Serve easily dynamic files with DataFromReader context method (#1304)

* Add DataFromReader context method

* Replace fmt by strconv.FormatInt

* Add pull request link to README

render/reader.go

@@ -0,0 +1,36 @@
+package render
+
+import (
+	"io"
+	"net/http"
+	"strconv"
+)
+
+type Reader struct {
+	ContentType   string
+	ContentLength int64
+	Reader        io.Reader
+	Headers       map[string]string
+}
+
+// Render (Reader) writes data with custom ContentType and headers.
+func (r Reader) Render(w http.ResponseWriter) (err error) {
+	r.WriteContentType(w)
+	r.Headers["Content-Length"] = strconv.FormatInt(r.ContentLength, 10)
+	r.writeHeaders(w, r.Headers)
+	_, err = io.Copy(w, r.Reader)
+	return
+}
+
+func (r Reader) WriteContentType(w http.ResponseWriter) {
+	writeContentType(w, []string{r.ContentType})
+}
+
+func (r Reader) writeHeaders(w http.ResponseWriter, headers map[string]string) {
+	header := w.Header()
+	for k, v := range headers {
+		if val := header[k]; len(val) == 0 {
+			header[k] = []string{v}
+		}
+	}
+}

render/render.go

@@ -25,6 +25,7 @@ var (
 	_ HTMLRender = HTMLProduction{}
 	_ Render     = YAML{}
 	_ Render     = MsgPack{}
+	_ Render     = Reader{}
 )
 
 func writeContentType(w http.ResponseWriter, value []string) {

render/render_test.go

@@ -11,6 +11,8 @@ import (
 	"html/template"
 	"net/http"
 	"net/http/httptest"
+	"strconv"
+	"strings"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -384,3 +386,24 @@ func TestRenderHTMLDebugPanics(t *testing.T) {
 	}
 	assert.Panics(t, func() { htmlRender.Instance("", nil) })
 }
+
+func TestRenderReader(t *testing.T) {
+	w := httptest.NewRecorder()
+
+	body := "#!PNG some raw data"
+	headers := make(map[string]string)
+	headers["Content-Disposition"] = `attachment; filename="filename.png"`
+
+	err := (Reader{
+		ContentLength: int64(len(body)),
+		ContentType:   "image/png",
+		Reader:        strings.NewReader(body),
+		Headers:       headers,
+	}).Render(w)
+
+	assert.NoError(t, err)
+	assert.Equal(t, body, w.Body.String())
+	assert.Equal(t, "image/png", w.HeaderMap.Get("Content-Type"))
+	assert.Equal(t, strconv.Itoa(len(body)), w.HeaderMap.Get("Content-Length"))
+	assert.Equal(t, headers["Content-Disposition"], w.HeaderMap.Get("Content-Disposition"))
+}