="This is an item description." },
+                new Item { Id = Guid.NewGuid().ToString(), Text = "Fourth item", Description="This is an item description." },
+                new Item { Id = Guid.NewGuid().ToString(), Text = "Fifth item", Description="This is an item description." },
+                new Item { Id = Guid.NewGuid().ToString(), Text = "Sixth item", Description="This is an item description." }
+            };
+        }
+
+        public async Task<bool> AddItemAsync(Item item)
+        {
+            items.Add(item);
+
+            return await Task.FromResult(true);
+        }
+
+        public async Task<bool> UpdateItemAsync(Item item)
+        {
+            var oldItem = items.Where((Item arg) => arg.Id == item.Id).FirstOrDefault();
+            items.Remove(oldItem);
+            items.Add(item);
+
+            return await Task.FromResult(true);
+        }
+
+        public async Task<bool> DeleteItemAsync(string id)
+        {
+            var oldItem = items.Where((Item arg) => arg.Id == id).FirstOrDefault();
+            items.Remove(oldItem);
+
+            return await Task.FromResult(true);
+        }
+
+        public async Task<Item> GetItemAsync(string id)
+        {
+            return await Task.FromResult(items.FirstOrDefault(s => s.Id == id));
+        }
+
+        public async Task<IEnumerable<Item>> GetItemsAsync(bool forceRefresh = false)
+        {
+            return await Task.FromResult(items);
+        }
+    }
+}

SignIn.xaml

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+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="MyMobileApp.SignIn">
+    <ContentPage.Content>
+        <StackLayout>
+            <Label Text="Привет!"
+                   Margin="0,100,0,0"
+                   VerticalOptions="Start"
+                   HorizontalOptions="CenterAndExpand"
+                   FontSize="30"
+                   TextColor="Black"/>
+            <Label Text="         Заполните Свои данные или&#10; продолжите через социальные медиа"
+                   VerticalOptions="Start"
+                   HorizontalOptions="CenterAndExpand"
+                   FontSize="20"
+                   TextColor="Gray"/>
+            <Grid>
+                <Label Text="Email"
+                       Margin="20,10,0,0"
+                       VerticalOptions="Start"
+                       HorizontalOptions="Start"
+                       FontSize="20"
+                       TextColor="Black" />
+                <Frame BackgroundColor="#f3f4f5"
+                       CornerRadius="25"
+                       Margin="20,50,20,0">
+                    <Label Text="xyz@gmail.com"
+                           VerticalOptions="Start"
+                           HorizontalOptions="Start"
+                           FontSize="20"
+                           TextColor="Gray" />
+                </Frame>
+            </Grid>
+            <Grid>
+                <Label Text="Пароль"
+                       Margin="20,10,0,0"
+                       VerticalOptions="Start"
+                       HorizontalOptions="Start"
+                       FontSize="20"
+                       TextColor="Black" />
+                <Frame BackgroundColor="#f3f4f5"
+                       CornerRadius="25"
+                       Margin="20,50,20,0">
+                    <Label Text="**********"
+                           VerticalOptions="Start"
+                           HorizontalOptions="Start"
+                           FontSize="20"
+                           TextColor="Gray" />
+                </Frame>
+            </Grid>
+            <Label Text="Восстановить"
+                   Margin="20,10,20,0"
+                   VerticalOptions="Start"
+                   HorizontalOptions="End"
+                   FontSize="15"
+                   TextColor="Gray" />
+            <Button Text="Войти"
+                    VerticalOptions="Center"
+                    HorizontalOptions="Start"
+                    Margin="21,20,0,0"
+                    FontSize="18"                    
+                    TextTransform="None"
+                    CornerRadius="15"
+                    WidthRequest="350"
+                    BackgroundColor="#48B2E7"
+                    Clicked="Button_Clicked"/>
+            <Grid>
+                <Label Text="Вы впервые?"
+                       Margin="0,190,230,0"
+                       VerticalOptions="End"
+                       HorizontalOptions="End"
+                       FontSize="18"
+                       TextColor="Gray" />
+                <Label Text="Создать пользователя"
+                       Margin="0,190,30,0"
+                       VerticalOptions="End"
+                       HorizontalOptions="End"
+                       FontSize="18"
+                       TextColor="Black" />
+            </Grid>
+        </StackLayout> 
+    </ContentPage.Content>
+</ContentPage>

