ios: Check the background state actively

It looks like the notifications for background state might not be
sent on some environments (e.g., iPod touch).

Updates #93

driver_darwin.go

@@ -23,6 +23,7 @@ package oto
 // void oto_render(void* inUserData, AudioQueueRef inAQ, AudioQueueBufferRef inBuffer);
 //
 // void oto_setNotificationHandler(AudioQueueRef audioQueue);
+// bool oto_isBackground(void);
 import "C"
 
 import (
@@ -50,9 +51,6 @@ type driver struct {
 	paused        bool
 	lastPauseTime time.Time
 
-	background     bool
-	backgroundCond *sync.Cond
-
 	err error
 
 	chWrite   chan []byte
@@ -138,7 +136,6 @@ func newDriver(sampleRate, channelNum, bitDepthInBytes, bufferSizeInBytes int) (
 		chWrite:    make(chan []byte),
 		chWritten:  make(chan int),
 	}
-	d.backgroundCond = sync.NewCond(&d.m)
 	runtime.SetFinalizer(d, (*driver).Close)
 	// Set the driver before setting the rendering callback.
 	setDriver(d)
@@ -257,7 +254,6 @@ func (d *driver) enqueueBuffer(buffer C.AudioQueueBufferRef) {
 
 	if osstatus := C.AudioQueueEnqueueBuffer(d.audioQueue, buffer, 0, nil); osstatus != C.noErr && d.err == nil {
 		d.err = fmt.Errorf("oto: AudioQueueEnqueueBuffer failed: %d", osstatus)
-		d.backgroundCond.Signal()
 		return
 	}
 }
@@ -279,11 +275,7 @@ func (d *driver) resume(afterSleep bool) bool {
 		}
 	}
 
-	for d.background && d.err == nil {
-		// As backgroundCond shares the same mutex as d.m, Wait unlocks the mutex.
-		d.backgroundCond.Wait()
-	}
-	if d.err != nil {
+	if C.oto_isBackground() {
 		return false
 	}
 
@@ -304,7 +296,6 @@ func (d *driver) pause() {
 	}
 	if osstatus := C.AudioQueuePause(d.audioQueue); osstatus != C.noErr && d.err == nil {
 		d.err = fmt.Errorf("oto: AudioQueuePause failed: %d", osstatus)
-		d.backgroundCond.Signal()
 		return
 	}
 	d.paused = true
@@ -319,21 +310,6 @@ func (d *driver) setError(err error) {
 		return
 	}
 	theDriver.err = err
-	d.backgroundCond.Signal()
-}
-
-func (d *driver) setForeground() {
-	d.backgroundCond.L.Lock()
-	d.background = false
-	d.backgroundCond.Signal()
-	d.backgroundCond.L.Unlock()
-}
-
-func (d *driver) setBackground() {
-	d.backgroundCond.L.Lock()
-	d.background = true
-	d.backgroundCond.Signal()
-	d.backgroundCond.L.Unlock()
 }
 
 func setNotificationHandler(driver *driver) {
@@ -355,13 +331,3 @@ func oto_setErrorByNotification(s C.OSStatus, from *C.char) {
 	gofrom := C.GoString(from)
 	theDriver.setError(fmt.Errorf("oto: %s at notification failed: %d", gofrom, s))
 }
-
-//export oto_setForeground
-func oto_setForeground() {
-	theDriver.setForeground()
-}
-
-//export oto_setBackground
-func oto_setBackground() {
-	theDriver.setBackground()
-}

driver_ios.m

@@ -24,8 +24,6 @@ @interface OtoNotificationObserver : NSObject {
 }
 
 - (void)onAudioSessionInterruption:(NSNotification *)notification;
-- (void)onApplicationDidEnterBackground:(NSNotification *)notification;
-- (void)onApplicationWillEnterForeground:(NSNotification *)notification;
 
 @end
 
@@ -58,14 +56,6 @@ - (void)onAudioSessionInterruption:(NSNotification *)notification {
   }
 }
 
-- (void)onApplicationDidEnterBackground:(NSNotification *)notification {
-  oto_setBackground();
-}
-
-- (void)onApplicationWillEnterForeground:(NSNotification *)notification {
-  oto_setForeground();
-}
-
 @end
 
 // oto_setNotificationHandler sets a handler for interruption events.
@@ -78,14 +68,23 @@ void oto_setNotificationHandler(AudioQueueRef audioQueue) {
          selector:@selector(onAudioSessionInterruption:)
              name:AVAudioSessionInterruptionNotification
            object:session];
-  [[NSNotificationCenter defaultCenter]
-      addObserver:observer
-         selector:@selector(onApplicationDidEnterBackground:)
-             name:UIApplicationDidEnterBackgroundNotification
-           object:session];
-  [[NSNotificationCenter defaultCenter]
-      addObserver:observer
-         selector:@selector(onApplicationWillEnterForeground:)
-             name:UIApplicationWillEnterForegroundNotification
-           object:session];
+
+  // The notifications UIApplicationDidEnterBackgroundNotification and
+  // UIApplicationWillEnterForegroundNotification were not reliable: at least,
+  // they were not notified at iPod touch A2178.
+  //
+  // Instead, check the background state via UIApplication actively.
+}
+
+bool oto_isBackground(void) {
+  if ([NSThread isMainThread]) {
+    return [[UIApplication sharedApplication] applicationState] ==
+           UIApplicationStateBackground;
+  }
+
+  __block bool background = false;
+  dispatch_sync(dispatch_get_main_queue(), ^{
+    background = oto_isBackground();
+  });
+  return background;
 }

driver_macos.m

@@ -51,3 +51,8 @@ void oto_setNotificationHandler(AudioQueueRef audioQueue) {
              name:NSWorkspaceDidWakeNotification
            object:NULL];
 }
+
+bool oto_isBackground(void) {
+  // TODO: Should this be implemented?
+  return false;
+}