MVVM binding is not updating UI element

Question

Hi everyone,
my issue is I don't update the user interface (TextBlock).

In summary:
by executing a method under translation (ImportData, which performs a series of Inserts in an MS SQL DB) the textblock should contain the time as the Insert proceeds in the ImportData method.

For this purpose I have created a Timer that is started before the transaction

This is the code involved.

<TextBlock Text="{Binding NumRig}" />

I have also used this statement but without success

<TextBlock Text="{Binding NumRig, UpdateSourceTrigger=PropertyChanged}" />

The property

private string _NumRig;
public string NumRig
{
get { return _NumRig; }
set
{
_NumRig= value;
base.OnPropertyChanged("NumRig");
}
}

The declaration of the Timer and the transaction that calls ImportData

{
Timer t = new Timer(TimerCallback, null, 0, 500);
using (System.Transactions.TransactionScope ts = new System.Transactions.TransactionScope(System.Transactions.TransactionScopeOption.Required, new System.TimeSpan(0, 10, 0)))
{
importResult = ImportData();
if (importResult)
ts.Complete();
}
t.Dispose();
}

and now the TimerCallback procedure

1st version of the procedure performed by the Timer (and which does not work)

void TimerCallback(object sender, EventArgs e)
{
NumRig= DateTime.Now.ToString("HH:mm:ss");
}

2nd version of the procedure performed by the Timer (and which does not work)

void TimerCallback(object sender, EventArgs e)
{
Application.Current.Dispatcher.Invoke(() =>
{
NumRig= DateTime.Now.ToString("HH:mm:ss");
});
}

3rd version of the procedure performed by the Timer (and which does not work)

void TimerCallback(object sender, EventArgs e)
{
Application.Current.Dispatcher.Invoke(() =>
{
NumRig= DateTime.Now.ToString("HH:mm:ss");
OnPropertyChanged("NumRig");
});
}

I'm wondering, why does the TextBlock never update?
What am I doing wrong ??

Thanks to who will answer me
Bye

Answer 1

Hi,
I cannot reproduce your problem with following code:

XAML:

<Window x:Class="WpfApp1.Window108"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp108"
        mc:Ignorable="d"
        Title="AntoBAro-5632_220216" Height="450" Width="800">
  <Window.DataContext>
    <local:ViewModel/>
  </Window.DataContext>
  <StackPanel>
    <TextBlock Text="{Binding NumRig}" />
  </StackPanel>
</Window>

and classes:

using System;
using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Threading;
using System.Windows;

namespace WpfApp108
{
  public class ViewModel : ViewModelBase
  {
    public ViewModel() => Start();
    private string _NumRig;
    public string NumRig
    {
      get { return _NumRig; }
      set
      {
        _NumRig = value;
        base.OnPropertyChanged("NumRig");
      }
    }

     void Start()
    {
      Timer t = new Timer(CB, null, 0, 500);
    }

    private void CB(object state)
    {
      NumRig = DateTime.Now.ToString("HH:mm:ss");
    }

  }

  public class ViewModelBase : INotifyPropertyChanged
  {
    public event PropertyChangedEventHandler PropertyChanged;
    internal void OnPropertyChanged([CallerMemberName] string nameProp = "") =>
      PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameProp));
  }
}

Answer 2

This is the code to better explain the situation

<Window x:Class="WpfApp6.MainWindow"
            xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
            xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
            xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
            xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
            xmlns:local="clr-namespace:WpfApp6"
            mc:Ignorable="d"
           Title="AntoBAro-5632_220216" Height="450" Width="800">

        <Window.DataContext>
            <local:ViewModel/>
        </Window.DataContext>

        <StackPanel>
            <Button Width="100" Height="100" Command="{Binding MyCommand}" Content="Start"/>
            <TextBlock Text="{Binding NumRig}" />
        </StackPanel>    
    </Window>



namespace WpfApp6
{
    public class ViewModel : ViewModelBase
    {
        public ViewModel() => Start();

        private string _NumRig;
        public string NumRig
        {
            get { return _NumRig; }
            set
            {
                _NumRig = value;
                base.OnPropertyChanged("NumRig");
            }
        }

        private ICommand _cmd;
        public ICommand MyCommand
        {
            get
            {
                if (_cmd == null)
                    _cmd = new RelayCommand(Command_Execute, Command_CanExecute);