SignIn.xaml.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+using Xamarin.Forms;
+using Xamarin.Forms.Xaml;
+
+namespace MyMobileApp
+{
+    [XamlCompilation(XamlCompilationOptions.Compile)]
+    public partial class SignIn : ContentPage
+    {
+        public SignIn()
+        {
+            InitializeComponent();
+        }
+
+        private async void Button_Clicked(object sender, EventArgs e)
+        {
+            await Navigation.PushAsync(new SecondPage());
+        }
+    }
+}

ViewModels/AboutViewModel.cs

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+using System;
+using System.Windows.Input;
+using Xamarin.Essentials;
+using Xamarin.Forms;
+
+namespace MyMobileApp.ViewModels
+{
+    public class AboutViewModel : BaseViewModel
+    {
+        public AboutViewModel()
+        {
+            Title = "About";
+            OpenWebCommand = new Command(async () => await Browser.OpenAsync("https://aka.ms/xamarin-quickstart"));
+        }
+
+        public ICommand OpenWebCommand { get; }
+    }
+}

ViewModels/BaseViewModel.cs

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+using MyMobileApp.Models;
+using MyMobileApp.Services;
+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using System.Runtime.CompilerServices;
+using Xamarin.Forms;
+
+namespace MyMobileApp.ViewModels
+{
+    public class BaseViewModel : INotifyPropertyChanged
+    {
+        public IDataStore<Item> DataStore => DependencyService.Get<IDataStore<Item>>();
+
+        bool isBusy = false;
+        public bool IsBusy
+        {
+            get { return isBusy; }
+            set { SetProperty(ref isBusy, value); }
+        }
+
+        string title = string.Empty;
+        public string Title
+        {
+            get { return title; }
+            set { SetProperty(ref title, value); }
+        }
+
+        protected bool SetProperty<T>(ref T backingStore, T value,
+            [CallerMemberName] string propertyName = "",
+            Action onChanged = null)
+        {
+            if (EqualityComparer<T>.Default.Equals(backingStore, value))
+                return false;
+
+            backingStore = value;
+            onChanged?.Invoke();
+            OnPropertyChanged(propertyName);
+            return true;
+        }
+
+        #region INotifyPropertyChanged
+        public event PropertyChangedEventHandler PropertyChanged;
+        protected void OnPropertyChanged([CallerMemberName] string propertyName = "")
+        {
+            var changed = PropertyChanged;
+            if (changed == null)
+                return;
+
+            changed.Invoke(this, new PropertyChangedEventArgs(propertyName));
+        }
+        #endregion
+    }
+}

ViewModels/ItemDetailViewModel.cs

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+using MyMobileApp.Models;
+using System;
+using System.Diagnostics;
+using System.Threading.Tasks;
+using Xamarin.Forms;
+
+namespace MyMobileApp.ViewModels
+{
+    [QueryProperty(nameof(ItemId), nameof(ItemId))]
+    public class ItemDetailViewModel : BaseViewModel
+    {
+        private string itemId;
+        private string text;
+        private string description;
+        public string Id { get; set; }
+
+        public string Text
+        {
+            get => text;
+            set => SetProperty(ref text, value);
+        }
+
+        public string Description
+        {
+            get => description;
+            set => SetProperty(ref description, value);
+        }
+
+        public string ItemId
+        {
+            get
+            {
+                return itemId;
+            }
+            set
+            {
+                itemId = value;
+                LoadItemId(value);
+            }
+        }
+
+        public async void LoadItemId(string itemId)
+        {
+            try
+            {
+                var item = await DataStore.GetItemAsync(itemId);
+                Id = item.Id;
+                Text = item.Text;
+                Description = item.Description;
+            }
+            catch (Exception)
+            {
+                Debug.WriteLine("Failed to Load Item");
+            }
+        }
+    }
+}

