从DataService MVVM Light持续更新ViewModel

Question

Originally my ViewModel had the following:

    public ObservableCollection<DataListItem> files { get; private set; }

    private object _filesLock = new object();

    public MainViewModel(IDataService dataService)
    {
        files = new ObservableCollection<DataListItem>();
        BindingOperations.EnableCollectionSynchronization(files, _filesLock);

        _dataService = dataService;
    }

One of the commands ran this code

            await Task.Run(() => {
                Parallel.ForEach(files, new ParallelOptions() { MaxDegreeOfParallelism = 2 }, (file) =>
                {                    
                    this.files.Add(new DataListItem(file));
                });
            });

But this doesn't work at design time. So I moved the code into the DataService.

    public async void ScanFolder()
    {
        if (!CanScanFolder()) return;

        files.Clear();

        await Task.Run(() =>
        {
            _dataService.GetData(SelectedFolder, filter, IncludeSubs, (myfiles, error) =>
                {
                    if (error != null)
                    {
                        return;
                    }
                    foreach (var file in myfiles.files)
                    {
                        files.Add(file);
                    }
                }

                );
        });            
    }

The DataService code looks like this

    public async void GetData(string folder, string filter, bool includeSubs, Action<DataItem, Exception> callback)
    {
        // Use this to connect to the actual data service
        var item = new DataItem(folder, filter, includeSubs);
        await item.ScanFolderAsync();
        callback(item, null);
    }

    public async Task<bool> ScanFolderAsync()
    {
        try
        {
            var ret = new List<DataListItem>();
            var files = Directory.EnumerateFiles(folder, filter, includeSubs ? SearchOption.AllDirectories : SearchOption.TopDirectoryOnly);
            await Task.Run(() => {
                Parallel.ForEach(files, new ParallelOptions() { MaxDegreeOfParallelism = 2 }, (file) =>
                {                    
                    this.files.Add(new DataListItem(file));
                });
            });

            return true;
        }
        catch (Exception)
        {
            return false;
        }
    }

But as far as I can tell there is no continuous communication channel between the DataService and the ViewModel. So with the new approach it reads all of the files and then displays it in the grid, but I want it to display each update as it goes.

My instinct is to add a Messenger within the foreach and subscribe to it in the ViewModel, but there does not seem to be a way to use a messenger within a data service.

How can the DataService send a message to the ViewModel each time it has scanned a file? Alternatively is there a better architecture for loading the data?

Answer 1

It turns out I was just missing the using statement for messenger and it is possible to update the ViewModel using it.

In the ViewModel constructor I added

Messenger.Default.Register<DataListItem>(this, (item) => { files.Add(item); });

and the ScanFolderAsync() method was updated to send the message

using GalaSoft.MvvmLight.Messaging;

public async Task<bool> ScanFolderAsync()
{
     try
     {
        var ret = new List<DataListItem>();
        var files = Directory.EnumerateFiles(folder, filter, includeSubs ? SearchOption.AllDirectories : SearchOption.TopDirectoryOnly);
        await Task.Run(() => {
                Parallel.ForEach(files, new ParallelOptions() { MaxDegreeOfParallelism = 1 }, (file) =>
            {
                var item = new DataListItem(file);
                Messenger.Default.Send<DataListItem>(item);
                this.files.Add(item);
            });
       });

        return true;
    }
    catch (Exception)
    {
        return false;
    }
}