                return _cmd;
            }
        }
        private bool Command_CanExecute(object param)
        {
            return true;
        }
        private void Command_Execute(object param)
        {
            Go();
        }

        void Start()  
        {
            NumRig = DateTime.Now.ToString("HH:mm:ss");  //Show in TextBlock current Time (the binding is ok)
        }

        void Go() //After click on button .....
        {
            Timer t = new Timer(CB, null, 0, 500);   

            bool is_ok = false;

            is_ok = ImportData(); //  I would like the TextBlock to be updated with the current time ("void CB") while this ImportData is running

            t.Dispose();

            if (is_ok == true)
            { MessageBox.Show(" Ok !!"); }
            else
            { MessageBox.Show("Failure"); }
        }

        private void CB(object state)
        {
            NumRig = DateTime.Now.ToString("HH:mm:ss");
        }

        public bool ImportData()
        {
            // ImportData populates 8 tables on MsSql Server, but in this case (for example) I create a text file 

            using (StreamWriter sw = File.CreateText(@"C:\temp\0000.txt"))
            {
                for (long i = 0; i < 6000000; i++) 
                {
                    sw.WriteLine("Values {0} Time {1} ", i.ToString(), DateTime.Now.ToString("HH:mm:ss"));
                }
            }
            return true;
        }
    }

    public class ViewModelBase : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        internal void OnPropertyChanged([CallerMemberName] string nameProp = "") =>
          PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameProp));
    }

    public class RelayCommand : ICommand
    {
        private readonly Action<object> _execute;
        private readonly Predicate<object> _canExecute;

        public RelayCommand(Action<object> execute)
            : this(execute, null)
        {
        }

        public RelayCommand(Action<object> execute, Predicate<object> canExecute)
        {
            if (execute == null)
            {
                throw new ArgumentNullException("execute");
            }

            _execute = execute;
            _canExecute = canExecute;
        }

        public bool CanExecute(object parameter)
        {
            return _canExecute == null || _canExecute((object)parameter);
        }
        public event EventHandler CanExecuteChanged
        {
            add
            {
                CommandManager.RequerySuggested += value;
            }
            remove
            {
                CommandManager.RequerySuggested -= value;
            }
        }
        public void Execute(object parameter)
        {
            _execute((object)parameter);
        }
    }
}

Answer 3

Hi,
if your method ImprtData executes in UI thread without waiting (async/await, 100% CPU) there's no time to refresh UI. The best way is to execute ImportData in seperate thread (Task) like in following demo. Replace your ViewModel with following ViewModel.

using System;
using System.ComponentModel;
using System.IO;
using System.Runtime.CompilerServices;
using System.Threading;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Input;

....

  public class ViewModel : ViewModelBase
  {
    public ViewModel() => Start();

    private string _NumRig;
    public string NumRig
    {
      get { return _NumRig; }
      set
      {
        _NumRig = value;
        base.OnPropertyChanged("NumRig");
      }
    }

    private ICommand _cmd;
    public ICommand MyCommand
    {
      get
      {
        if (_cmd == null)
          _cmd = new RelayCommand(Command_Execute, Command_CanExecute);

        return _cmd;
      }
    }
    private bool Command_CanExecute(object param) => true;

    private void Command_Execute(object param) => Task.Factory.StartNew(Go);

    void Start()
    {
      NumRig = DateTime.Now.ToString("HH:mm:ss");  //Show in TextBlock current Time (the binding is ok)
    }

    void Go() //After click on button .....
    {
      Timer t = new Timer(CB, null, 0, 500);

      bool is_ok = false;

      is_ok = ImportData(); //  I would like the TextBlock to be updated with the current time ("void CB") while this ImportData is running

      t.Dispose();

      if (is_ok == true)
      { MessageBox.Show(" Ok !!"); }
      else
      { MessageBox.Show("Failure"); }
    }

    private void CB(object state)
    {
      NumRig = DateTime.Now.ToString("HH:mm:ss");
    }

    public bool ImportData()
    {
      // ImportData populates 8 tables on MsSql Server, but in this case (for example) I create a text file 

      using (StreamWriter sw = File.CreateText(@"C:\temp\0000.txt"))
      {
        for (long i = 0; i < 6000000; i++)
        {
          sw.WriteLine("Values {0} Time {1} ", i.ToString(), DateTime.Now.ToString("HH:mm:ss"));
        }
      }
      return true;
    }

  }