ViewModels/ItemsViewModel.cs

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+using MyMobileApp.Models;
+using MyMobileApp.Views;
+using System;
+using System.Collections.ObjectModel;
+using System.Diagnostics;
+using System.Threading.Tasks;
+using Xamarin.Forms;
+
+namespace MyMobileApp.ViewModels
+{
+    public class ItemsViewModel : BaseViewModel
+    {
+        private Item _selectedItem;
+
+        public ObservableCollection<Item> Items { get; }
+        public Command LoadItemsCommand { get; }
+        public Command AddItemCommand { get; }
+        public Command<Item> ItemTapped { get; }
+
+        public ItemsViewModel()
+        {
+            Title = "Browse";
+            Items = new ObservableCollection<Item>();
+            LoadItemsCommand = new Command(async () => await ExecuteLoadItemsCommand());
+
+            ItemTapped = new Command<Item>(OnItemSelected);
+
+            AddItemCommand = new Command(OnAddItem);
+        }
+
+        async Task ExecuteLoadItemsCommand()
+        {
+            IsBusy = true;
+
+            try
+            {
+                Items.Clear();
+                var items = await DataStore.GetItemsAsync(true);
+                foreach (var item in items)
+                {
+                    Items.Add(item);
+                }
+            }
+            catch (Exception ex)
+            {
+                Debug.WriteLine(ex);
+            }
+            finally
+            {
+                IsBusy = false;
+            }
+        }
+
+        public void OnAppearing()
+        {
+            IsBusy = true;
+            SelectedItem = null;
+        }
+
+        public Item SelectedItem
+        {
+            get => _selectedItem;
+            set
+            {
+                SetProperty(ref _selectedItem, value);
+                OnItemSelected(value);
+            }
+        }
+
+        private async void OnAddItem(object obj)
+        {
+            await Shell.Current.GoToAsync(nameof(NewItemPage));
+        }
+
+        async void OnItemSelected(Item item)
+        {
+            if (item == null)
+                return;
+
+            // This will push the ItemDetailPage onto the navigation stack
+            await Shell.Current.GoToAsync($"{nameof(ItemDetailPage)}?{nameof(ItemDetailViewModel.ItemId)}={item.Id}");
+        }
+    }
+}

ViewModels/LoginViewModel.cs

@@ -0,0 +1,24 @@
+using MyMobileApp.Views;
+using System;
+using System.Collections.Generic;
+using System.Text;
+using Xamarin.Forms;
+
+namespace MyMobileApp.ViewModels
+{
+    public class LoginViewModel : BaseViewModel
+    {
+        public Command LoginCommand { get; }
+
+        public LoginViewModel()
+        {
+            LoginCommand = new Command(OnLoginClicked);
+        }
+
+        private async void OnLoginClicked(object obj)
+        {
+            // Prefixing with `//` switches to a different navigation stack instead of pushing to the active one
+            await Shell.Current.GoToAsync($"//{nameof(AboutPage)}");
+        }
+    }
+}

ViewModels/NewItemViewModel.cs

@@ -0,0 +1,65 @@
+using MyMobileApp.Models;
+using System;
+using System.Collections.Generic;
+using System.Text;
+using System.Windows.Input;
+using Xamarin.Forms;
+
+namespace MyMobileApp.ViewModels
+{
+    public class NewItemViewModel : BaseViewModel
+    {
+        private string text;
+        private string description;
+
+        public NewItemViewModel()
+        {
+            SaveCommand = new Command(OnSave, ValidateSave);
+            CancelCommand = new Command(OnCancel);
+            this.PropertyChanged +=
+                (_, __) => SaveCommand.ChangeCanExecute();
+        }
+
+        private bool ValidateSave()
+        {
+            return !String.IsNullOrWhiteSpace(text)
+                && !String.IsNullOrWhiteSpace(description);
+        }
+
+        public string Text
+        {
+            get => text;
+            set => SetProperty(ref text, value);
+        }
+
+        public string Description
+        {
+            get => description;
+            set => SetProperty(ref description, value);
+        }
+
+        public Command SaveCommand { get; }
+        public Command CancelCommand { get; }
+
+        private async void OnCancel()
+        {
+            // This will pop the current page off the navigation stack
+            await Shell.Current.GoToAsync("..");
+        }
+
+        private async void OnSave()
+        {
+            Item newItem = new Item()
+            {
+                Id = Guid.NewGuid().ToString(),
+                Text = Text,
+                Description = Description
+            };
+
+            await DataStore.AddItemAsync(newItem);
+
+            // This will pop the current page off the navigation stack
+            await Shell.Current.GoToAsync("..");
+        }
+    }
+}

