add optional DownloadCallback when calling ToFile

resp.go

@@ -97,8 +97,8 @@ func (r *Resp) ToXML(v interface{}) error {
 	return xml.Unmarshal(data, v)
 }
 
-// ToFile download the response body to file
-func (r *Resp) ToFile(name string) error {
+// ToFile download the response body to file with optional download callback
+func (r *Resp) ToFile(name string, callback ...DownloadCallback) error {
 	file, err := os.Create(name)
 	if err != nil {
 		return err
@@ -110,10 +110,42 @@ func (r *Resp) ToFile(name string) error {
 		return err
 	}
 
+	if len(callback) > 0 && r.resp.ContentLength > 0 {
+		return r.download(file, r.resp.ContentLength, callback...)
+	}
+
 	_, err = io.Copy(file, r.resp.Body)
 	return err
 }
 
+func (r *Resp) download(file *os.File, totalLength int64, callback ...DownloadCallback) error {
+	p := make([]byte, 1024)
+	b := r.resp.Body
+	var currentLength int64
+	for {
+		l, err := b.Read(p)
+		if l > 0 {
+			_, _err := file.Write(p[:l])
+			if _err != nil {
+				return _err
+			}
+			currentLength += int64(l)
+			for _, cb := range callback {
+				cb.OnProgress(currentLength, totalLength)
+			}
+		}
+		if err != nil {
+			if err == io.EOF {
+				for _, cb := range callback {
+					cb.OnComplete(file)
+				}
+				return nil
+			}
+			return err
+		}
+	}
+}
+
 var regNewline = regexp.MustCompile(`\n|\r`)
 
 func (r *Resp) autoFormat(s fmt.State) {
@@ -177,5 +209,10 @@ func (r *Resp) Format(s fmt.State, verb rune) {
 	} else { // auto
 		r.autoFormat(s)
 	}
+}
 
+// DownloadCallback is the callback when downloading
+type DownloadCallback interface {
+	OnComplete(file *os.File)
+	OnProgress(currentLength int64, totalLength int64)
 }