Add new method Encode to Viper that allows loading rawVal into config and merging it with the current config map

viper.go

@@ -1704,6 +1704,16 @@ func (v *Viper) SafeWriteConfigAs(filename string) error {
 	return v.writeConfig(filename, false)
 }
 
+// Encode will load the rawVal into config, then you can write to file
+func (v *Viper) Encode(rawVal interface{}, opts ...DecoderConfigOption) error {
+	var cfg map[string]interface{}
+	err := decode(rawVal, defaultDecoderConfig(&cfg, opts...))
+	if err != nil {
+		return err
+	}
+	return v.MergeConfigMap(cfg)
+}
+
 // Marshal will return a Buffer containing the content that should have been written to the file
 func (v *Viper) Marshal() (*bytes.Buffer, error) {
 	data, err := v.encoderRegistry.Encode(v.getConfigType(), v.AllSettings())

viper_test.go

@@ -1979,6 +1979,18 @@ func TestSafeWriteConfigAsWithExistingFile(t *testing.T) {
 	assert.True(t, ok, "Expected ConfigFileAlreadyExistsError")
 }
 
+func TestEncode(t *testing.T) {
+	type config struct {
+		Age   int
+		Beard bool
+	}
+	C := &config{35, true}
+	v := New()
+	require.NoError(t, v.Encode(C))
+	expected := map[string]interface{}{"age": 35, "beard": true}
+	assert.Equal(t, v.config, expected)
+}
+
 func TestMarshal(t *testing.T) {
 	v := New()
 	v.SetConfigType("yaml")
@@ -1988,6 +2000,20 @@ func TestMarshal(t *testing.T) {
 	assert.Equal(t, c.Bytes(), yamlWriteExpected)
 }
 
+func TestEncodeAndMarshal(t *testing.T) {
+	type config struct {
+		Age   int
+		Beard bool
+	}
+	C := &config{35, true}
+	v := New()
+	require.NoError(t, v.Encode(C))
+	v.SetConfigType("yaml")
+	c, err := v.Marshal()
+	require.NoError(t, err)
+	assert.Equal(t, c.Bytes(), yamlEncodeWriteExpected)
+}
+
 func TestWriteHiddenFile(t *testing.T) {
 	v := New()
 	fs := afero.NewMemMapFs()

viper_yaml_test.go

@@ -51,3 +51,7 @@ emails:
         created: 01/02/03
         active: true
 `)
+
+var yamlEncodeWriteExpected = []byte(`age: 35
+beard: true
+`)