Views/AboutPage.xaml

@@ -0,0 +1,52 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="MyMobileApp.Views.AboutPage"
+             xmlns:vm="clr-namespace:MyMobileApp.ViewModels"
+             Title="{Binding Title}">
+    
+    <ContentPage.BindingContext>
+        <vm:AboutViewModel />
+    </ContentPage.BindingContext>
+    
+    <ContentPage.Resources>
+        <ResourceDictionary>
+            <Color x:Key="Accent">#96d1ff</Color>
+        </ResourceDictionary>
+    </ContentPage.Resources>
+
+    <Grid>
+        <Grid.RowDefinitions>
+            <RowDefinition Height="Auto" />
+            <RowDefinition Height="*" />
+        </Grid.RowDefinitions>
+        <StackLayout BackgroundColor="{StaticResource Accent}" VerticalOptions="FillAndExpand" HorizontalOptions="Fill">
+            <StackLayout Orientation="Horizontal" HorizontalOptions="Center" VerticalOptions="Center">
+                <ContentView Padding="0,40,0,40" VerticalOptions="FillAndExpand">
+                    <Image Source="xamarin_logo.png" VerticalOptions="Center" HeightRequest="64" />
+                </ContentView>
+            </StackLayout>
+        </StackLayout>
+        <ScrollView Grid.Row="1">
+            <StackLayout Orientation="Vertical" Padding="30,24,30,24" Spacing="10">
+                <Label Text="Start developing now" FontSize="Title"/>
+                <Label Text="Make changes to your XAML file and save to see your UI update in the running app with XAML Hot Reload. Give it a try!" FontSize="16" Padding="0,0,0,0"/>
+                <Label FontSize="16" Padding="0,24,0,0">
+                    <Label.FormattedText>
+                        <FormattedString>
+                            <FormattedString.Spans>
+                                <Span Text="Learn more at "/>
+                                <Span Text="https://aka.ms/xamarin-quickstart" FontAttributes="Bold"/>
+                            </FormattedString.Spans>
+                        </FormattedString>
+                    </Label.FormattedText>
+                </Label>
+                <Button Margin="0,10,0,0" Text="Learn more"
+                        Command="{Binding OpenWebCommand}"
+                        BackgroundColor="{StaticResource Primary}"
+                        TextColor="White" />
+            </StackLayout>
+        </ScrollView>
+    </Grid>
+
+</ContentPage>

Views/AboutPage.xaml.cs

@@ -0,0 +1,15 @@
+using System;
+using System.ComponentModel;
+using Xamarin.Forms;
+using Xamarin.Forms.Xaml;
+
+namespace MyMobileApp.Views
+{
+    public partial class AboutPage : ContentPage
+    {
+        public AboutPage()
+        {
+            InitializeComponent();
+        }
+    }
+}

Views/ItemDetailPage.xaml

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="MyMobileApp.Views.ItemDetailPage"
+             Title="{Binding Title}">
+
+    <StackLayout Spacing="20" Padding="15">
+        <Label Text="Text:" FontSize="Medium" />
+        <Label Text="{Binding Text}" FontSize="Small"/>
+        <Label Text="Description:" FontSize="Medium" />
+        <Label Text="{Binding Description}" FontSize="Small"/>
+    </StackLayout>
+    
+</ContentPage>

Views/ItemDetailPage.xaml.cs

@@ -0,0 +1,15 @@
+using MyMobileApp.ViewModels;
+using System.ComponentModel;
+using Xamarin.Forms;
+
+namespace MyMobileApp.Views
+{
+    public partial class ItemDetailPage : ContentPage
+    {
+        public ItemDetailPage()
+        {
+            InitializeComponent();
+            BindingContext = new ItemDetailViewModel();
+        }
+    }
+}

Views/ItemsPage.xaml

@@ -0,0 +1,44 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="MyMobileApp.Views.ItemsPage"
+             Title="{Binding Title}"
+             xmlns:local="clr-namespace:MyMobileApp.ViewModels"  
+             xmlns:model="clr-namespace:MyMobileApp.Models"  
+             x:Name="BrowseItemsPage">
+
+    <ContentPage.ToolbarItems>
+        <ToolbarItem Text="Add" Command="{Binding AddItemCommand}" />
+    </ContentPage.ToolbarItems>
+    <!--
+      x:DataType enables compiled bindings for better performance and compile time validation of binding expressions.
+      https://docs.microsoft.com/xamarin/xamarin-forms/app-fundamentals/data-binding/compiled-bindings
+    -->
+    <RefreshView x:DataType="local:ItemsViewModel" Command="{Binding LoadItemsCommand}" IsRefreshing="{Binding IsBusy, Mode=TwoWay}">
+        <CollectionView x:Name="ItemsListView"
+                ItemsSource="{Binding Items}"
+                SelectionMode="None">
+            <CollectionView.ItemTemplate>
+                <DataTemplate>
+                    <StackLayout Padding="10" x:DataType="model:Item">
+                        <Label Text="{Binding Text}" 
+                            LineBreakMode="NoWrap" 
+                            Style="{DynamicResource ListItemTextStyle}" 
+                            FontSize="16" />
+                        <Label Text="{Binding Description}" 
+                            LineBreakMode="NoWrap"
+                            Style="{DynamicResource ListItemDetailTextStyle}"
+                            FontSize="13" />
+                        <StackLayout.GestureRecognizers>
+                            <TapGestureRecognizer 
+                                NumberOfTapsRequired="1"
+                                Command="{Binding Source={RelativeSource AncestorType={x:Type local:ItemsViewModel}}, Path=ItemTapped}"		
+                                CommandParameter="{Binding .}">
+                            </TapGestureRecognizer>
+                        </StackLayout.GestureRecognizers>
+                    </StackLayout>
+                </DataTemplate>
+            </CollectionView.ItemTemplate>
+        </CollectionView>
+    </RefreshView>
+</ContentPage>

