audio: Add AudioContext.Update (#177)

examples/audio/main.go

@@ -35,6 +35,7 @@ var (
 )
 
 func update(screen *ebiten.Image) error {
+	audioContext.Update()
 	if !audioLoaded {
 		select {
 		case <-audioLoadingDone:

examples/pcm/main.go

@@ -131,6 +131,7 @@ func addNote() error {
 }
 
 func update(screen *ebiten.Image) error {
+	audioContext.Update()
 	defer func() {
 		frames++
 	}()

examples/piano/main.go

@@ -201,6 +201,7 @@ func init() {
 }
 
 func update(screen *ebiten.Image) error {
+	audioContext.Update()
 	updateInput()
 	for i, key := range keys {
 		if keyStates[key] != 1 {

examples/sinewave/main.go

@@ -79,6 +79,7 @@ func (s *stream) Seek(offset int64, whence int) (int64, error) {
 var player *audio.Player
 
 func update(screen *ebiten.Image) error {
+	audioContext.Update()
 	if player == nil {
 		var err error
 		player, err = audioContext.NewPlayer(&stream{})

exp/audio/audio.go

@@ -24,6 +24,7 @@ import (
 // TODO: In JavaScript, mixing should be done by WebAudio for performance.
 type mixedPlayersStream struct {
 	context      *Context
+	writtenBytes int
 }
 
 func min(a, b int) int {
@@ -37,9 +38,19 @@ func (s *mixedPlayersStream) Read(b []byte) (int, error) {
 	s.context.Lock()
 	defer s.context.Unlock()
 
+	// TODO: 60 (FPS) is a magic number
+	bytesPerFrame := s.context.sampleRate * 4 / 60
+	x := s.context.frames*bytesPerFrame + len(b)
+	if x <= s.writtenBytes {
+		return 0, nil
+	}
+
 	l := len(b) / 4 * 4
 	if len(s.context.players) == 0 {
-		return 0, nil
+		l := min(len(b), x-s.writtenBytes)
+		copy(b, make([]byte, l))
+		s.writtenBytes += l
+		return l, nil
 	}
 	closed := []*Player{}
 	ll := l
@@ -76,6 +87,7 @@ func (s *mixedPlayersStream) Read(b []byte) (int, error) {
 	for _, p := range closed {
 		delete(s.context.players, p)
 	}
+	s.writtenBytes += ll
 	return ll, nil
 }
 
@@ -86,6 +98,7 @@ type Context struct {
 	stream      *mixedPlayersStream
 	players     map[*Player]struct{}
 	innerPlayer *player
+	frames      int
 	sync.Mutex
 }
 
@@ -95,7 +108,9 @@ func NewContext(sampleRate int) *Context {
 		sampleRate: sampleRate,
 		players:    map[*Player]struct{}{},
 	}
-	c.stream = &mixedPlayersStream{c}
+	c.stream = &mixedPlayersStream{
+		context: c,
+	}
 	p, err := startPlaying(c.stream, c.sampleRate)
 	if err != nil {
 		panic(fmt.Sprintf("audio: NewContext error: %v", err))
@@ -104,6 +119,18 @@ func NewContext(sampleRate int) *Context {
 	return c
 }
 
+// Update proceeds the inner (logical) time of the context by 1/60 second.
+// This is expected to be called in the game's updating function (sync mode)
+// or an independent goroutine with timers (unsync mode).
+// In sync mode, the game logical time syncs the audio logical time and
+// you will find audio stops when the game stops e.g. when the window is deactivated.
+// In unsync mode, the audio never stops even when the game stops.
+func (c *Context) Update() {
+	c.Lock()
+	defer c.Unlock()
+	c.frames++
+}
+
 type Player struct {
 	context *Context
 	src     io.ReadSeeker

exp/audio/audio_js.go

@@ -79,11 +79,6 @@ func (p *player) proceed() error {
 	const bytesPerSample = channelNum * 16 / 8
 	bufferSize := p.sampleRate * bytesPerSample / 60
 	c := int64(p.context.Get("currentTime").Float() * float64(p.sampleRate))
-	// Buffer size is relatively big and it is needed to check that c.positionInSample doesn't
-	// proceed too far away (#180).
-	if c+int64(bufferSize) < p.positionInSamples {
-		return nil
-	}
 	if p.positionInSamples < c {
 		p.positionInSamples = c
 	}

exp/audio/vorbis.go

@@ -30,6 +30,8 @@ type VorbisStream struct {
 	buf *bytes.Reader
 }
 
+// TODO: Rename to DecodeVorbis or Decode?
+
 func (c *Context) NewVorbisStream(src io.Reader) (*VorbisStream, error) {
 	decoded, channels, sampleRate, err := vorbis.Decode(src)
 	if err != nil {