Invoke MainPage.SendButton_Clicked in async function

I write a network chat client app with Xamarin.Forms.
To build network receive loop I need to run it at a async function.
In the function 'Device.BeginInvokeOnMainThread' I want to invoke the 'MainPage.SendButton_Clicked' but there is no instance for me to reference it.
I want to know how to do this in Xamarin.Forms is recommended.

        public partial class App : Application
        {
            ...
            public App()
            {
              ...
              MainManager.StartWorkThread();
            }
            ...
        }

        class MainManager
        {
            public static void StartWorkThread()
            {
                System.Threading.Tasks.Task.Run(() =>
                {
                    int n = 0;
                    while(true)
                    {
                        n++;
                        int id = Thread.CurrentThread.ManagedThreadId;
                        Debug.WriteLine("MainManager.cs CurrentThreadId = {0} [{1}]", id, n);
                        Thread.Sleep(3000);

                        Device.BeginInvokeOnMainThread(() => 
                        {
                            // I don't know how to invoke the MainPage.SendButton_Clicked at here
                        });
                    }
                });
            }
        }

    public partial class MainPage : ContentPage
    {
        public ObservableCollection<TextMessage> textMessages;

        public MainPage()
        {
            InitializeComponent();

             textMessages = new ObservableCollection<TextMessage>();
            for (int i = 0; i < 30; i++)
            {
                var msg = new TextMessage();
                msg.Nickname = "我的六字昵称";
                msg.Ip = "127.0.0.1";
                msg.Message = "这是一条聊天信息";
                msg.MessageNumber = i + 1;
                textMessages.Add(msg);
            }

            this.BindingContext = textMessages;
        }

        private void SendButton_Clicked(object sender, EventArgs e)
        {
            var msg = new TextMessage();
            msg.Nickname = "我的六字昵称";
            msg.Ip = "127.0.0.1";
            msg.Message = sendText.Text;
            msg.MessageNumber = 100;
            textMessages.Add(msg);

            sendText.Text = "";
            listView.ScrollTo(msg, ScrollToPosition.End, true);
        }

    }