Earlier in November, the ASP.NET Monsters had the opportunity to take part in the ASP.NET Core hackathon at the Microsoft MVP Summit. In past years, we have used the hackathon as an opportunity to spend some time working on GenFu. This year, we wanted to try something a little different.
A few months ago, we had Taylor Mullen on The Monsters Weekly to chat about Razor in ASP.NET Core. At some point during that interview, it was pointed that MVC is designed in a way that a new view engine could easily be plugged into the framework. It was also noted that implementing a view engine is a really big job. This got us to thinking…what if we could find an existing view engine of some sort. How easy would it be to get actually put a new view engine in MVC?
And so, that was our goal for the hackathon. Find a way to replace Razor with an alternate view engine in a single day of hacking.
We wanted to pick something that in no way resembled Razor. Simon suggested Pug (previously known as Jade), a popular view template engine used in Express. In terms of syntax, Pug is about as different from Razor as it possibly could be. Pug uses whitespace to indicate nesting of elements and does away with angle brackets all together. For example, the following template:
would generate this HTML:
To our surpise, this not only worked, it was also easy! We created a very simple file called pugcompile.js.
model is the view model we want to bind to the template and
mytemplate.pug is the name of the file containing the pug template:
Now that we had proven this was possible, it was time to integrate this with MVC by creating a new MVC View Engine.
We decided to call our view engine Pugzor which is a combination of Pug and Razor. Of course, this doesn’t really make much sense since our view engine really has nothing to do with Razor but naming is hard and we thought we were being funny.
Keeping in mind our goal of implenting a view engine in a single day, we wanted to do this with the simplest way possible. After spending some time digging through the source code for MVC, we determined that we needed to implement the
IViewEngine interface as well as implement a custom
IViewEngine is responsible for locating a view based on a
ActionContext and a
ViewName. When a controller returns a
View, it is the
FindView method that is responsible for finding a view based on some convetions. The
FindView method returns a
ViewEngineResult which is a simple class containing a
boolean Success property indicating whether or not a view was found and an
IView View property containing the view if it was found.
We decided to use the same view location conventions as Razor. That is, a view is located in
Here is a simplified version of the FindView method for the
You can view the complete implentation on GitHub.
Next, we created a class called
PugzorView which implements
PugzorView takes in a path to a pug template and an instance of
INodeServices. The MVC framework calls the
RenderAsync when it is wants the view to be rendered. In this method, we call out to
pugcompile and then write the resulting HTML out to the view context.
The only thing left was to configure MVC to use our new view engine. At first, we thought we could easy add a new view engine using the
AddViewOptions extension method when adding MVC to the service collection.
This is where we got stuck. We can’t add a concrete instance of the
PugzorViewEngine to the
ViewEngines collection in the
Startup.ConfigureServices method because the view engine needs to take part in dependency injection. The
PugzorViewEngine has a dependency on
INodeServices and we want that to be injected by ASP.NET Core’s dependency injection framework. Luckily, the all knowning Razor master Taylor Mullen was on hand to show us the right way to register our view engine.
The recommended approach for adding a view engine to MVC is to create a custom setup class that implements
IConfigureOptions<MvcViewOptions>. The setup class takes in an instance of our
IPugzorViewEngine via constructor injection. In the configure method, that view engine is added to the list of view engines in the
Now all we need to do is register the setup class and view engine the
Like magic, we now have a working view engine. Here’s a simple example:
So we reached our goal of creating an alternate view engine for MVC in a single day. We had some time left so we thought we would try to take this one step further and create a NuGet package. There were some challenges here, specifically related to including the required node modules in the NuGet package. Simon is planning to write a separate blog post on that topic.
You can give it a try yourself. Add a reference to the
pugzor.core NuGet package then call
.AddMvc() in the
Razor still works as the default but if no Razor view is found, the MVC framework will try using the PugzorViewEngine. If a matching pug template is found, that template will be rendered.
We had a blast working on this project. While this started out as a silly excercise, we sort of ended up with something that could be useful. We were really surprised at how easy it was to create a new view engine for MVC. We don’t expect that Pugzor will be wildly popular but since it works we thought we would put it out there and see what people think.
We have some open issues and some ideas for how to extend the
PugzorViewEngine. Let us know what you think or jump in and contribute some code. We accept pull requests :-)