Add text drawing utility

exp/util/drawText.go

@@ -0,0 +1,91 @@
+package util
+
+import (
+	"image"
+	"image/color"
+	"image/draw"
+	"strings"
+
+	"golang.org/x/image/font"
+	"golang.org/x/image/font/basicfont"
+	"golang.org/x/image/math/fixed"
+
+	"golang.org/x/mobile/event/size"
+	"golang.org/x/mobile/exp/gl/glutil"
+	"golang.org/x/mobile/geom"
+)
+
+func drawText(img *image.RGBA, x, y int, s string) {
+	col := color.RGBA{0, 0, 0, 255}
+	point := fixed.Point26_6{fixed.I(x), fixed.I(y)}
+
+	d := &font.Drawer{
+		Dst:  img,
+		Src:  image.NewUniform(col),
+		Face: basicfont.Face7x13,
+		Dot:  point,
+	}
+	d.DrawString(s)
+}
+
+// Text draws text to screen
+type Text struct {
+	sz     size.Event
+	images *glutil.Images
+	m      *glutil.Image
+	// TODO: store *gl.Context
+}
+
+// NewText creates a Text tied to the current GL context.
+func NewText(images *glutil.Images) *Text {
+	return &Text{
+		images: images,
+	}
+}
+
+// Draw draws text at the x, y coordinates at the scaleX, scaleY specified by user
+// Draw accepts strings with newline characters
+func (t *Text) Draw(sz size.Event, x, y int, scaleX, scaleY geom.Pt, s string) {
+	if sz.WidthPx == 0 && sz.HeightPx == 0 {
+		return
+	} else if t.sz != sz {
+		t.sz = sz
+		if t.m != nil {
+			t.m.Release()
+		}
+		t.m = t.images.NewImage(sz.WidthPx, sz.HeightPx)
+	}
+
+	// split string by newline
+	lines := strings.Split(s, "\n")
+
+	// draw each string on a separate line
+	for i, v := range lines {
+		drawText(t.m.RGBA, int(geom.Pt(x)/scaleX), int(geom.Pt(y)/scaleY)+i*10, v)
+	}
+
+	// copy img data to GL device
+	t.m.Upload()
+
+	t.m.Draw(
+		sz,
+		geom.Point{0, 0},                    // topLeft
+		geom.Point{sz.WidthPt * scaleX, 0},  // topRight
+		geom.Point{0, sz.HeightPt * scaleY}, // bottomLeft
+		t.m.RGBA.Bounds(),
+	)
+}
+
+func (t *Text) Clear() {
+	if t.m != nil {
+		draw.Draw(t.m.RGBA, t.m.RGBA.Bounds(), image.Transparent, image.Point{}, draw.Src)
+	}
+}
+
+func (t *Text) Release() {
+	if t.m != nil {
+		t.m.Release()
+		t.m = nil
+		t.images = nil
+	}
+}