audio: More intellegent suspending/resuming (#617)

Before this change, the audio is suspended when the game stops for
1/12[s]. However, as game often stops for more than 1/12[s]
especially on mobiles, this implemntation caused some audio
glitches.

This change fixes this problem by re-implementing suspending/
resumeing audio by detecting the window is active/focused or not.

audio/audio.go

@@ -151,9 +151,6 @@ func (p *players) hasSource(src io.ReadCloser) bool {
 // For a typical usage example, see examples/wav/main.go.
 type Context struct {
 	players    *players
-	initCh     chan struct{}
-	initedCh   chan struct{}
-	pingCount  int
 	sampleRate int
 	err        error
 
@@ -218,26 +215,20 @@ func CurrentContext() *Context {
 	return c
 }
 
-func (c *Context) ping() {
-	if c.initCh != nil {
-		close(c.initCh)
-		c.initCh = nil
-	}
-	<-c.initedCh
-
-	c.m.Lock()
-	c.pingCount = 5
-	c.m.Unlock()
-}
-
 func (c *Context) loop() {
-	c.initCh = make(chan struct{})
-	c.initedCh = make(chan struct{})
+	initCh := make(chan struct{})
 
-	// Copy the channel since c.initCh can be set as nil after clock.RegisterPing.
-	initCh := c.initCh
-
-	clock.RegisterPing(c.ping)
+	suspendCh := make(chan struct{}, 1)
+	resumeCh := make(chan struct{}, 1)
+	hooks.OnSuspendAudio(func() {
+		suspendCh <- struct{}{}
+	})
+	hooks.OnResumeAudio(func() {
+		resumeCh <- struct{}{}
+	})
+	clock.OnStart(func() {
+		close(initCh)
+	})
 
 	// Initialize oto.Player lazily to enable calling NewContext in an 'init' function.
 	// Accessing oto.Player functions requires the environment to be already initialized,
@@ -255,21 +246,14 @@ func (c *Context) loop() {
 	}
 	defer p.Close()
 
-	close(c.initedCh)
-
 	bytesPerFrame := c.sampleRate * bytesPerSample * channelNum / clock.FPS
 	written := int64(0)
 	prevWritten := int64(0)
 	for {
-		c.m.Lock()
-		if c.pingCount == 0 {
-			c.m.Unlock()
-			time.Sleep(10 * time.Millisecond)
-			continue
-		}
-		c.pingCount--
-		c.m.Unlock()
-
+		select {
+		case <-suspendCh:
+			<-resumeCh
+		default:
 			const n = 2048
 			if _, err := io.CopyN(p, c.players, n); err != nil {
 				c.err = err
@@ -281,6 +265,7 @@ func (c *Context) loop() {
 			clock.ProceedAudioTimer(fs)
 			prevWritten = written
 		}
+	}
 }
 
 // Update is deprecated as of 1.6.0-alpha.

internal/clock/clock.go

@@ -43,7 +43,8 @@ var (
 	lastFPSUpdated int64
 	framesForFPS   int64
 
-	ping func()
+	started bool
+	onStart func()
 
 	m sync.Mutex
 )
@@ -55,9 +56,9 @@ func CurrentFPS() float64 {
 	return v
 }
 
-func RegisterPing(pingFunc func()) {
+func OnStart(f func()) {
 	m.Lock()
-	ping = pingFunc
+	onStart = f
 	m.Unlock()
 }
 
@@ -87,11 +88,14 @@ func Update() int {
 	m.Lock()
 	defer m.Unlock()
 
-	n := now()
-
-	if ping != nil {
-		ping()
+	if !started {
+		if onStart != nil {
+			onStart()
 		}
+		started = true
+	}
+
+	n := now()
 
 	// Initialize lastSystemTime if needed.
 	if lastSystemTime == 0 {

internal/hooks/hooks.go

@@ -14,15 +14,26 @@
 
 package hooks
 
+import (
+	"sync"
+)
+
+var m sync.Mutex
+
 var onBeforeUpdateHooks = []func() error{}
 
 // AppendHookOnBeforeUpdate appends a hook function that is run before the main update function
 // every frame.
 func AppendHookOnBeforeUpdate(f func() error) {
+	m.Lock()
 	onBeforeUpdateHooks = append(onBeforeUpdateHooks, f)
+	m.Unlock()
 }
 
