cli: fixed spinner glitches on fast execution

cmd/triangle/main.go

@@ -54,7 +54,10 @@ const (
 	TriangleMessage
 )
 
-var imgurl *os.File
+var (
+	imgurl  *os.File
+	spinner *utils.Spinner
+)
 
 // version indicates the current build version.
 var version string
@@ -104,6 +107,12 @@ func main() {
 		BgColor:         *bgColor,
 	}
 
+	spinnerText := fmt.Sprintf("%s %s",
+		decorateText("▲ TRIANGLE", TriangleMessage),
+		decorateText("is generating the triangulated image...", DefaultMessage))
+
+	spinner = utils.NewSpinner(spinnerText, time.Millisecond*80, true)
+
 	// Supported input image file types.
 	srcExts := []string{".jpg", ".jpeg", ".png"}
 
@@ -331,12 +340,7 @@ func processor(in, out string, proc *triangle.Processor, fn func()) (
 	defer src.(*os.File).Close()
 	defer dst.(*os.File).Close()
 
-	spinnerText := fmt.Sprintf("%s %s",
-		decorateText("▲ TRIANGLE", TriangleMessage),
-		decorateText("is generating the triangulated image...", DefaultMessage))
-
-	s := utils.NewSpinner(spinnerText, time.Millisecond*100, true)
-	s.Start()
+	spinner.Start()
 
 	if filepath.Ext(out) == ".svg" {
 		svg := &triangle.SVG{
@@ -354,10 +358,12 @@ func processor(in, out string, proc *triangle.Processor, fn func()) (
 		}
 		_, triangles, points, err = tri.Draw(src, dst, fn)
 	}
-	s.Stop()
-	fmt.Fprintf(os.Stderr, fmt.Sprintf("%s %s",
+	stopMsg := fmt.Sprintf("%s %s",
 		decorateText("▲ TRIANGLE", TriangleMessage),
-		decorateText("is generating the triangulated image... ✔", SuccessMessage)))
+		decorateText("is generating the triangulated image... ✔", SuccessMessage))
+	spinner.StopMsg = stopMsg
+
+	spinner.Stop()
 
 	return triangles, points, err
 }

utils/spinner.go

@@ -18,6 +18,7 @@ type Spinner struct {
 	writer     io.Writer
 	message    string
 	lastOutput string
+	StopMsg    string
 	hideCursor bool
 	stopChan   chan struct{}
 }
@@ -49,7 +50,6 @@ func (s *Spinner) Start() {
 					return
 				default:
 					s.mu.Lock()
-					s.erase(s.lastOutput) // clear the last line
 
 					output := fmt.Sprintf("\r%s%s %c%s", s.message, SuccessColor, r, DefaultColor)
 					fmt.Fprintf(s.writer, output)
@@ -71,21 +71,26 @@ func (s *Spinner) Stop() {
 		// makes the cursor visible
 		fmt.Fprint(s.writer, "\033[?25h")
 	}
-	s.erase(s.lastOutput)
+	s.erase()
+	if len(s.StopMsg) > 0 {
+		fmt.Fprintf(s.writer, s.StopMsg)
+	}
 	s.stopChan <- struct{}{}
 }
 
 // erase deletes the last terminal line.
 // Caller must hold the the locker.
-func (s *Spinner) erase(lastOutput string) {
-	n := utf8.RuneCountInString(lastOutput)
+func (s *Spinner) erase() {
+	n := utf8.RuneCountInString(s.lastOutput)
 	if runtime.GOOS == "windows" {
 		clearString := "\r" + strings.Repeat(" ", n) + "\r"
 		fmt.Fprint(s.writer, clearString)
+		s.lastOutput = ""
 		return
 	}
 	for _, c := range []string{"\b", "\127", "\b", "\033[K"} { // "\033[K" for macOS Terminal
 		fmt.Fprint(s.writer, strings.Repeat(c, n))
 	}
 	fmt.Fprintf(s.writer, "\r\033[K") // erases to end of line
+	s.lastOutput = ""
 }