Add text framer interface

frame.go

@@ -84,6 +84,15 @@ func (f DataFrame) Bytes() []byte {
 	return f.data
 }
 
+// TextFramer represents frames that contain encoded text
+type TextFramer interface {
+	Framer
+	Encoding() string
+	SetEncoding(string) error
+	Text() string
+	SetText(string) error
+}
+
 // TextFrame represents frames that contain encoded text
 type TextFrame struct {
 	FrameHead

id3v2.go

@@ -197,12 +197,10 @@ func (t *Tag) SetGenre(text string) {
 	t.setTextFrameText(t.commonMap["Genre"], text)
 }
 
-func (t *Tag) textFrame(id string) *TextFrame {
+func (t *Tag) textFrame(id string) TextFramer {
 	if frame := t.Frame(id); frame != nil {
-		switch frame.(type) {
-		case (*TextFrame):
-			return frame.(*TextFrame)
-		default:
+		if textFramer, ok := frame.(TextFramer); ok {
+			return textFramer
 		}
 	}