It's a github.com/julienschmidt/httprouter drop in replacement. Basically all methods from the original library are wrapped. It makes easy to us the httprouter on App Engine and outside of App Engine.

The problem with App Engine context is that it requires your app to hardcode the appengine.NewContext in your handlers. As stated in https://blog.golang.org/context inside Google's infrastructure they pass Context around. It's pretty useful for puting timeouts and other limits on a request and passing user permissions.

On App Engine the init code would look:

func SomeHandler(c context.Context, w http.ResponseWriter, r *http.Request) { /* some code */ }

func GaeContext(r *http.Request) context.Context {
	return appengine.NewContext(r)
}

func init() {
	r := New()
	r.ContextFactory = GaeContext
	r.GET("/page/:pageid/:mod", SomeHandler)
	http.Handler("", r)  // r may be replaced by r.R, the underlaying httprouter.
}

For more details please the doc of the original repo:

The extra useful method indluce:

func (r *Router) WithMiddleware(m Middleware) *Router

the returned Router keeps the underlying httprouter.Router but extends the currently registered middlewares by passed generator.

Under the hood it keeps all added Middleware generators in list and builds the desired handle method when one adds a path to be handled.