WPF UI not refreshing

Question

I have a problem to refresh my UI in a wpf / mvvm project.

When I log out there is a action on UI to display "waiting" object and then do a long process.

All codes I present are not complete

private void btnPower_Click(object sender, RoutedEventArgs e)
{
      // IHM changes
      btnRightMenuShow.IsEnabled = false;
      btnRightMenuShow.Visibility = Visibility.Hidden;
      StationWindowViewModel stnWindowVM = (StationWindowViewModel)DataContext;
      stnWindowVM.ProgressMode = LoadingProcessViewModel.ProgressMode.Sending;

      // PRocess
      if (stnWindowVM.PowerOffCmd.CanExecute(null))
      {
          stnWindowVM.PowerOffCmd.Execute(null);
      }
}

Like this the ihm is like frozen during the process and never refresh() before the application ended.

I first try to use a background worker like

private void btnPower_Click(object sender, RoutedEventArgs e)
{
      // IHM changes
      btnRightMenuShow.IsEnabled = false;
      btnRightMenuShow.Visibility = Visibility.Hidden;
      StationWindowViewModel stnWindowVM = (StationWindowViewModel)DataContext;
      stnWindowVM.ProgressMode = LoadingProcessViewModel.ProgressMode.Sending;

      // PRocess
      powerOffWorker.RunWorkerAsync(stnWindowVM);
}

private void powerOffWorker_DoWork(object Sender, System.ComponentModel.DoWorkEventArgs e)
{
      if (stnWindowVM.PowerOffCmd.CanExecute(null))
      {
          stnWindowVM.PowerOffCmd.Execute(null);
      }
}

In this case the IHM is refreshed but i have threading errors with some code trying to access object created by parent thread on main process (The calling thread cannot access this object because a different thread owns it)

So i try to use a Dispatcher like

private void btnPower_Click(object sender, RoutedEventArgs e)
{
      // IHM changes
      btnRightMenuShow.IsEnabled = false;
      btnRightMenuShow.Visibility = Visibility.Hidden;
      StationWindowViewModel stnWindowVM = (StationWindowViewModel)DataContext;
      stnWindowVM.ProgressMode = LoadingProcessViewModel.ProgressMode.Sending;

      // PRocess
      this.Dispatcher.Invoke(new Action(() =>
      {
          if (stnWindowVM.PowerOffCmd.CanExecute(null))
          {
              stnWindowVM.PowerOffCmd.Execute(null);
          }
      }));
}

I also try dispatcher inside backgrounworker but with those dispatcher i don't have anymore problem with calling thread but the UI is not updating.

EDIT

This is my test for async/await implementation

private void btnPower_Click(object sender, RoutedEventArgs e)
{
  // IHM changes
  btnRightMenuShow.IsEnabled = false;
  btnRightMenuShow.Visibility = Visibility.Hidden;
  StationWindowViewModel stnWindowVM = (StationWindowViewModel)DataContext;
  stnWindowVM.ProgressMode = LoadingProcessViewModel.ProgressMode.Sending;

  // PRocess
  PowerOffAsync(stnWindowVM);
}

private async void PowerOffAsync(StationWindowViewModel stnWindowVM)
{
    await Task.Run(() =>
    {
        if (stnWindowVM.PowerOffCmd.CanExecute(null))
         {
            stnWindowVM.PowerOffCmd.Execute(null);
        }
    });
}

In the PowerOffCmd process ther is some operation on object like list which gives error with calling thread...

hi, sorry by IHM i mean the "visual" / window seen by the user. — lavoyech, Jul 11 '18 at 13:07
Any particular reasons why you're not using `async/await` instead ? — scharette, Jul 11 '18 at 13:08
i'm not expert on the subject and i didn't try to use async/await so i will — lavoyech, Jul 11 '18 at 13:09
I wrote an answer that is not specific to MVVM, but you'll see a complete answer for using async/await and progressbar while processing a long task. It might be of interest. See this https://stackoverflow.com/a/51212664/7692463 — scharette, Jul 11 '18 at 13:15
At the moment i reproduce the same behaviour if i set async/await the window is refreshed but i have a problem with The calling thread which cannot access this object because a different thread owns it. And if i add a dispatcher inside the async/await i solve the thread problem but the window is not refreshed anymore... — lavoyech, Jul 11 '18 at 13:30

score 1 · Accepted Answer · answered Jul 11 '18 at 14:00

1

Your probem is that you try to invoke the dispatcher in the awaiting call instead of calling it where you need to work on UI.

Basically, if you do

await Task.Run(() =>
{
    this.Dispatcher.Invoke(new Action(() =>
    {
        if (stnWindowVM.PowerOffCmd.CanExecute(null))
        {
            stnWindowVM.PowerOffCmd.Execute(null);
        }
    }));
});

You're doing it wrong since you're processing your async work on the UI thread the whole time.

One work around you can try is invoke the dispatcher in your PowerOffCmd where it gives you the error. This way, everything except where you need to work on UI thread (modifying controls for example) will execute on a separate thread which will assure that you're UI stay responsive.

answered Jul 11 '18 at 14:00

scharette

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So if i understand i should be more accurate and use Dispatcher.invoke() for every line which gives thread error inside PowerOffCmd? – lavoyech Jul 11 '18 at 14:08
I can't say if this is the best solution for your particular case since I'm not familiar with your architecture, but it will definitely work. – scharette Jul 11 '18 at 14:11
ok i'm going to try and come back to you after. thanks for your time – lavoyech Jul 11 '18 at 14:13
@lavoyech If you're always doing the samething in a lot of place in your code, consider writing a static method that invoke the dispatcher and does the work on your object passed as a parameter. Then call it when you need in your `PowerOffCmd`. – scharette Jul 11 '18 at 14:15

WPF UI not refreshing

1 Answers1