Add io.Writer-based feed generation methods

The current ToXML method returns a string, meaning the content is
buffered into memory before being written to some output. With this
change, XML can be written directly to the output, which may make
a difference with large feeds.

This is a backwards compatible change, as methods are only added.

feed.go

@@ -2,6 +2,7 @@ package feeds
 
 import (
 	"encoding/xml"
+	"io"
 	"time"
 )
 
@@ -41,7 +42,7 @@ func (f *Feed) Add(item *Item) {
 	f.Items = append(f.Items, item)
 }
 
-// returns the first non-zero time formatted as a string or "" 
+// returns the first non-zero time formatted as a string or ""
 func anyTimeFormat(format string, times ...time.Time) string {
 	for _, t := range times {
 		if !t.IsZero() {
@@ -69,14 +70,37 @@ func ToXML(feed XmlFeed) (string, error) {
 	return s, nil
 }
 
+// Write a feed object (either a Feed, AtomFeed, or RssFeed) as XML into
+// the writer. Returns an error if XML marshaling fails.
+func WriteXML(feed XmlFeed, w io.Writer) error {
+	x := feed.FeedXml()
+	// write default xml header, without the newline
+	if _, err := w.Write([]byte(xml.Header[:len(xml.Header)-1])); err != nil {
+		return err
+	}
+	e := xml.NewEncoder(w)
+	e.Indent("", "  ")
+	return e.Encode(x)
+}
+
 // creates an Atom representation of this feed
 func (f *Feed) ToAtom() (string, error) {
 	a := &Atom{f}
 	return ToXML(a)
 }
 
+// Writes an Atom representation of this feed to the writer.
+func (f *Feed) WriteAtom(w io.Writer) error {
+	return WriteXML(&Atom{f}, w)
+}
+
 // creates an Rss representation of this feed
 func (f *Feed) ToRss() (string, error) {
 	r := &Rss{f}
 	return ToXML(r)
 }
+
+// Writes an RSS representation of this feed to the writer.
+func (f *Feed) WriteRss(w io.Writer) error {
+	return WriteXML(&Rss{f}, w)
+}

feed_test.go

@@ -1,6 +1,7 @@
 package feeds
 
 import (
+	"bytes"
 	"testing"
 	"time"
 )
@@ -112,14 +113,33 @@ func TestFeed(t *testing.T) {
 		},
 	}
 
-	atom, _ := feed.ToAtom()
-	rss, _ := feed.ToRss()
-
+	atom, err := feed.ToAtom()
+	if err != nil {
+		t.Error("unexpected error encoding Atom: %v", err)
+	}
 	if atom != atomOutput {
 		t.Errorf("Atom not what was expected.  Got:\n%s\n\nExpected:\n%s\n", atom, atomOutput)
 	}
+	var buf bytes.Buffer
+	if err := feed.WriteAtom(&buf); err != nil {
+		t.Errorf("unexpected error writing Atom: %v", err)
+	}
+	if got := buf.String(); got != atomOutput {
+		t.Errorf("Atom not what was expected.  Got:\n%s\n\nExpected:\n%s\n", got, rssOutput)
+	}
 
+	rss, err := feed.ToRss()
+	if err != nil {
+		t.Errorf("unexpected error encoding RSS: %v", err)
+	}
 	if rss != rssOutput {
 		t.Errorf("Rss not what was expected.  Got:\n%s\n\nExpected:\n%s\n", rss, rssOutput)
 	}
+	buf.Reset()
+	if err := feed.WriteRss(&buf); err != nil {
+		t.Errorf("unexpected error writing RSS: %v", err)
+	}
+	if got := buf.String(); got != rssOutput {
+		t.Errorf("Rss not what was expected.  Got:\n%s\n\nExpected:\n%s\n", got, rssOutput)
+	}
 }