为什么不等待这种方法？

我有一个使用rg plugins popup扩展的弹出窗口，该窗口应该使用消息传递中心发送布尔值，但是调用此弹出窗口的方法永远不会等待结果。

在下方，您可以看到DisplayAlert.cs文件，该文件具有一种显示模式窗口并使用消息传递中心接收布尔值的方法：

using Rg.Plugins.Popup.Extensions;
using Xamarin.Forms;
using System.Threading.Tasks;

namespace MasterDetailPageNavigation.XAML
{
    public class Alerta
    {
        public Alerta(){}

        public class Retorno
        {
            public bool valor { get; set; }
        }

        public bool retorno;
        public async Task<bool> ShowAlert(string tipo, string titulo, string msg, string btnconfirm, string btncancel)
        {
            await App.Current.MainPage.Navigation.PushPopupAsync(
                new DisplayAlert(tipo, titulo, msg, btnconfirm, btncancel)
            );

            MessagingCenter.Subscribe<Retorno>(this, "DisplayAlert", (value) =>
            {
                retorno = value.valor;
            });

            return retorno;
        }
    }
}

这是弹出式ContentPage的代码，正如我所说，它必须由消息传递中心返回true或false：

using Xamarin.Forms;
using Rg.Plugins.Popup.Pages;
using Rg.Plugins.Popup.Extensions;

namespace MasterDetailPageNavigation.XAML
{
    public partial class DisplayAlert : PopupPage
    {
        public DisplayAlert(string tipo, string titulo, string msg,string btnconfirm=null,string btncancel=null)
        {
            InitializeComponent();
            switch (tipo)
            {
                case "ok":
                    XIcon.Text = "\uf058";
                    XIcon.TextColor = Color.FromHex("#009570");
                    break;

                case "error":
                    XIcon.Text = "\uf06a";
                    XIcon.TextColor = Color.FromHex("#FF0000");
                    break;

                case "confirm":
                    XIcon.Text = "\uf059";
                    XIcon.TextColor = Color.FromHex("#2181DF");
                    XBotoes.IsVisible = true;
                    XOk.IsVisible = false;
                    XConfirmar.Text = btnconfirm != null ? btnconfirm : XConfirmar.Text;
                    XCancelar.Text = btncancel != null ? btncancel : XCancelar.Text;
                    break;
            }

            XTitulo.Text = titulo;
            XMsg.Text = msg;
        }

        void XOk_Clicked(System.Object sender, System.EventArgs e)
        {
            Navigation.PopPopupAsync();
        }

        public class Retorno
        {
            public bool valor { get; set; }
        }

        void XConfirmar_Clicked(System.Object sender, System.EventArgs e)
        {
            MessagingCenter.Send(new Retorno() { valor = true }, "DisplayAlert");
        }

        void XCancelar_Clicked(System.Object sender, System.EventArgs e)
        {
            MessagingCenter.Send(new Retorno() { valor = false }, "DisplayAlert");
        }
    }
}

最后，在主窗口中，我调用并等待该方法：

Alerta alerta = new Alerta();

// IT SHOULD AWAIT HERE BUT DON'T
bool opt = await alerta.ShowAlert("confirm", "Do you confirm?", "Message asking for confirmation","Exit","Continue here");

if (opt) //it's always false
{
   Application.Current.Properties.Clear();
   await Application.Current.SavePropertiesAsync();
   Application.Current.MainPage = new Login();
}

我在哪里做错了或错过了？