Xamarin.Forms ActivityIndicator and Binding

我有一个非常简单的页面（堆栈布局中的按钮和活动指示器）。我尝试使用绑定来激活/停用活动指示器，使用我用 INotifyPropertyChanged 编写的通知类。

当我按下按钮时，我希望活动指示器可见并运行。

按钮单击事件中的代码运行（它是一个 ftp...我想异步完成，但现在我没有成功），"绑定"属性（可见和正在运行的通知类）似乎改变了状态，但没有出现活动指示器。

我不知道这是绑定问题，布局问题，还是其他什么。谁能帮我？

这是页面

    using System;
using Xamarin.Forms;
using FlagFtp;
using System.ComponentModel;
using System.Runtime.CompilerServices;
namespace Geco
{
    public class PageFtp: ContentPage
    {
        private Notification _notification = new Notification ();
        public PageFtp ()
        {
            this.Title = "Carico database";
            var stacklayout = new StackLayout ();
            stacklayout.HorizontalOptions = LayoutOptions.Center;
            stacklayout.VerticalOptions = LayoutOptions.Center;
            var activityIndicator = new ActivityIndicator ();
            activityIndicator.IsEnabled = true;
            activityIndicator.SetBinding (ActivityIndicator.IsVisibleProperty, "Visible");
            activityIndicator.SetBinding (ActivityIndicator.IsRunningProperty, "Running");
            activityIndicator.BindingContext = _notification;
            bool okFtp = true;
            string errorFtp = "";
            // Verifico se ho il database
            var filename = DependencyService.Get<IFiles> ().GetFileName (App.DB_FILENAME);
            #if DEBUG
            DependencyService.Get<IFiles>().Delete(filename);
            #endif
            var buttonRetry = new Button ();
            buttonRetry.Text = "Procedere con il carico del database";
            buttonRetry.Clicked += (object sender, EventArgs e) =>{
                okFtp = ftp (filename, ref errorFtp);
                if(okFtp)
                    DependencyService.Get<IOpenActivity>().OpenActivity(App.EnumForms.Login);
                else{
                    DisplayAlert("Errore",errorFtp,"OK");
                }
            };
            stacklayout.Children.Add (buttonRetry);
            stacklayout.Children.Add (activityIndicator);
            this.Content = stacklayout;
        }

        private bool ftp(string filename, ref string error) {
            System.Net.NetworkCredential credentials = new System.Net.NetworkCredential ();
            credentials.UserName = "alessandro";
            credentials.Password = "ireland";
            bool ok = false;
            try
            {
                _notification.Visible = true;
                _notification.Running = true;
                FlagFtp.FtpClient ftpClient = new FtpClient (credentials);
                string uri = "ftp://192.168.0.102/GECOl.sqlite";
                FtpFileInfo ftpFileInfo = ftpClient.GetFileInfo (new Uri (uri));
                FtpStream ftpstream = ftpClient.OpenRead (ftpFileInfo);
                byte[] buffer = new byte[ftpFileInfo.Length];
                ftpstream.Read (buffer, 0,(int) ftpFileInfo.Length);
                DependencyService.Get<IFiles> ().SaveBytes (filename, buffer);
                ok = true;
            }
            catch(Exception ex) {
                error = ex.Message;
            }
            finally {
                _notification.Visible = false;
                _notification.Running = false;
            }
            return ok;
        }
        public class Notification : INotifyPropertyChanged
        {
            private bool _visible = false ;
            public bool Visible {
                get { return _visible; }
                set { 
                    if (value.Equals (_visible))
                        return;
                    _visible = value;
                    OnPropertyChanged ();
                }
            }
            private bool _running = false;
            public bool Running  {
                get { return _running; }
                set { 
                    if (value.Equals (_running))
                        return;
                    _running = value;
                    OnPropertyChanged ();
                }
            }
            public event PropertyChangedEventHandler PropertyChanged;
            void OnPropertyChanged([CallerMemberName]String propertyName=null)
            {
                var handler=PropertyChanged;
                if(handler != null)
                {
                    handler(this, new PropertyChangedEventArgs(propertyName));
                }
            }
        }
    }
}

using System;
using Xamarin.Forms;
#if __ANDROID__
using Android.App;
#endif
namespace SEEForgeX.Helpers
{
    public class BaseContentPage : ContentPage
    {
        #region PRIVATE VARIABLES
#if __ANDROID__
        ProgressDialog p = null;
#endif
        #endregion
        #region PROPERTIES
        public bool IsShowing { get; set; }
        public bool LoadingViewFlag {
            get {
                return (bool)GetValue (LoadingProperty);
            }
            set {
                SetValue (LoadingProperty, value);
#if __ANDROID__
                if (value == true)
                {
                    p = new ProgressDialog(Forms.Context);
                    p.SetMessage("Loading...");
                    p.SetCancelable(false);
                    p.Show();
                    IsShowing = true;
                }
                else
                {
                    if (p != null)
                    {
                        p.Dismiss();
                        p = null;
                        IsShowing = false;
                    }
                }
#endif
            }
        }
        public static readonly BindableProperty LoadingProperty = 
            BindableProperty.Create ((BaseContentPage w) => w.LoadingViewFlag, false);
        #endregion
        public BaseContentPage ()
        {
        }
    }
}