Passing in parameters to a http.HandlerFunc

Question

I'm using Go's built in http server and pat to respond to some URLs:

mux.Get("/products", http.HandlerFunc(index))

func index(w http.ResponseWriter, r *http.Request) {
    // Do something.
}

---

I need to pass in an extra parameter to this handler function - an interface.

func (api Api) Attach(resourceManager interface{}, route string) {
    // Apply typical REST actions to Mux.
    // ie: Product - to /products
    mux.Get(route, http.HandlerFunc(index(resourceManager)))

    // ie: Product ID: 1 - to /products/1
    mux.Get(route+"/:id", http.HandlerFunc(show(resourceManager)))
}

func index(w http.ResponseWriter, r *http.Request, resourceManager interface{}) {
    managerType := string(reflect.TypeOf(resourceManager).String())
    w.Write([]byte(fmt.Sprintf("%v", managerType)))
}

func show(w http.ResponseWriter, r *http.Request, resourceManager interface{}) {
    managerType := string(reflect.TypeOf(resourceManager).String())
    w.Write([]byte(fmt.Sprintf("%v", managerType)))
}

How can I send in an extra paramter to the handler function?

Answer 1

You should be able to do what you wish by using closures.

Change func index() to the following (untested):

func index(resourceManager interface{}) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        managerType := string(reflect.TypeOf(resourceManager).String())
        w.Write([]byte(fmt.Sprintf("%v", managerType)))
    }
}

And then do the same to func show()

Answer 2

Another option is to use types implementing http.Handler directly rather than only using functions. For example:

type IndexHandler struct {
    resourceManager interface{}
}

func (ih IndexHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    managerType := string(reflect.TypeOf(ih.resourceManager).String())
    w.Write([]byte(fmt.Sprintf("%v", managerType)))
}

...
mux.Get(route, IndexHandler{resourceManager})

This kind of pattern can be useful if you want to refactor your ServeHTTP handler method into multiple methods.