MvvmLight：从 RelayCommand 执行中设置属性时视图未更新

Question

By lack of any response on the developer's GitHub I will repeat my question here. I hope someone can help me in some way.

This is the first time I am using MvvmLight, so I hope I am not overlooking something obvious.

In my WPF ViewModel I have something like:

  private ICommand readFileCommand;
  public ICommand ReadFileCommand => readFileCommand ?? (readFileCommand = new RelayCommand(ReadFile));

  private void ReadFile()
  {
        FileMessage = "Message.";
  }

  private string fileMessage;
  public string FileMessage
  {
       get { return fileMessage; }
       set
       {
           //Set(ref fileMessage, value);

           fileMessage = value;
           RaisePropertyChanged();
       }
   }

I have a couple of problems with it.

• Main problem is that setting a property like FileMessage from within a method like ReadFile() does not result in an update of the view until ReadFile is completed.
• There is a difference between using RaisePropertyChanged() which succeeds at that moment, and using Set() which does nothing at all. Though the latter did work outside such a method.
• The problem extends to other elements like a DataGrid on a DataView.

In wondered if the called methods should be asynchronous, but that does not seem logical. I have not tried that yet as that does not really fit into what I want to achieve.

So what is happening? Am I overlooking something? Is this a limitation of the framework? Or is this a bug?

Thanks!

Answer 1

Main problem is that setting a property like FileMessage from within a method like ReadFile() does not result in an update of the view until ReadFile is completed.

This makes sense as you cannot both execute your ReadFile method and update the UI on the same thread simultaneously. This has nothing to do with MvvmLight or commands.

If you set the property before you run any potentially long-running code, either asynchronously or synchronously on a background thread, it should work as expected.

Try this for example:

private async void ReadFile()
{
    FileMessage = "Message.";
    await Task.Delay(5000); //simulate...
    FileMessage = "Done!";
}

Or this:

private async void ReadFile()
{
    FileMessage = "Message.";
    await Task.Run(() => Thread.Sleep(5000));
    FileMessage = "Done!";
}

Answer 2

I 'solved' this the way below, which does for me what I ultimately wanted, but still leaves me with questions.

The code is sort of a complete example to make my point. One remark. Using the debugger with breaks can be very misleading in the behaviour.

Observations

• What DOES work are the 'Run' messages.
• However, all of the other messages are not displayed. And that leaves me dumbfounded.

The ReadFile method is synchronous and directly on the UI thread (I checked by the name), and does nothing with threading outside of the Task. So why is it unable to display anything? I even superfluously used Invoke, which did not help.

So that still leaves me doubting whether the RelayCommand properly works.

I even considered switching to Windows Community Toolkit, but that seemed too big a step.

It may end there anyway, as MVVVM Light is already dropped by Windows Template Studio, and development stopped december 2018!

If I overlook anything, clarifications are still appreciated.

using GalaSoft.MvvmLight;
using Application.Contracts.ViewModels;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Threading;

namespace ViewModels
{
    public class ViewModel : ViewModelBase, INavigationAware
    {
        public ViewModel()
        {
            uiDispatcher = Dispatcher.CurrentDispatcher;
        }

        private Dispatcher uiDispatcher;

        public async void OnNavigatedTo(object parameter)
        {
            uiDispatcher.Thread.Name = "OnNavigatedTo";
        }

        public void OnNavigatedFrom()
        { }

        private string fileMessage = "No file yet";
        public string FileMessage
        {
            get { return fileMessage; }
            set
            {
                // Using this simple way instead of Set, which did not work.
                fileMessage = value;
                RaisePropertyChanged();
            }
        }

        private void ReadFile()
        {
            FileMessage = "ReadFile 1.";

            Thread.Sleep(1000);

            uiDispatcher.Invoke(() => FileMessage = "ReadFile Invoke 1.", DispatcherPriority.Send);

            Thread.Sleep(1000);

            // Use Run on a non UI-thread and Dispatcher to enable intermediate updates back on the UI-thread.
            Task.Run(() =>
            {
                uiDispatcher.Invoke(() => FileMessage = "Run 1.", DispatcherPriority.Send);

                Thread.Sleep(1000);

                uiDispatcher.Invoke(() => FileMessage = "Run 2.", DispatcherPriority.Send);

                Thread.Sleep(1000);
            });

            Thread.Sleep(1000);

            FileMessage = "ReadFile 2.";

            Thread.Sleep(1000);

            uiDispatcher.Invoke(() => FileMessage = "ReadFile Invoke 2.", DispatcherPriority.Send);
        }
    }
}

Answer 3

Try a slightly modified implementation:

   private async void ReadFile()
    {
        FileMessage = "ReadFile 1.";

        await Task.Delay(1000);

        await uiDispatcher.BeginInvoke(() => FileMessage = "ReadFile Invoke 1.", DispatcherPriority.Send);

        await Task.Delay(1000);

        // Use Run on a non UI-thread and Dispatcher to enable intermediate updates back on the UI-thread.
        await Task.Run(async () =>
        {
            uiDispatcher.Invoke(() => FileMessage = "Run 1.", DispatcherPriority.Send);

            await Task.Delay(1000);

            uiDispatcher.Invoke(() => FileMessage = "Run 2.", DispatcherPriority.Send);

            await Task.Delay(1000);
        });

        await Task.Delay(1000);

        FileMessage = "ReadFile 2.";

        await Task.Delay(1000);

        await uiDispatcher.BeginInvoke(() => FileMessage = "ReadFile Invoke 2.", DispatcherPriority.Send);
    }
}

In general, this code is absurd.
But I didn't make any big changes to maintain continuity.

PS The code was written here in the post editor.
Sorry - minor mistakes are possible.