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I'm currently putting together a multi-tenancy web app using MVC 3. At least 30 different Web sites will share a common codebase, and while also sharing similar under-the-hood functionality, they are need to look significantly different. As a consequence I'm using the URL to internally separate out clients, and an overridden Razor view engine (at least in terms of finding the view) to automatically select either a customised view for a given action, or the default view.

To help 'compartmentalise' a complex page, any particular view will make use of several partials, usually rendered as self-contained actions, so that generally a custom view or partial view will only have small HTML differences, helping to minimise any code specific to a client's site.

I hope you followed that background!

So one HTML page might be made up of lots of little smatterings of HTML partial views, which could come from specific folders for the client, or a general-purpose version. I'm hoping to make it easier for our designer to make minor changes to a page by easily seeing where in the folder structure the bit of HTML he wants to change are.

My proposal then is that each partial will be 'bracketed' with HTML comments such as:

{ Content of partial }

Obviously I could put these in manually, but that's just asking for trouble, for typos, for copied and then modified client versions not being updated with the correct URL. It should be possible to get this from some context and inject it, I think.

At the same time, I need to be able to not do this for certain Actions. Eg, a partial might be generating text inside a textarea, say, so the comments wouldn't be appropriate there. On the whole I'm happy to put these comments in unless I specify that it's not appropriate.

For me this suggests an ActionFilter on an Action, which I can apply site wide and then turn off for certain Actions. I'd hope that one of the overridable events would let me ascertain this path, but I can't seem to find anywhere it's stored. Furthermore, OnResultExecuting seems to fire before the Partial has been selected, and OnResultExecuted seems to have already written out the contents of the Partial, so I can't insert the starting comment here. I also can't find any reference to the path of the selected partial.

Just for completeness, it's my intention that this attribute would only write these comments when compiled in Debug mode.

So, does anyone know how I might be able to get the path to the selected View without any kind of hack between FindPartialView and the Attribute? Is my Attribute method the best choice or is there an easier way to do this? Perhaps something's built in already!

Many thanks for your help.

Steve Owen
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1 Answers1

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Well, I've never forgotten about wanting this, and always hoped I'd solve it one day, and thankfully I have.

I've overridden the default WebViewPage (I use the Razor engine), and in particular ExecutePageHierarchy to inject the comments:

public abstract class PaladinWebViewPage : PaladinWebViewPage<dynamic>
{
}

public abstract class PaladinWebViewPage<TModel> : WebViewPage<TModel>
{
    public bool DisplaySourceCodeComments
    {
        get { return ((bool?) ViewBag.__DisplaySourceCodeComments) ?? false; }
        set { ViewBag.__DisplaySourceCodeComments = value; }
    }

    public override void ExecutePageHierarchy()
    {
        base.ExecutePageHierarchy();

        // Filters can be used to set and clear this value so we can decide when to show this comment
        if (!DisplaySourceCodeComments) return;

        var sw = Output as StringWriter;
        if (sw == null) return;

        var sb = sw.GetStringBuilder();
        sb.Insert(0, string.Format("<!-- Start of {0} -->", VirtualPath));
        sb.AppendFormat("<!-- End of {0} -->", VirtualPath);
    }

VirtualPath tells us the exact file used to build the HTML, so we can inject the filename before and after. This isn't doing anything at the moment, since the default is to not show comments (the "?? false" in DisplaySourceCodeComments).

Also to use this view page you need to edit Views/Web.config and change the pageBaseType to this type.

I want to selectively turn these comments on and off so I've created an ActionFilter:

public class DisplaySourceCodeCommentsAttribute : ActionFilterAttribute
{
    private readonly bool _displaceSourceCodeComments;

    public DisplaySourceCodeCommentsAttribute(bool displaceSourceCodeComments)
    {
        _displaceSourceCodeComments = displaceSourceCodeComments;
    }

    public override void OnActionExecuted(ActionExecutedContext filterContext)
    {
        base.OnActionExecuted(filterContext);

        var viewResult = filterContext.Result as ViewResultBase;
        if (viewResult == null) return;

        viewResult.ViewBag.__DisplaySourceCodeComments = _displaceSourceCodeComments;
    }
}

I'm slightly unhappy that I've had to use the ViewBag here and also separately in the view page override, as they aren't tightly linked, but I can't find a way for the filter to directly interact with the view page, so this is something of a necessary fudge. It does have the benefit that displaying source code for a view or partial also automatically displays it for any child partials until you turn it off again, since the ViewBag is passed down the chain.

With this in place, any action can turn on the source code comments with [DisplaySourceCodeComments(true)]

or, obviously turn it off again with false

The Attribute checks that the context result is a ViewResultBase, which means just Views and Partials, so Json or Content or redirects aren't affected, which is very handy too.

Finally, I make this action filter a global when running in debug mode so that every view, and partial has the source comment included, by adding the following line to global.asax.cs:

[#]if DEBUG

        // When in debug mode include HTML comments showing where a view or partial has come from
        GlobalFilters.Filters.Add(new DisplaySourceCodeCommentsAttribute(true));

[#]endif

I'm really happy I've finally got it sorted so I hope this is useful for someone else.

Steve Owen
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