2 files changed, 79 insertions, 0 deletions
diff --git a/go/dummy-ui/main.go b/go/dummy-ui/main.go
index a3e90f48..fd936466 100644
--- a/go/dummy-ui/main.go
+++ b/go/dummy-ui/main.go
@@ -34,6 +34,8 @@ func (a *app) build() {
 	a.AddWidget(tI)
 
 	a.AddWidget(ui.NewTimerBar(0, 10, a.windowWidth, 5, 30*time.Second))
+
+	a.AddWidget(ui.NewSpinner(100, 400, 50))
 }
 
 func (a *app) Layout(width, height int) (int, int) {
diff --git a/go/ui/spinner.go b/go/ui/spinner.go
new file mode 100644
index 00000000..083ec292
--- /dev/null
+++ b/go/ui/spinner.go
@@ -0,0 +1,77 @@
+package ui
+
+import (
+	"image/color"
+	"math"
+
+	"github.com/hajimehoshi/ebiten/v2"
+	"github.com/hajimehoshi/ebiten/v2/vector"
+)
+
+var (
+	SPINNER_DEFAULT_FOREGROUND = color.White
+	SPINNER_DEFAULT_STROKE     = 5
+)
+
+type Spinner struct {
+	WidgetBase
+	fg    color.Color
+	alpha uint64
+}
+
+func NewSpinner(x, y int, diameter int) *Spinner {
+	s := &Spinner{
+		WidgetBase: NewWidgetBase(x, y, diameter, diameter),
+		fg:         SPINNER_DEFAULT_FOREGROUND,
+	}
+	s.renderImpl = func() *ebiten.Image { return s.render() }
+	s.EventHandlersMap["update"] = func() error {
+		s.alpha = s.alpha + 3
+		s.ForceRedraw()
+		return nil
+	}
+	return s
+}
+
+func (s *Spinner) Fg(fg color.Color) *Spinner {
+	s.fg = fg
+	return s
+}
+
+func (s *Spinner) render() *ebiten.Image {
+	img := ebiten.NewImage(s.Width, s.Height)
+	var path vector.Path
+
+	r := float64((s.Width - 2*SPINNER_DEFAULT_STROKE) / 2)
+	cx := s.Width/2 + SPINNER_DEFAULT_STROKE
+	cy := cx
+	a1 := float64(s.alpha%360) * math.Pi / 180
+	a2 := float64((s.alpha+90)%360) * math.Pi / 180
+
+	path.Arc(float32(cx), float32(cy), float32(r), float32(a2), float32(a1), vector.Clockwise)
+
+	strokeOp := &vector.StrokeOptions{}
+	strokeOp.Width = float32(SPINNER_DEFAULT_STROKE)
+	vertices := []ebiten.Vertex{}
+	indices := []uint16{}
+	vertices, indices = path.AppendVerticesAndIndicesForStroke(vertices, indices, strokeOp)
+
+	{
+		r, g, b, a := s.fg.RGBA()
+		for i := range vertices {
+			vertices[i].SrcX = 1
+			vertices[i].SrcY = 1
+			vertices[i].ColorR = float32(r)
+			vertices[i].ColorG = float32(g)
+			vertices[i].ColorB = float32(b)
+			vertices[i].ColorA = float32(a)
+		}
+	}
+
+	op := &ebiten.DrawTrianglesOptions{}
+	op.AntiAlias = true
+	op.FillRule = ebiten.FillRuleNonZero
+	op.ColorScaleMode = ebiten.ColorScaleModePremultipliedAlpha
+	img.DrawTriangles(vertices, indices, whiteSubImage, op)
+	return img
+}