Create your own middleware in Chi

Link to original article (recommended): https://gabriellazcano.com/blog/create-your-own-middleware-in-chi/

Introduction

chi is a lightweight, idiomatic and composable router for building Go HTTP services.

I've had some trouble recently in finding out how the chi library works. It is definitely powerful and in my opinion one of the best alternatives out there. You can check out the features and the performance if you want to make an informed opinion for choosing it.

In the GitHub they briefly show an example of how they are using it but they don't dive much into it. It's pretty straightforward actually.

You call a middleware using r.Use where r is the router. Either a nested router like in this example.

r.Route("/user", func(r chi.Router) {
    r.Use(user.UserContextBody)
    r.Route("/{username}", func(r chi.Router) {
        // your code
    })
})

Or the main router

r := chi.NewRouter()

// middleware stack
r.Use(middleware.RequestID)
r.Use(middleware.RealIP)
r.Use(middleware.Logger)
r.Use(middleware.Recoverer)

What is a middleware in Chi?

A middleware is a function that receives a http.Handler and returns a http.Handler. Yeah it's not quite self-explanatory.

Anything you can do with the standard library you can do. We just have to serve the http.Handler from the parameter, you can find it as next in the Chi library.

func UserContextBody(next http.Handler) http.Handler {
    return http.HandlerFunc(func(rw http.ResponseWriter, r *http.Request) {
        var user User 

        err := json.NewDecoder(r.Body).Decode(&user)
        if err != nil {
            fmt.Println(err)
            return
        }

        ctx := context.WithValue(r.Context(), "user", user) 
        next.ServeHTTP(rw, r.WithContext(ctx))
    })
}

In the example above I'm getting the body of the request, decoding it into a struct and adding it to the next handler, this is how we can chain multiple middlewares.

And as you may have noticed we are using context this is to pass values throughout the handlers. We have to pass the context (for passing it from previous middlewares), a key and the value. You can check the documentation for context

Get the context

In order to get the context either from another middleware or from a route handler, we have to call the context function from the http.Request and get the key we used while defining the context, then we have to pass type of the context we are passed. In this case we were passing a User struct but it could be a string, and int or whatever type it's being passed in the context.

// example for getting a string from context
ctx := r.Context()
key := ctx.Value("key").(string)

r.Use(UserContextBody)
r.Route("/{username}", func(r chi.Router) {
    r.Get("/", func(rw http.ResponseWriter, r *http.Request) {
        // your code
    })
})

r.Post("/", func(rw http.ResponseWriter, r *http.Request) {
    u := r.Context().Value("user").(User)
    // your code
})