handler.go

package api

import (
	"reflect"

	"gitlab.com/uafrica/go-utils/errors"
)

type handler struct {
	RequestParamsType reflect.Type
	RequestBodyType   reflect.Type
	ResponseType      reflect.Type
	FuncValue         reflect.Value
}

func NewHandler(fnc interface{}) (handler, error) {
	h := handler{}

	fncType := reflect.TypeOf(fnc)
	if fncType.NumIn() < 2 || fncType.NumIn() > 3 {
		return h, errors.Errorf("takes %d args instead of (Context, Params[, Body])", fncType.NumIn())
	}
	if fncType.NumOut() < 1 || fncType.NumOut() > 2 {
		return h, errors.Errorf("returns %d results instead of ([Response,] error)", fncType.NumOut())
	}

	//arg[0] must implement interface lambda_helpers.Context
	if _, ok := reflect.New(fncType.In(0)).Interface().(IContext); !ok {
		return h, errors.Errorf("first arg %v does not implement lambda_helpers.IContext", fncType.In(0))
	}

	//arg[1] must be a struct for params. It may be an empty struct, but
	//all public fields require a json tag which we will use to math the URL param name
	if err := validateStructType(fncType.In(1)); err != nil {
		return h, errors.Errorf("second arg %v is not valid params struct type", fncType.In(1))
	}
	h.RequestParamsType = fncType.In(1)

	//arg[2] is optional and must be a struct for request body. It may be an empty struct, but
	//all public fields require a json tag which we will use to unmarshal the request body from JSON
	if fncType.NumIn() >= 3 {
		if fncType.In(2).Kind() == reflect.Slice {
			if err := validateStructType(fncType.In(2).Elem()); err != nil {
				return h, errors.Errorf("third arg %v is not valid body []struct type", fncType.In(2))
			}
		} else {
			if err := validateStructType(fncType.In(2)); err != nil {
				return h, errors.Errorf("third arg %v is not valid body struct type", fncType.In(2))
			}
		}

		//todo: check special fields for claims, and see if also applies to params struct...
		//AccountID must be int64 or *int64 with tag =???
		//UserID must be int64 or *int64 with tag =???
		//Username must be string with tag =???

		h.RequestBodyType = fncType.In(2)
	}

	//if 2 results, first must be response struct or array of response structs that will be marshalled to JSON
	if fncType.NumOut() > 1 {
		if fncType.Out(0).Kind() == reflect.Slice {
			if err := validateStructType(fncType.Out(0).Elem()); err != nil {
				return h, errors.Errorf("first result %v is not valid response []struct type", fncType.Out(0))
			}
		} else {
			if err := validateStructType(fncType.Out(0)); err != nil {
				return h, errors.Errorf("first result %v is not valid response struct type", fncType.Out(0))
			}
		}