App 01: Tally

<?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="Tally.MainPage" BackgroundColor="Black">

    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto"/>
            <RowDefinition Height="*"/>
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>

        <Label Text="Tap to count" TextColor="White" FontSize="40" HorizontalOptions="Center" HorizontalTextAlignment="Center" Grid.Row="0"/>
        <Label x:Name="CountLabel" Grid.Row="1" HorizontalTextAlignment="Center" VerticalOptions="Center" Text="" FontSize="60" TextColor="White"/>
        <Button x:Name="ResetButton" Grid.Row="2" Clicked="ResetButton_OnClicked" 
                Text="Reset" CornerRadius="15" HeightRequest="40" 
                BackgroundColor="Black" BorderColor="White" BorderWidth="2" TextColor="White" 
                Padding="10,0,10,0" Margin="10,0,10,0"></Button>

        <ContentView HorizontalOptions="FillAndExpand"
                     VerticalOptions="FillAndExpand"
                     Grid.Row="1"
                     Grid.Column="0">
            <ContentView.GestureRecognizers>
                <TapGestureRecognizer Tapped="OnTapped"/>
            </ContentView.GestureRecognizers>
        </ContentView>

    </Grid>

</ContentPage>

using System;
using Xamarin.Forms;

namespace Tally
{
    public partial class MainPage
    {
        private int _count;

        public MainPage()
        {
            InitializeComponent();
            if (Application.Current.Properties.ContainsKey("Count"))
            {
                _count = Int32.Parse(Application.Current.Properties["Count"].ToString());
                UpdateCountLabelText();
            }
        }

        private void ResetButton_OnClicked(object sender, EventArgs e)
        {
            _count = 0;
            UpdateCountLabelText();
        }

        private void OnTapped(object sender, EventArgs e)
        {
            _count++;
            UpdateCountLabelText();
        }

        private void UpdateCountLabelText()
        {
            CountLabel.Text = _count.ToString("N0");
            Application.Current.Properties["Count"] = _count;
        }

    }
}