encoding/xml: add (*Encoder).Indent

Exposing this on the Encoder allows streaming generation of indented XML.

R=golang-dev, rogpeppe
CC=golang-dev
https://golang.org/cl/7221075

src/pkg/encoding/xml/example_test.go

@@ -50,6 +50,46 @@ func ExampleMarshalIndent() {
 	//   </person>
 }
 
+func ExampleEncoder() {
+	type Address struct {
+		City, State string
+	}
+	type Person struct {
+		XMLName   xml.Name `xml:"person"`
+		Id        int      `xml:"id,attr"`
+		FirstName string   `xml:"name>first"`
+		LastName  string   `xml:"name>last"`
+		Age       int      `xml:"age"`
+		Height    float32  `xml:"height,omitempty"`
+		Married   bool
+		Address
+		Comment string `xml:",comment"`
+	}
+
+	v := &Person{Id: 13, FirstName: "John", LastName: "Doe", Age: 42}
+	v.Comment = " Need more details. "
+	v.Address = Address{"Hanga Roa", "Easter Island"}
+
+	enc := xml.NewEncoder(os.Stdout)
+	enc.Indent("  ", "    ")
+	if err := enc.Encode(v); err != nil {
+		fmt.Printf("error: %v\n", err)
+	}
+
+	// Output:
+	//   <person id="13">
+	//       <name>
+	//           <first>John</first>
+	//           <last>Doe</last>
+	//       </name>
+	//       <age>42</age>
+	//       <Married>false</Married>
+	//       <City>Hanga Roa</City>
+	//       <State>Easter Island</State>
+	//       <!-- Need more details. -->
+	//   </person>
+}
+
 // This example demonstrates unmarshaling an XML excerpt into a value with
 // some preset fields. Note that the Phone field isn't modified and that
 // the XML <Company> element is ignored. Also, the Groups field is assigned

src/pkg/encoding/xml/marshal.go

@@ -81,8 +81,7 @@ func Marshal(v interface{}) ([]byte, error) {
 func MarshalIndent(v interface{}, prefix, indent string) ([]byte, error) {
 	var b bytes.Buffer
 	enc := NewEncoder(&b)
-	enc.prefix = prefix
-	enc.indent = indent
+	enc.Indent(prefix, indent)
 	if err := enc.Encode(v); err != nil {
 		return nil, err
 	}
@@ -99,6 +98,14 @@ func NewEncoder(w io.Writer) *Encoder {
 	return &Encoder{printer{Writer: bufio.NewWriter(w)}}
 }
 
+// Indent sets the encoder to generate XML in which each element
+// begins on a new indented line that starts with prefix and is followed by
+// one or more copies of indent according to the nesting depth.
+func (enc *Encoder) Indent(prefix, indent string) {
+	enc.prefix = prefix
+	enc.indent = indent
+}
+
 // Encode writes the XML encoding of v to the stream.
 //
 // See the documentation for Marshal for details about the conversion