codec: add ability to control codec via struct tags

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

codec/codec.go

@@ -28,6 +28,8 @@ var (
 	DefaultMaxMsgSize int = 1024 * 1024 * 4 // 4Mb
 	// DefaultCodec is the global default codec
 	DefaultCodec Codec = NewCodec()
+	// DefaultTagName specifies struct tag name to control codec Marshal/Unmarshal
+	DefaultTagName = "codec"
 )
 
 // MessageType specifies message type for codec

util/reflect/reflect.go

@@ -12,6 +12,7 @@ import (
 var (
 	ErrInvalidStruct = errors.New("invalid struct specified")
 	ErrInvalidValue  = errors.New("invalid value specified")
+	ErrNotFound      = errors.New("struct field not found")
 )
 
 type Option func(*Options)

util/reflect/struct.go

@@ -14,6 +14,84 @@ var ErrInvalidParam = errors.New("invalid url query param provided")
 
 var bracketSplitter = regexp.MustCompile(`\[|\]`)
 
+func StructFieldByTag(src interface{}, tkey string, tval string) (interface{}, error) {
+	sv := reflect.ValueOf(src)
+	if sv.Kind() == reflect.Ptr {
+		sv = sv.Elem()
+	}
+	if sv.Kind() != reflect.Struct {
+		return nil, ErrInvalidStruct
+	}
+
+	typ := sv.Type()
+	for idx := 0; idx < typ.NumField(); idx++ {
+		fld := typ.Field(idx)
+		val := sv.Field(idx)
+		if !val.CanSet() || len(fld.PkgPath) != 0 {
+			continue
+		}
+		switch val.Kind() {
+		default:
+			ts, ok := fld.Tag.Lookup(tkey)
+			if !ok {
+				continue
+			}
+			for _, p := range strings.Split(ts, ",") {
+				if p == tval {
+					return val.Interface(), nil
+				}
+			}
+		case reflect.Ptr:
+			if val = val.Elem(); val.Kind() == reflect.Struct {
+				if iface, err := StructFieldByTag(val.Interface(), tkey, tval); err == nil {
+					return iface, nil
+				}
+			}
+		case reflect.Struct:
+			if iface, err := StructFieldByTag(val.Interface(), tkey, tval); err == nil {
+				return iface, nil
+			}
+		}
+	}
+	return nil, ErrNotFound
+}
+
+func StructFieldByName(src interface{}, tkey string) (interface{}, error) {
+	sv := reflect.ValueOf(src)
+	if sv.Kind() == reflect.Ptr {
+		sv = sv.Elem()
+	}
+	if sv.Kind() != reflect.Struct {
+		return nil, ErrInvalidStruct
+	}
+
+	typ := sv.Type()
+	for idx := 0; idx < typ.NumField(); idx++ {
+		fld := typ.Field(idx)
+		val := sv.Field(idx)
+		if !val.CanSet() || len(fld.PkgPath) != 0 {
+			continue
+		}
+		switch val.Kind() {
+		default:
+			if fld.Name == tkey {
+				return val.Interface(), nil
+			}
+		case reflect.Ptr:
+			if val = val.Elem(); val.Kind() == reflect.Struct {
+				if iface, err := StructFieldByName(val.Interface(), tkey); err == nil {
+					return iface, nil
+				}
+			}
+		case reflect.Struct:
+			if iface, err := StructFieldByName(val.Interface(), tkey); err == nil {
+				return iface, nil
+			}
+		}
+	}
+	return nil, ErrNotFound
+}
+
 // StructFields returns slice of struct fields
 func StructFields(src interface{}) ([]reflect.StructField, error) {
 	var fields []reflect.StructField

util/reflect/struct_test.go

@@ -5,6 +5,44 @@ import (
 	"testing"
 )
 
+func TestStructByTag(t *testing.T) {
+	type Str struct {
+		Name []string `json:"name" codec:"flatten"`
+	}
+
+	val := &Str{Name: []string{"first", "second"}}
+
+	iface, err := StructFieldByTag(val, "codec", "flatten")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if v, ok := iface.([]string); !ok {
+		t.Fatalf("not []string %v", iface)
+	} else if len(v) != 2 {
+		t.Fatalf("invalid number %v", iface)
+	}
+}
+
+func TestStructByName(t *testing.T) {
+	type Str struct {
+		Name []string `json:"name" codec:"flatten"`
+	}
+
+	val := &Str{Name: []string{"first", "second"}}
+
+	iface, err := StructFieldByName(val, "Name")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if v, ok := iface.([]string); !ok {
+		t.Fatalf("not []string %v", iface)
+	} else if len(v) != 2 {
+		t.Fatalf("invalid number %v", iface)
+	}
+}
+
 func TestStructURLValues(t *testing.T) {
 	type Str struct {
 		Str  *Str   `json:"str"`