examples/mobile: Add playing sound

examples/mobile/mobile/update.go

@@ -15,11 +15,13 @@
 package mobile
 
 import (
+	"errors"
 	"fmt"
 	"log"
 	"math"
 
 	"github.com/hajimehoshi/ebiten"
+	"github.com/hajimehoshi/ebiten/audio"
 	"github.com/hajimehoshi/ebiten/ebitenutil"
 	"github.com/hajimehoshi/ebiten/examples/common"
 )
@@ -42,7 +44,83 @@ func init() {
 	}
 }
 
+const (
+	screenWidth  = 320
+	screenHeight = 240
+	sampleRate   = 44100
+	frequency    = 440
+)
+
+var audioContext *audio.Context
+
+func init() {
+	var err error
+	// Let's lazy context creation!
+	audioContext, err = audio.NewContext(sampleRate)
+	if err != nil {
+		log.Fatal(err)
+	}
+}
+
+type stream struct {
+	position int64
+}
+
+func (s *stream) Read(data []byte) (int, error) {
+	if len(data)%4 != 0 {
+		return 0, errors.New("len(data) % 4 must be 0")
+	}
+	const length = sampleRate / frequency // TODO: This should be integer?
+	p := s.position / 4
+	for i := 0; i < len(data)/4; i++ {
+		const max = (1<<15 - 1) / 2
+		b := int16(math.Sin(2*math.Pi*float64(p)/length) * max)
+		data[4*i] = byte(b)
+		data[4*i+1] = byte(b >> 8)
+		data[4*i+2] = byte(b)
+		data[4*i+3] = byte(b >> 8)
+		p++
+	}
+	s.position += int64(len(data))
+	s.position %= length * 4
+	return len(data), nil
+}
+
+func (s *stream) Seek(offset int64, whence int) (int64, error) {
+	const length = sampleRate / frequency
+	switch whence {
+	case 0:
+		s.position = offset
+	case 1:
+		s.position += offset
+	case 2:
+		return 0, errors.New("whence must be 0 or 1")
+	}
+	s.position %= length * 4
+	return s.position, nil
+}
+
+func (s *stream) Close() error {
+	return nil
+}
+
+var player *audio.Player
+
 func Update(screen *ebiten.Image) error {
+	if player == nil {
+		var err error
+		player, err = audio.NewPlayer(audioContext, &stream{})
+		if err != nil {
+			return err
+		}
+		if err := player.Play(); err != nil {
+			return err
+		}
+	}
+	if err := audioContext.Update(); err != nil {
+		return err
+	}
+
 	count++
 	w, h := gophersImage.Size()
 	op := &ebiten.DrawImageOptions{}