Issue with Community MVVM messaging and storage service.

[tomhornyak]

The problem is I get an exception "No response was received for the given request message." after sending it in the view model. But I do hit the breakpoint at the storage provider and it's not null, although the dialog is not shown. I am still learning how Avaloina and the community.mvvm works, so any help would be appreciated.

The Message

public class FilePickerRequestMessage : AsyncRequestMessage<IEnumerable<IStorageFile>?>

{
    public FilePickerOpenOptions Options { get; }
    public FilePickerRequestMessage(FilePickerOpenOptions options) => Options = options;
}

Register in OnLoaded

WeakReferenceMessenger.Default.Register<MainWindow, FilePickerRequestMessage>
(this, async (r, m) =>

{
    // Use the top-level window's StorageProvider
    // Note: If I put a breakpoint here - it hits it after the viewmodel sends the message
    var files = await r.StorageProvider.OpenFilePickerAsync(m.Options);
    m.Reply(files); // Send the result back to the ViewModel
});

The View Model

        [RelayCommand]
        public async Task SelectFile()
        {
            var options = new FilePickerOpenOptions
            {
                Title = "Select a File",
                AllowMultiple = false,
                FileTypeFilter =
                [
                    new FilePickerFileType("Card Files")
                    {
                        Patterns = new[] { "*.ikx" }
                    },
                    new FilePickerFileType("All Files")
                    {
                        Patterns = new[] { "*.*" }
                    }
                ]
            };
            // Send the request and wait for the result
            var file = await WeakReferenceMessenger.Default.Send(new FilePickerRequestMessage(options));
            if (file != null)
            {
                // Process the file (e.g., file.Path)
            }

[LaurentInSeattle]

1 - Receiving class must comply to IRecipient, Example:
IRecipient<>

2 - Then implement Receive, Example:
public void Receive(LanguageChangedMessage _)
{
// Do stuff here
}

Edit: Struggling with tags: IRecipient generic of received type

[tomhornyak]

That was not it. Still got the same exception exception "No response was received for the given request message." and the file picker dialog did now show.

[LaurentInSeattle]

Maybe related to: CommunityToolkit/dotnet#898

[tomhornyak]

Your are correct. It seems to happen calling an async task. Here is my latest test. It also fails.

public class FilePickerRequestMessage : AsyncRequestMessage<string>

WeakReferenceMessenger.Default.Register<MainWindow, FilePickerRequestMessage>(this, async (r, m) =>
{
    //var files = await r.StorageProvider.OpenFilePickerAsync(m.Options);
    var files = await TestTask();
    m.Reply(files);
});

private async Task<string> TestTask()
{
    {
        var url = "https://www.google.com.com";
        using var client = new HttpClient();
        // Control returns to the caller here until the download is complete
        string content = await client.GetStringAsync(url);
        return content;
    }

I guess I'm off to the mvvm group.

[LaurentInSeattle]

Does not seem to be an Avalonia issue...

[tomhornyak]

oops - replied to the wrong message.

[maxkatz6]

I use Community MVVM messaging a lot, and see a problem here.

In this snippet:

WeakReferenceMessenger.Default.Register<MainWindow, FilePickerRequestMessage>
(this, async (r, m) =>

{
    // Use the top-level window's StorageProvider
    // Note: If I put a breakpoint here - it hits it after the viewmodel sends the message
    var files = await r.StorageProvider.OpenFilePickerAsync(m.Options);
    m.Reply(files); // Send the result back to the ViewModel
});

Messenger callback should NOT be async.
Instead, AsyncRequestMessage.Reply accepts Task.
In your case - your async callback is not awaited, and reply is called AFTER message awaiting was completed.

One of ways to fix it is introducing another local function:

WeakReferenceMessenger.Default.Register<MainWindow, FilePickerRequestMessage>
     (this, static (r, m) =>  m.Reply(Handle(r, m.Options)));

static async Task<IEnumerable<IStorageFile>> HandleFilePickerRequestMessage(MainWindow w, FilePickerOpenOptions o)
{
    return await w.StorageProvider.OpenFilePickerAsync(o);
}

(I also used "static" where possible to avoid leaked state/closures).

[tomhornyak]

Thank you very much for you help. That did it.

[tomhornyak]

MaxKatz6 reply fixed it

[tomhornyak]

I created working examples in https://github.com/tomhornyak/TomsAvaloniaExamples. Hopefully it can help others.