 func RunBeforeUpdateHooks() error {
+	m.Lock()
+	defer m.Unlock()
+
 	for _, f := range onBeforeUpdateHooks {
 		if err := f(); err != nil {
 			return err
@@ -30,3 +41,45 @@ func RunBeforeUpdateHooks() error {
 	}
 	return nil
 }
+
+var (
+	audioSuspended bool
+	onSuspendAudio func()
+	onResumeAudio  func()
+)
+
+func OnSuspendAudio(f func()) {
+	m.Lock()
+	onSuspendAudio = f
+	m.Unlock()
+}
+
+func OnResumeAudio(f func()) {
+	m.Lock()
+	onResumeAudio = f
+	m.Unlock()
+}
+
+func SuspendAudio() {
+	m.Lock()
+	defer m.Unlock()
+	if audioSuspended {
+		return
+	}
+	audioSuspended = true
+	if onSuspendAudio != nil {
+		onSuspendAudio()
+	}
+}
+
+func ResumeAudio() {
+	m.Lock()
+	defer m.Unlock()
+	if !audioSuspended {
+		return
+	}
+	audioSuspended = false
+	if onResumeAudio != nil {
+		onResumeAudio()
+	}
+}

internal/ui/ui_glfw.go

@@ -29,6 +29,7 @@ import (
 	"github.com/go-gl/glfw/v3.2/glfw"
 
 	"github.com/hajimehoshi/ebiten/internal/devicescale"
+	"github.com/hajimehoshi/ebiten/internal/hooks"
 	"github.com/hajimehoshi/ebiten/internal/input"
 	"github.com/hajimehoshi/ebiten/internal/opengl"
 )
@@ -552,7 +553,9 @@ func (u *userInterface) update(g GraphicsContext) error {
 
 	_ = u.runOnMainThread(func() error {
 		u.pollEvents()
+		defer hooks.ResumeAudio()
 		for !u.isRunnableInBackground() && u.window.GetAttrib(glfw.Focused) == 0 {
+			hooks.SuspendAudio()
 			// Wait for an arbitrary period to avoid busy loop.
 			time.Sleep(time.Second / 60)
 			u.pollEvents()

internal/ui/ui_js.go

@@ -23,6 +23,7 @@ import (
 	"github.com/gopherjs/gopherjs/js"
 
 	"github.com/hajimehoshi/ebiten/internal/devicescale"
+	"github.com/hajimehoshi/ebiten/internal/hooks"
 	"github.com/hajimehoshi/ebiten/internal/input"
 	"github.com/hajimehoshi/ebiten/internal/opengl"
 	"github.com/hajimehoshi/ebiten/internal/web"
@@ -39,11 +40,13 @@ type userInterface struct {
 
 	sizeChanged bool
 	windowFocus bool
+	pageVisible bool
 }
 
 var currentUI = &userInterface{
 	sizeChanged: true,
 	windowFocus: true,
+	pageVisible: true,
 }
 
 func MonitorSize() (int, int) {
@@ -167,10 +170,17 @@ func (u *userInterface) updateGraphicsContext(g GraphicsContext) {
 	}
 }
 
+func (u *userInterface) suspended() bool {
+	return !u.runnableInBackground && (!u.windowFocus || !u.pageVisible)
+}
+
 func (u *userInterface) update(g GraphicsContext) error {
-	if !u.runnableInBackground && !u.windowFocus {
+	if u.suspended() {
+		hooks.SuspendAudio()
 		return nil
 	}
+	hooks.ResumeAudio()
+
 	if opengl.GetContext().IsContextLost() {
 		opengl.GetContext().RestoreContext()
 		g.Invalidate()
@@ -232,9 +242,27 @@ func initialize() error {
 	}
 	window.Call("addEventListener", "focus", func() {
 		currentUI.windowFocus = true
+		if currentUI.suspended() {
+			hooks.SuspendAudio()
+		} else {
+			hooks.ResumeAudio()
+		}
 	})
 	window.Call("addEventListener", "blur", func() {
 		currentUI.windowFocus = false
+		if currentUI.suspended() {
+			hooks.SuspendAudio()
+		} else {
+			hooks.ResumeAudio()
+		}
+	})
+	doc.Call("addEventListener", "visibilitychange", func() {
+		currentUI.pageVisible = !doc.Get("hidden").Bool()
+		if currentUI.suspended() {
+			hooks.SuspendAudio()
+		} else {
+			hooks.ResumeAudio()
+		}
 	})
 	window.Call("addEventListener", "resize", func() {
 		currentUI.updateScreenSize()

internal/ui/ui_mobile.go

@@ -31,6 +31,7 @@ import (
 	"golang.org/x/mobile/gl"
 
 	"github.com/hajimehoshi/ebiten/internal/devicescale"
+	"github.com/hajimehoshi/ebiten/internal/hooks"
 	"github.com/hajimehoshi/ebiten/internal/input"
 	"github.com/hajimehoshi/ebiten/internal/opengl"
 )
@@ -202,7 +203,18 @@ func (u *userInterface) scaleImpl() float64 {
 }
 
 func (u *userInterface) update(g GraphicsContext) error {
-	<-renderCh
+render:
+	for {
+		select {
+		case <-renderCh:
+			break render
+		case <-time.After(500 * time.Millisecond):
+			hooks.SuspendAudio()
+			continue
+		}
+	}
+	hooks.ResumeAudio()
+
 	defer func() {
 		renderChEnd <- struct{}{}
 	}()