问在xamarin ios中打开摄像头时出现白屏

EN

namespace Kefi.Controls
{
    public class CustomScanPage : ContentPage
    {
        ZXingScannerView zxing;
        public CustomScanPage() : base()
        {
            zxing = new ZXingScannerView
            {
                HorizontalOptions = LayoutOptions.Center,
                VerticalOptions = LayoutOptions.Center

            };
            NavigationPage.SetHasNavigationBar(this,false);
            zxing.OnScanResult += (result) =>
                Device.BeginInvokeOnMainThread(() =>
                {
                    zxing.IsAnalyzing = false;
                    zxing.IsScanning = false;
                    MessagingCenter.Send(result, "Scanresult");
                });
            var grid = new Grid
            {
                VerticalOptions = LayoutOptions.FillAndExpand,
                HorizontalOptions = LayoutOptions.FillAndExpand,
            };
        }
        protected  override void OnAppearing()
        {
            base.OnAppearing();
            zxing.Options.DelayBetweenAnalyzingFrames = 5;
            zxing.Options.DelayBetweenContinuousScans = 5;
            zxing.IsScanning = true;

        }

        protected override void OnDisappearing()
        {
            zxing.IsScanning = false;
            base.OnDisappearing();
        }

    }
}