main view model for a test app.

MainViewModel.cs

using GalaSoft.MvvmLight;
using GalaSoft.MvvmLight.Command;
using GalaSoft.MvvmLight.Threading;
using Microsoft.AspNet.SignalR.Client.Hubs;
using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Input;
using Windows.ApplicationModel.Core;

namespace ChatApp.WindowsStore.ViewModel
{
    /// <summary>
    /// This class contains properties that the main View can data bind to.
    /// <para>
    /// Use the <strong>mvvminpc</strong> snippet to add bindable properties to this ViewModel.
    /// </para>
    /// <para>
    /// You can also use Blend to data bind with the tool's support.
    /// </para>
    /// <para>
    /// See http://www.galasoft.ch/mvvm
    /// </para>
    /// </summary>
    public class MainViewModel : ViewModelBase
    {

        HubConnection _hubConnection;
        IHubProxy _proxy;
        ObservableCollection<string> _messages;

        /// <summary>
        /// Initializes a new instance of the MainViewModel class.
        /// </summary>
        public MainViewModel()
        {
            _hubConnection = new HubConnection("http://localhost:18479/");
            _proxy = _hubConnection.CreateHubProxy("ChatHub");
            _messages = new ObservableCollection<string>();
            _messages.Add("test");
            _messages.Add("test2");
            _proxy.On<string, string>("SendMessage", (username, message) => SendMessage(username, message));


            this.SendMessageCommand = new RelayCommand(async() => await this.SendToServer());
            _hubConnection.Start();
        }

        private void SendMessage(string username, string message)
        {
            try
            {
                DispatcherHelper.CheckBeginInvokeOnUI(() =>
                {
                    _messages.Add(string.Format("{0}: {1}", username, message));
                });
            }
            catch (Exception ex)
            {
                _messages.Add(ex.Message);
            }
        }

        private async Task SendToServer()
        {
            await _proxy.Invoke("SendMessage", Username, Message);
        }

        public IList<string> Messages
        {
            get { return _messages; }
        }

        public ICommand SendMessageCommand { get; private set; }

        private string _message;
        public string Message
        {
            get { return _message; }
            set
            {
                _message = value;
                RaisePropertyChanged(() => Message);
            }
        }

        private string _username;
        public string Username
        {
            get { return _username; }
            set
            {
                _username = value;
                RaisePropertyChanged(() => Username);
            }
        }
    }
}