Automatically switching views for AMP in ASP.NET MVC

I want to create and AMP version of my website in ASP.NET MVC using .NET Core 2.0. Previously I had done some work with DisplayModeProvider instances in tha past on .Net framework, but that does not seem to be an option in .NET Core.

What I want to be able to do is alter the view names to be index.amp.cshtml rather than index.cshtml when my URL starts iwth /amp. What's the best way to achieve this in .NET Core?

Solution

You can do something like this using IViewLocationExpander. As it happens, I was playing with this a few days ago so I have some code samples to hand. If you create something like this:

public class AmpViewLocationExpander : IViewLocationExpander
{
    public void PopulateValues(ViewLocationExpanderContext context)
    {
        var contains = context.ActionContext.HttpContext.Request.Query.ContainsKey("amp");
        context.Values.Add("AmpKey", contains.ToString());

        var containsStem = context.ActionContext.HttpContext.Request.Path.StartsWithSegments("/amp");
        context.Values.Add("AmpStem", containsStem.ToString());
    }

    public IEnumerable<string> ExpandViewLocations(ViewLocationExpanderContext context, IEnumerable<string> viewLocations)
    {
        if (!(context.ActionContext.ActionDescriptor is ControllerActionDescriptor descriptor)) { return viewLocations; }

        if (context.ActionContext.HttpContext.Request.Query.ContainsKey("amp")
            || context.ActionContext.HttpContext.Request.Path.StartsWithSegments("/amp")
        )
        {
            return viewLocations.Select(x => x.Replace("{0}", "{0}.amp"));
        }

        return viewLocations;
    }
}

iViewLocationExpander can be found in Microsoft.AspNetCore.Mvc.Razor

Then in your Configure Services method in Startup.cs, add the following:

  services.Configure<RazorViewEngineOptions>(options =>
        {
            options.ViewLocationExpanders.Add(new AmpViewLocationExtender());
        });

What this will do is update the view locations per request to insert .amp before .cshtml any time the URL either starts with /amp or there is a query string key of amp. If your AMP views don't exist, it might blow-up a little, I've not fully tested it, but it should get you started.