added an example for a gomobile (gomobile build) implementation

README.md

@@ -22,7 +22,7 @@ The sdlcanvas and glfwcanvas subpackages provide a very simple way to get starte
 - Android
 - iOS
 
-Unfortunately using full Go apps using gomobile doesn't work since gomobile does not seem to create a GL view with a stencil buffer, and the canvas package makes heavy use of the stencil buffer. Therefore the ```gomobile bind``` command has to be used together with platform specific projects.
+Using gomobile to build a full Go app using ```gomobile build``` now works by using an offscreen texture to render to and then rendering that to the screen. See the example in examples/gomobile. The offscreen texture is necessary since gomobile automatically creates a GL context without a stencil buffer, which this library requires.
 
 # Example
 

examples/gomobile/gomobile.go

@@ -0,0 +1,59 @@
+package main
+
+import (
+	"math"
+	"time"
+
+	"github.com/tfriedel6/canvas"
+	"github.com/tfriedel6/canvas/glimpl/xmobile"
+	"golang.org/x/mobile/app"
+	"golang.org/x/mobile/event/lifecycle"
+	"golang.org/x/mobile/event/paint"
+	"golang.org/x/mobile/event/size"
+	"golang.org/x/mobile/gl"
+)
+
+func main() {
+	app.Main(func(a app.App) {
+		var cv, painter *canvas.Canvas
+		var w, h int
+
+		var glctx gl.Context
+		for e := range a.Events() {
+			switch e := a.Filter(e).(type) {
+			case lifecycle.Event:
+				switch e.Crosses(lifecycle.StageVisible) {
+				case lifecycle.CrossOn:
+					glctx, _ = e.DrawContext.(gl.Context)
+					canvas.LoadGL(glimplxmobile.New(glctx))
+					cv = canvas.NewOffscreen(0, 0)
+					painter = canvas.New(0, 0, 0, 0)
+					a.Send(paint.Event{})
+				case lifecycle.CrossOff:
+					glctx = nil
+				}
+			case size.Event:
+				w, h = e.WidthPx, e.HeightPx
+			case paint.Event:
+				if glctx != nil {
+					glctx.ClearColor(0, 0, 0, 0)
+					glctx.Clear(gl.COLOR_BUFFER_BIT)
+
+					cv.SetBounds(0, 0, w, h)
+					painter.SetBounds(0, 0, w, h)
+
+					fw, fh := float64(w), float64(h)
+					color := math.Sin(float64(time.Now().UnixNano())*0.000000002)*0.3 + 0.7
+
+					cv.SetFillStyle(color*0.2, color*0.2, color*0.8)
+					cv.FillRect(fw*0.25, fh*0.25, fw*0.5, fh*0.5)
+
+					painter.DrawImage(cv)
+
+					a.Publish()
+					a.Send(paint.Event{})
+				}
+			}
+		}
+	})
+}