Previewer Unable to find public constructor for type [view model] about avalonia-docs HOT 5 CLOSED

But MidiForMacrosViewModel doesn't have a public constructor, while you try to use it as a parameterless DataContext:
https://github.com/SimonORorke/FalconProgrammer/blob/main/FalconProgrammer/Views/MidiForMacrosView.axaml#L12

from avalonia-docs.

If you want to resolve previewing data context, you should define this context in C# code where you can access your DI container.

from avalonia-docs.

Thanks, @maxkatz6! I got it working. For anyone who encounters the same problem, here is an example. As I mentioned, the public view model constructors in my application all have parameters, like this:

public MidiForMacrosViewModel(IDialogService dialogService,
  IDispatcherService dispatcherService) : base(dialogService, dispatcherService) { }

So I had to define Design.DataContext in code behind instead of in XAML, like this:

public MidiForMacrosView() {
  InitializeComponent();
  // This only sets the DataContext for the previewer in the IDE.
  Design.SetDataContext(this,
    new MidiForMacrosViewModel(new DialogService(), new DispatcherService()));
}

I expect view model constructors with parameters are common, as my understanding is that it is part of the usual way dependency injection works. That is the purpose of the view model constructors with parameters in my project. But I could not find any documentation that covers using the previewer in such a scenario. I would expect to find it in the How To Use Live Preview page of Avalonia's documentation, but I don't see anything there.

Have I missed something in the documentation? If not, I'd be happy to have a go at improving the How To Use Live Preview page and submitting a pull request to the documentation project.

from avalonia-docs.

@SimonORorke

• parameterless constructor may be internal, but it must be there
• or as you said, add a designtime one
• or use a static one with {x:Static my:MyDesignerVM.Instance}
• or use if(Design.IsDesignTime) in code behind.

a PR to the docs may be a good idea, could also be cross-linked to this one: https://docs.avaloniaui.net/docs/guides/implementation-guides/how-to-implement-dependency-injection

from avalonia-docs.

@timunie
My pull request commit incorporates some of the points you make, though there were a couple I was not sure about and did not mention. If applicable, please let me know how I may improve the update.

from avalonia-docs.