MainPage.xaml

<?xml version="1.0" encoding="utf-8"?>
<ContentPage Title="Barcode Demo" 
             xmlns="http://xamarin.com/schemas/2014/forms" 
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
             xmlns:local="clr-namespace:BarcodeScanner" 
             xmlns:viewmodels="clr-namespace:BarcodeScanner.ViewModels;assembly=BarcodeScanner"
             x:Class="BarcodeScanner.Views.MainPage">
    <ContentPage.BindingContext>
        <viewmodels:MainPageViewModel />
    </ContentPage.BindingContext>
    <StackLayout Padding="20">
        <!-- Place new controls here -->
        <Button Command="{Binding ButtonCommand}" HorizontalOptions="FillAndExpand" VerticalOptions="EndAndExpand" BackgroundColor="Teal" TextColor="White" FontSize="Medium" Text="Click to open scanner" />
        <Label Text="{Binding Result}" HorizontalOptions="Center" VerticalOptions="StartAndExpand" />
    </StackLayout>
</ContentPage>

MainPageViewModel.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Windows.Input;
using Xamarin.Forms;
using ZXing;
using ZXing.Mobile;
using ZXing.Net.Mobile.Forms;

namespace BarcodeScanner.ViewModels
{
    public class MainPageViewModel : INotifyPropertyChanged
    {
        private string _result;
        public string Result
        {
            get => _result;
            set
            {
                _result = value;
                OnPropertyChanged(nameof(Result));
            }
        }

        public ICommand ButtonCommand { get; private set; }

        public MainPageViewModel()
        {
            ButtonCommand = new Command(OnButtomCommand);
        }

        public event PropertyChangedEventHandler PropertyChanged;

        private void OnPropertyChanged([CallerMemberName] string propertyName = "")
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }

        private void OnButtomCommand()
        {
            var options = new MobileBarcodeScanningOptions();
            options.PossibleFormats = new List<BarcodeFormat>
            {
                BarcodeFormat.QR_CODE,
                BarcodeFormat.CODE_128,
                BarcodeFormat.EAN_13
            };
            var page = new ZXingScannerPage(options) { Title = "Scanner" };
            var closeItem = new ToolbarItem { Text = "Close" };
            closeItem.Clicked += (object sender, EventArgs e) =>
            {
                page.IsScanning = false;
                Device.BeginInvokeOnMainThread(() =>
                {
                    Application.Current.MainPage.Navigation.PopModalAsync();
                });
            };
            page.ToolbarItems.Add(closeItem);
            page.OnScanResult += (result) =>
            {
                page.IsScanning = false;

                Device.BeginInvokeOnMainThread(() => {
                    Application.Current.MainPage.Navigation.PopModalAsync();
                    if (string.IsNullOrEmpty(result.Text))
                    {
                        Result = "No valid code has been scanned";
                    }
                    else
                    {
                        Result = $"Result: {result.Text}";
                    }
                });
            };
            Application.Current.MainPage.Navigation.PushModalAsync(new NavigationPage(page) { BarTextColor = Color.White, BarBackgroundColor = Color.CadetBlue }, true);
        }
    }
}