public class MockSinaService : ISinaService
    {
        public List<Status> StatusList{get;set;}

        public void GetStatuses(System.Action<IEnumerable<Status>> onGetStatusesCompleted = null, System.Action<System.Exception> onError = null, System.Action onFinally = null)
        {           
            DispatcherTimer timer = new DispatcherTimer();
            timer.Interval = TimeSpan.FromSeconds(2);
            timer.Tick += delegate(object sender, EventArgs e)
            {                    
                Status testStatus =
                    new Status
                    {
                        Text = "this is test status",
                        CreatedAt = DateTime.Now                        
                    };

                onGetStatusesCompleted(new List<Status>() { testStatus });
                timer.Stop(); 
            };
            timer.Start();
        }
    }

        public ISinaService SinaService
        {
            get 
            {
                if (_sinaService == null)
                {
                    _sinaService = new SinaService(); 
                }
                return _sinaService; 
            }
            set
            {
                if (value != _sinaService)
                {
                    _sinaService = value;                    
                }
            }
        }       

        public void RefreshHomeList()
        {
            Trace.DetailMsg("RefreshHomeList");
            IsHomeRefreshing = true;
            SinaService.GetStatuses(
                delegate(IEnumerable<Status> statuses)
                {
                    IsHomeRefreshing = false;

                    foreach (Status status in statuses)
                    {
                        HomeList.Add(status);
                    }
                },
                delegate(Exception exception)
                {
                    IsHomeRefreshing = false;
                });            
        }  

        [TestInitialize]
        public void Initialize()
        {
            _mainViewModel = new MainPageViewModel();
            _mockSinaService = new MockSinaService();
            _mainViewModel.SinaService = _mockSinaService;
        }

        [TestMethod]
        [Asynchronous]
        public void Refresh_HomeList_Success()
        {
            bool isHomeListRefreshed = false;
            _mainViewModel.HomeList.CollectionChanged +=
                (s, e) =>
                {
                    isHomeListRefreshed = true;
                    Assert.AreEqual(NotifyCollectionChangedAction.Add, e.Action);
                    Assert.AreEqual(1, e.NewItems.Count, "only should be +1 item");
                };
            _mainViewModel.RefreshHomeList();            
            EnqueueConditional(() => isHomeListRefreshed);
            EnqueueCallback(() => Assert.AreEqual(_mainViewModel.HomeList.Count, 1, "Expected non-empty products list."));                     
            EnqueueTestComplete();    

    public class User
    {
        public ulong Id { get; set; }
        public string ScreenName { get; set; }
        public string ImageUrl { get; set; }
        public string Name { get; set; }
        public string Url { get; set; }
        public string Description { get; set; }
        public string Location { get; set; }
        public int Followers { get; set; }
        public int Friends { get; set; }
        public DateTime CreatedAt { get; set; }
    }

    public class Status
    {
        public Status()
        {
            User = new User();
        }

        public DateTime CreatedAt { get; set; }        

        public ulong Id { get; set; }       
                     
        public string Text { get; set; }

        public string Source { get; set; }

        public User User { get; set; }

        public bool Favorited { get; set; }

        public int RetweetCount { get; set; }

        public bool Retweeted { get; set; }

    }

    /// <summary>
    /// A simple Sina service interface
    /// </summary>
    public interface ISinaService
    {
        /// <summary>
        /// Get statuses from sina weibo
        /// </summary>
        /// <param name="onGetStatusesCompleted">the callback method</param>
        /// <param name="onError"></param>
        /// <param name="onFinally"></param>
        void GetStatuses(Action<IEnumerable<Status>> onGetStatusesCompleted = null, Action<Exception> onError = null, Action onFinally = null);
    }

    public class MainPageViewModel : ViewModelBase
    {
        internal readonly ISinaService SinaService;   
        private bool _isHomeRefreshing = false;        
               
        public MainPageViewModel()
        {
            SinaService = new SinaService();
            HomeList = new ObservableCollection<Status>();
           
        }

        public ObservableCollection<Status> HomeList { get; internal set; }
        
        /// <summary>
        /// Indicates whether there is an indeterminate operation in progress
        /// </summary>
        public bool IsHomeRefreshing
        {
            get { return _isHomeRefreshing; }
            set
            {
                if (value != _isHomeRefreshing)
                {
                    _isHomeRefreshing = value;
                    OnNotifyPropertyChanged("IsHomeRefreshing");
                }
            }
        }         

    }

        [TestMethod]
        [ExpectedException(typeof(NullReferenceException))]
        public void NullInstance()
        {
            _mainViewModel = null;
            bool x = _mainViewModel.IsHomeRefreshing; 
        }

        [TestMethod]
        public void Init_MainPageViewModel_Success()
        {
            Assert.IsNotNull(_mainViewModel);
            Assert.IsNotNull(_mainViewModel.HomeList);
            Assert.IsNotNull(_mainViewModel.SinaService); 
            Assert.IsInstanceOfType(_mainViewModel, typeof(INotifyPropertyChanged));
        }