add in encoding hook

encode.go

@@ -29,6 +29,7 @@ type jsonNumber interface {
 type encoder struct {
 	emitter yaml_emitter_t
 	event   yaml_event_t
+	hook    func(in interface{}) (ok bool, out interface{}, err error)
 	out     []byte
 	flow    bool
 	// doneInit holds whether the initial stream_start_event has been
@@ -100,7 +101,22 @@ func (e *encoder) marshal(tag string, in reflect.Value) {
 		e.nilv()
 		return
 	}
+
 	iface := in.Interface()
+	if e.hook != nil {
+		ok, out, err := e.hook(iface)
+		if ok {
+			if err != nil {
+				fail(err)
+			}
+			if out == nil {
+				e.nilv()
+				return
+			}
+			iface = out
+		}
+	}
+
 	switch m := iface.(type) {
 	case jsonNumber:
 		integer, err := m.Int64()

encode_test.go

@@ -625,6 +625,38 @@ func (s *S) TestSortedOutput(c *C) {
 	}
 }
 
+type secret string
+
+func (s secret) MarshalYAML() (interface{}, error) {
+	return "<secret>", nil
+}
+
+type account struct {
+	Username string `yaml:"username"`
+	Password secret `yaml:"password"`
+}
+
+func (s *S) TestEncoderHook(c *C) {
+	user := account{Username: "admin", Password: secret("supersecret")}
+
+	var buf bytes.Buffer
+	e := yaml.NewEncoder(&buf)
+	e.SetHook(func(in interface{}) (ok bool, out interface{}, err error) {
+		switch v := in.(type) {
+		case secret:
+			return true, string(v), nil
+		default:
+			return false, nil, nil
+		}
+	})
+
+	err := e.Encode(user)
+	c.Assert(err, IsNil)
+
+	expect := "username: admin\npassword: supersecret\n"
+	c.Assert(string(buf.Bytes()), Equals, expect)
+}
+
 func newTime(t time.Time) *time.Time {
 	return &t
 }

yaml.go

@@ -220,6 +220,13 @@ func NewEncoder(w io.Writer) *Encoder {
 	}
 }
 
+// SetHook adds a marshaling hook into the Encoder. The function f will be
+// called for each node to be marshalled. f must return true if the hook
+// "handled" the node, in which case the output value will be used.
+func (e *Encoder) SetHook(hook func(in interface{}) (ok bool, out interface{}, err error)) {
+	e.encoder.hook = hook
+}
+
 // Encode writes the YAML encoding of v to the stream.
 // If multiple items are encoded to the stream, the
 // second and subsequent document will be preceded