Prism: Using Xamarin.Forms Previewer with Page that has ViewModelLocaltor enable throws at design-time

[ghost]

Using Xamarin.Forms Previewer with Page that has ViewModelLocaltor enable throws at design-time

Create a new XF project using Prism

Enable ViewModelLocator on the MainPage

Build the solution

Open the VS XF Previewer

Exception throw.

Discussion:
VML needs to handle design-time scenarios in XF. The code can't throw an exception at design-time. If Xamarin can't provide reliable design-time API, this code will have to handle this scenario and not throw.

[dansiegel]

@kdawg1406 I'm not sure what exactly you're doing but, I can File -> New a project add the ViewModelLocator to a View and I get no exception thrown by the Xamarin Forms Previewer.

Can you provide some more information like which versions of Prism, Xamarin Forms and your IDE.

[ghost]

Use AppMap to create the solution.

On May 10, 2017 9:53 PM, "Dan" [e-mail] wrote:

@kdawg1406 https://github.com/kdawg1406 I'm not sure what exactly
you're doing but, I can File -> New a project add the ViewModelLocator to a
View and I get no exception thrown by the Xamarin Forms Previewer.

Can you provide some more information like which versions of Prism,
Xamarin Forms and your IDE.

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[dansiegel]

@kdawg1406 if you remove the ViewModelLocator does the Previewer in VS succeed?

[ghost]

Yes

[dansiegel]

Ok... with the Attached property removed try the following.

public class ViewModelLocatorExtension : IMarkupExtension
{
    public object View { get; set; }

    public object ProvideValue(IServiceProvider serviceProvider)
    {
        object viewModel = null;
        ViewModelLocationProvider.AutoWireViewModelChanged(View, (v, vm) => viewModel = vm);
        return viewModel;
    }
}

xmlns:xaml="{local import to the above class}"
x:Name="view"
BindingContext="{xaml:ViewModelLocator {x:Reference view}}"

[brianlagunas]

The default behavior works fine for me. The only issue that was noticed is that the App.xaml.cs requires an empty default ctor. I added this, and everything worked fine:

public App() : this(null) { }

I found no issues of exceptions in the previewer with VML. The bindings worked and displayed all the default values defined for the properties.

When PrismAplication was written, I did not consider anything creating it using reflection, so I was not concerned with having a default ctor. Since it is obvious that the Previewer needs this, I will update the PrismApplication class to have a default ctor.

[brianlagunas]

Actually, this isn't something that needs to be done in Prism itself. The templates just have to be updated to provide the ctor snippet.

[lock[bot]]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.