Views/ItemsPage.xaml.cs

@@ -0,0 +1,32 @@
+using MyMobileApp.Models;
+using MyMobileApp.ViewModels;
+using MyMobileApp.Views;
+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Xamarin.Forms;
+using Xamarin.Forms.Xaml;
+
+namespace MyMobileApp.Views
+{
+    public partial class ItemsPage : ContentPage
+    {
+        ItemsViewModel _viewModel;
+
+        public ItemsPage()
+        {
+            InitializeComponent();
+
+            BindingContext = _viewModel = new ItemsViewModel();
+        }
+
+        protected override void OnAppearing()
+        {
+            base.OnAppearing();
+            _viewModel.OnAppearing();
+        }
+    }
+}

Views/LoginPage.xaml

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             xmlns:d="http://xamarin.com/schemas/2014/forms/design"
+             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+             mc:Ignorable="d"
+             x:Class="MyMobileApp.Views.LoginPage"
+             Shell.NavBarIsVisible="False">
+    <ContentPage.Content>
+        <StackLayout Padding="10,0,10,0" VerticalOptions="Center">
+            <Button VerticalOptions="Center" Text="Login" Command="{Binding LoginCommand}"/>
+        </StackLayout>
+    </ContentPage.Content>
+</ContentPage>

Views/LoginPage.xaml.cs

@@ -0,0 +1,21 @@
+using MyMobileApp.ViewModels;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Xamarin.Forms;
+using Xamarin.Forms.Xaml;
+
+namespace MyMobileApp.Views
+{
+    [XamlCompilation(XamlCompilationOptions.Compile)]
+    public partial class LoginPage : ContentPage
+    {
+        public LoginPage()
+        {
+            InitializeComponent();
+            this.BindingContext = new LoginViewModel();
+        }
+    }
+}

Views/NewItemPage.xaml

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
+             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+             x:Class="MyMobileApp.Views.NewItemPage"
+             Shell.PresentationMode="ModalAnimated"
+             Title="New Item"
+             xmlns:ios="clr-namespace:Xamarin.Forms.PlatformConfiguration.iOSSpecific;assembly=Xamarin.Forms.Core"
+             ios:Page.UseSafeArea="true">
+    <ContentPage.Content>
+        <StackLayout Spacing="3" Padding="15">
+            <Label Text="Text" FontSize="Medium" />
+            <Entry Text="{Binding Text, Mode=TwoWay}" FontSize="Medium" />
+            <Label Text="Description" FontSize="Medium" />
+            <Editor Text="{Binding Description, Mode=TwoWay}" AutoSize="TextChanges" FontSize="Medium" Margin="0" />
+            <StackLayout Orientation="Horizontal">
+                <Button Text="Cancel" Command="{Binding CancelCommand}" HorizontalOptions="FillAndExpand"></Button>
+                <Button Text="Save" Command="{Binding SaveCommand}" HorizontalOptions="FillAndExpand"></Button>
+            </StackLayout>
+        </StackLayout>
+    </ContentPage.Content>
+    
+</ContentPage>

Views/NewItemPage.xaml.cs

@@ -0,0 +1,21 @@
+using MyMobileApp.Models;
+using MyMobileApp.ViewModels;
+using System;
+using System.Collections.Generic;
+using System.ComponentModel;
+using Xamarin.Forms;
+using Xamarin.Forms.Xaml;
+
+namespace MyMobileApp.Views
+{
+    public partial class NewItemPage : ContentPage
+    {
+        public Item Item { get; set; }
+
+        public NewItemPage()
+        {
+            InitializeComponent();
+            BindingContext = new NewItemViewModel();
+        }
+    }
+}