QrCode.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:prism="clr-namespace:Prism.Mvvm;assembly=Prism.Forms"
             xmlns:forms="clr-namespace:ZXing.Net.Mobile.Forms;assembly=ZXing.Net.Mobile.Forms"
             x:Class="QrCodeSample.Views.VerificaQrCode">
    <Grid HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand">
        <forms:ZXingScannerView IsScanning="{Binding IsScanning}" IsAnalyzing="{Binding IsAnalyzing}" Result="{Binding Result, Mode=TwoWay}" ScanResultCommand="{Binding QRScanResultCommand}"></forms:ZXingScannerView>
        <forms:ZXingDefaultOverlay TopText="QCLUB - Leitor QrCode" BottomText="Passe o leitor sobre o QrCode" ShowFlashButton="False" Opacity="0.9"></forms:ZXingDefaultOverlay>
    </Grid>
</ContentPage>

QrCodeViewModel.cs

public class VerificaQrCodeViewModel : BaseViewModel
    {
        public ZXing.Result Result { get; set; }
        private INavigationService _navigationService;
        private IPageDialogService _dialogService;
        public DelegateCommand QRScanResultCommand { get; set; }

        public VerificaQrCodeViewModel(INavigationService navigationService, IPageDialogService dialogService)
        {
            _navigationService = navigationService;
            _dialogService = dialogService;
            QRScanResultCommand = new DelegateCommand(ExecuteQRScanResultCommand);
        }
        private bool isAnalyzing = true;
        public bool IsAnalyzing
        {
            get => isAnalyzing;
            set => SetProperty(ref isAnalyzing, value);
        }

        private bool isScanning = true;
        public bool IsScanning
        {
            get => isScanning;
            set => SetProperty(ref isScanning, value);
        }

        public async void ExecuteQRScanResultCommand()
        {
            //tem que chamar pela mainThread pra trocar de tela fechando o leitor
            Device.BeginInvokeOnMainThread(async () =>
            {
                //do your job here - Result.Text contains QR CODE
                //await _dialogService.DisplayAlertAsync("Scanned Barcode", Result.Text, "OK");

            });
        }

        
    }

Usando o plugin ZXing com XAML e MVVM