Xamarin Forms : Basic ViewModel (MVVM)

ItemCountViewModel.cs

using System;
using System.ComponentModel;
using Xamarin.Forms;

namespace XamarinFormsTest01
{
    public class ItemCountViewModel : INotifyPropertyChanged
    {
        private static ItemCountViewModel instance = new ItemCountViewModel();
        public static ItemCountViewModel GetInstance()
        {
            return instance;
        }

        int itemCount;
        public event PropertyChangedEventHandler PropertyChanged = delegate { };
        public ItemCountViewModel()
        {
            ItemCount = 0;
            /*Device.StartTimer(TimeSpan.FromSeconds(1), () =>
            {
                ItemCount++;
                return true;
            }
            );*/
        }
        public int ItemCount
        {
            set
            {
                itemCount = value;
                PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("ItemCount"));
            }
            get
            {
                return itemCount;
            }
        }
    }
}

MainPage.xaml

<?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"
             xmlns:maps="clr-namespace:Xamarin.Forms.Maps;assembly=Xamarin.Forms.Maps"
             xmlns:local="clr-namespace:XamarinFormsTest01"
             mc:Ignorable="d"
             x:Class="XamarinFormsTest01.MainPage">

    <StackLayout>
        <Label x:Name="lblMain" Text="{Binding ItemCount}" 
           HorizontalOptions="Center"
           VerticalOptions="CenterAndExpand" >
            <Label.BindingContext>
                <local:ItemCountViewModel />
            </Label.BindingContext>
        </Label>
        
        <Button x:Name="BtnStart" Text="start" Pressed="BtnStart_Pressed" />
        <Button x:Name="BtnStop" Text="stop" Pressed="BtnStop_Pressed" />

        <!--<maps:Map></maps:Map>-->
    </StackLayout>

</ContentPage>

MainPage.xaml.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Diagnostics;
using System.Linq;
using System.Net;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using Xamarin.Essentials;
using Xamarin.Forms;
using Xamarin.Forms.Maps;
using Map = Xamarin.Forms.Maps.Map;

namespace XamarinFormsTest01
{
    // Learn more about making custom code visible in the Xamarin.Forms previewer
    // by visiting https://aka.ms/xamarinforms-previewer
    [DesignTimeVisible(false)]
    public partial class MainPage : ContentPage
    {
        public MainPage()
        {
            InitializeComponent();
            lblMain.BindingContext = ItemCountViewModel.GetInstance();
        }

        private void BtnStart_Pressed(object sender, EventArgs e)
        {
            httprequest();
        }
        private void BtnStop_Pressed(object sender, EventArgs e)
        {
        }

        private async void showmsg()
        {
            var action = await DisplayActionSheet("Select an action", "OK", "CANCEL", "Action 1", "Action 2", "Action 3");
            await DisplayAlert("Alert", "action = " + action, "OK");
        }


        private void httprequest()
        {
            Device.StartTimer(TimeSpan.FromSeconds(1), () =>
                {
                    ItemCountViewModel.GetInstance().ItemCount++;
                    HttpWebRequest h = WebRequest.Create("https://www.google.com/search?q=reuniware") as HttpWebRequest;
                    HttpWebResponse r = h.GetResponse() as HttpWebResponse;
                    using (var reader = new System.IO.StreamReader(r.GetResponseStream(), Encoding.UTF8))
                    {
                        string responseText = reader.ReadToEnd();
                    }
                    return true;
                }
            );

        }

    }
}