RazorViewEngine custom search paths for Umbraco grid partial views

I have been trying to add some new functionality to my BlockPreview package to remove some magic strings.

As of version 1.1.2, the package searches for partial views by using magic strings of the actual paths in the file system:

if (isGrid)
{
    partialPath = $"/Views/Partials/blockgrid/Components/{documentTypeAlias}.cshtml";
}
else
{
    partialPath = $"/Views/Partials/blocklist/Components/{documentTypeAlias}.cshtml";
}

An issue raised by 37Stars on GitHub posed a great question...

Instead of hardcoding the path for views to "/Views/Partials/blockgrid/Components/{contentAlias}.cshtml" or "/Views/Partials/blocklist/Components/{contentAlias}.cshtml", use the Razor Engine to find the view.

Changing line 120 in BackOfficePreviewService.cs:

string partialPath = _razorViewEngine.FindView(controllerContext, contentAlias, false).ViewName;

I gave this a go! But unfortunately the RazorViewEngine was never able to find the files inside /Views/Partials/blockgrid because they aren't a default search path. Typically it's searching for things inside /Views, /Views/Partials, /Views/Shared, etc.

So how do we tell it to search the paths we want? After a bit of Googling and talking to ChatGPT, I landed on adding a custom ViewLocationExpander. Here's my first attempt at it:

public class BlockViewEngineOptionsSetup : IConfigureOptions<RazorViewEngineOptions>
{
    public void Configure(RazorViewEngineOptions options)
    {
        if (options == null)
        {
            throw new ArgumentNullException(nameof(options));
        }

        options.ViewLocationExpanders.Add(new BlockViewLocationExpander());
    }

    public class BlockViewLocationExpander : IViewLocationExpander
    {
        public IEnumerable<string> ExpandViewLocations(
            ViewLocationExpanderContext context, IEnumerable<string> viewLocations)
        {
            string[] blockViewLocations =
            {
                "/Views/Partials/blockgrid/Components/{0}.cshtml",
                "/Views/Partials/blocklist/Components/{0}.cshtml"
            };

            viewLocations = blockViewLocations.Concat(viewLocations);

            return viewLocations;
        }

        // not a dynamic expander
        public void PopulateValues(ViewLocationExpanderContext context)
        {
        }
    }
}

What this code is doing is telling the RazorViewEngine to check the paths I'm adding when it's trying to run FindView().

I attempted to compose this in a few ways as shown in many examples you'll see across the web:

public class Startup : IComposer
{
    public void Compose(IUmbracoBuilder builder)
    {
        builder.Services.ConfigureOptions<BlockViewEngineOptionsSetup>();
        builder.Configure<RazorViewEngineOptions>(options =>
        {
            options.ViewLocationExpanders.Add(new BlockViewLocationExpander());
        });
    }
}

None of this was cutting it. The code was being hit in my Startup but the actual code running inside of BlockViewEngineOptionsSetup was never hit when trying to call FindView().

After some more Googling, this GitHub issue pointed me to the fact that it would have to be composed before AddWebsite() is called in ConfigureServices in the main Startup.cs file.

I reworked the code and ended up needing to use an extension method for IUmbracoBuilder to make it work.

public static class BlockPreviewUmbracoBuilderExtensions
{
    public static IUmbracoBuilder AddBlockPreview(this IUmbracoBuilder builder)
    {
        ...
        builder.Services.ConfigureOptions<BlockViewEngineOptionsSetup>();
        return builder;
    }
}

And then composed in my site's ConfigureServices in Startup.cs...

public void ConfigureServices(IServiceCollection services)
{
    services.AddUmbraco(_env, _config)
        .AddBackOffice()
+       .AddBlockPreview()
        .AddWebsite()
        .AddComposers()
        .Build();
}

And it works! When calling FindView, I now get results from the /blockgrid and /blocklist folders.

Watch out for this landing in v1.2.0 of Umbraco.Community.BlockPreview soon!