Why binding is not enough to update the interface in WP7 (MVVM)

Question

I am trying to create a WP7 application based on the MVVM pattern, but I have a problem updating the content of the TextBlock binding. In the current state, I need to open the page again to update the content. I think this is due to setting the data context, but I could not fix it.

PropertyChangedEventHandler Property in ViewModel.cs

public class ViewModel : INotifyPropertyChanged { public event PropertyChangedEventHandler PropertyChanged; private void NotifyPropertyChanged(string propertyName) { if (null != PropertyChanged) { PropertyChanged(this, new PropertyChangedEventArgs(propertyName)); } } private string _txtStatus = ""; public string TxtStatus { get { return _txtStatus; } set { _txtStatus = value; NotifyPropertyChanged("TxtStatus"); } } 

ViewModel property in App.xaml.cs

 public partial class App : Application { private static ViewModel _viewModel { get; set; } public static ViewModel ViewModel { get { return _viewModel ?? (_viewModel = new ViewModel()); } } 

Setting DataContext in StatusPage.xaml.cs

 public partial class Status : PhoneApplicationPage { public Status() { InitializeComponent(); DataContext = App.ViewModel; } 

Binding in StatusPage.xaml

 <TextBlock x:Name="TxtStatus" Text="{Binding Path=TxtStatus, Mode=OneWay}" Width="450" TextWrapping="Wrap" HorizontalAlignment="Left" VerticalAlignment="Top" /> 

UPDATE 1

TxtStatus parameter value in MqttService.cs

 public class MqttService { private readonly ViewModel _viewModel; public MqttService(ViewModel viewModel) { _viewModel = viewModel; } private void Log(string log) { _viewModel.TxtStatus = _viewModel.TxtStatus + log; } private void Connect() { _client.Connect(true); Log(MsgConnected + _viewModel.TxtBrokerUrl + ", " + _viewModel.TxtClientId + "\n"); _viewModel.IsConnected = true; } 

MqttService property in ViewModel.cs

  private MqttService _mqttService; public MqttService MqttService { get { return _mqttService ?? (_mqttService = new MqttService(this)); } } 

Now I'm wondering, maybe I have some kind of problem with a circular link (MqttService-ViewModel). I'm not sure this looks good to me.

Answer 1

Thanks to everyone. After Erno de Weerd and ken2k wrote comments about the threads, I did some research and found this: Notify the user interface thread from the background thread . I changed the way the TxtStatus value is set and now it works fine. :)

(bad) The value of the TxtStatus parameter in MqttService.cs

 private void Log(string log) { _viewModel.TxtStatus = _viewModel.TxtStatus + log; } 

(good) The value of the TxtStatus parameter in MqttService.cs

 private void Log(string log) { Deployment.Current.Dispatcher.BeginInvoke(() => { App.ViewModel.TxtStatus = log + App.ViewModel.TxtStatus; }); } 

Answer 2

Your code works fine for me in WPF .NET4.0

so maybe your TxtStatus property TxtStatus never get a string

 _txtStatus ="new status"; // wrong TxtStatus = "new status"; // right 

or you will interfere with your x:Name="TxtStatus" , but this will be a problem with Windows-Phone-7