Convenient return handler for Martini JSON API calls

jsonreturnhandler.go

import (
	"encoding/json"
	"net/http"
	"reflect"

	"github.com/codegangsta/inject"
	"github.com/go-martini/martini"
)

const (
	contentType = "application/json; charset=UTF-8"
)

type APIResponse struct {
	S int
	E string      `json:",omitempty"`
	D interface{} `json:",omitempty"`
}

// Allows responses from request handles to be in the following form:
//
//    (status, <json-encodable-object>)
//    (status, error)
//
func MartiniReturnHandler() martini.ReturnHandler {
	return func(ctx martini.Context, vals []reflect.Value) {
		// No return value, we assume the function wrote out the response.
		if len(vals) == 0 {
			return
		}

		res := ctx.Get(inject.InterfaceOf((*http.ResponseWriter)(nil))).Interface().(http.ResponseWriter)
		// req := ctx.Get(reflect.TypeOf(&http.Request{})).Interface().(*http.Request)

		var responseVal reflect.Value
		status := 200
		if len(vals) > 1 && vals[0].Kind() == reflect.Int {
			status = int(vals[0].Int())
			responseVal = vals[1]
			if res.Header().Get("Content-Type") == "" {
				switch responseVal.Interface().(type) {
				case []byte:
					res.Header().Set("Content-Type", "application/octet-stream")
				case string:
					res.Header().Set("Content-Type", "text/html; charset=utf-8")
				case error:
					res.Header().Set("Content-Type", contentType)
				default:
					res.Header().Set("Content-Type", contentType)
				}
			}
			res.WriteHeader(status)
		} else if len(vals) > 0 {
			responseVal = vals[0]
		}
		if canDeref(responseVal) {
			responseVal = responseVal.Elem()
		}

		enc := json.NewEncoder(res)
		switch v := responseVal.Interface().(type) {
		case string:
			res.Write([]byte(v))

		case []byte:
			res.Write(v)

		case error:
			enc.Encode(&APIResponse{
				S: status,
				E: v.Error(),
			})

		default:
			enc.Encode(&APIResponse{
				S: status,
				D: v,
			})
		}
	}
}