add validator wrapper (#370)

all requests that have method Validate() error can be used to validate incoming request

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

validator.go

@@ -0,0 +1,25 @@
+package validator
+
+import (
+	"context"
+
+	"github.com/micro/go-micro/errors"
+	"github.com/micro/go-micro/server"
+)
+
+type Validator interface {
+	Validate() error
+}
+
+func NewHandlerWrapper() server.HandlerWrapper {
+	return func(fn server.HandlerFunc) server.HandlerFunc {
+		return func(ctx context.Context, req server.Request, rsp interface{}) error {
+			if v, ok := req.Body().(Validator); ok {
+				if err := v.Validate(); err != nil {
+					return errors.BadRequest(req.Service(), "%v", err)
+				}
+			}
+			return fn(ctx, req, rsp)
+		}
+	}
+}