Allow a way to modify locations in which views are looked for without having to modify ViewEngine

[pranavkm]

In Mvc 5, the VirtualPathProviderViewEngine that was the base for all view engines that shipped in the box looked for views in a very specific set of locations that were exposed as properties of that type.

The primary way to use customize rules for discovering view locations was by sub typing and changing the result of these properties. This feature work is designed towards finding a nicer way to go about this:

Proposal: View Location Expanders

1. We start with a set of seed formats: ["/Views/{1}/{0}", "/Views/Shared/{0}"]
2. We introduce a contract for view location expanders as

interface IViewLocationExpander
{
     IEnumerable<string> ExpandViewLocations(IEnumerable<string> seed, ActionContext context);
}

3. Individual expanders can add tokens, expand previously inserted tokens etc

public class LocaleViewLocationExpander : IViewLocationExpander
{
     public IEnumerable<string> ExpandViewLocations(IEnumerable<string> seed, ActionContext context)
     {
        var locale = context.RouteData["locale"];
        if (!string.IsNullOrEmpty(locale))
        {
            // Views/{1}/{0}.en-gb
            // Views/Shared/{0}.en-us
            // Views/{1}/{0}
            // Views/Shared/{0}

            return seed.Select(s => areaName + '/' + s)
                       .Concat(seed);
        }
     }
 }

4. Expanders are registered via Options so they can be re-ordered, removed etc.The RazorViewEngine calls into these expanders to get all possible locations to locate views as part of FindView and FindPartialView.

cc @yishaigalatzer / @loudej

[yishaigalatzer]

Also:

1. Seed formats can be changed.
2. We need to decide what default expanders to build
3. What default expanders to register
4. Is registration indeed in Options or directly on the RazorViewEngine?

[yishaigalatzer]

@pranavkm please set up a design meeting for this.