Add template Value() for JSONs

json.go

@@ -26,8 +26,9 @@ func (fv *JSON) String() string {
 	return fv.Text
 }
 
-// JSONs is a `flag.Value` for JSON arguments.
+// JSONs is a `flag.Value` for JSON arguments. If non-nil, the `Value` field is used to generate template values.
 type JSONs struct {
+	Value  func() interface{}
 	Values []interface{}
 	Texts  []string
 }
@@ -38,9 +39,14 @@ func (fv *JSONs) Help() string {
 }
 
 // Set is flag.Value.Set
-func (fv *JSONs) Set(v string) error {
+func (fv *JSONs) Set(v string) (err error) {
 	var value interface{}
-	err := json.Unmarshal([]byte(v), &value)
+	if fv.Value != nil {
+		value = fv.Value()
+		err = json.Unmarshal([]byte(v), value)
+	} else {
+		err = json.Unmarshal([]byte(v), &value)
+	}
 	if err == nil {
 		fv.Texts = append(fv.Texts, v)
 		fv.Values = append(fv.Values, value)

json_test.go

@@ -47,6 +47,28 @@ func TestJSONs(t *testing.T) {
 	}
 }
 
+func TestJSONsValue(t *testing.T) {
+	type example struct {
+		A string
+		B int
+	}
+	fv := flagvar.JSONs{
+		Value: func() interface{} {
+			return &example{}
+		},
+	}
+	var fs flag.FlagSet
+	fs.Var(&fv, "json", "")
+
+	err := fs.Parse([]string{"-json", `{"A":"abc","B":123}`})
+	if err != nil {
+		t.Fail()
+	}
+	if !reflect.DeepEqual(fv.Values, []interface{}{&example{A: "abc", B: 123}}) {
+		t.Fail()
+	}
+}
+
 func TestJSONsFail(t *testing.T) {
 	fv := flagvar.JSONs{}
 	var fs flag.FlagSet