Add observer for AVAudioSession.outputVolume

BUG=webrtc:7696

Review-Url: https://codereview.webrtc.org/2895263006
Cr-Commit-Position: refs/heads/master@{#18267}

webrtc/modules/audio_device/ios/objc/RTCAudioSession.h

@@ -74,6 +74,10 @@ RTC_EXPORT
  */
 - (void)audioSessionDidStopPlayOrRecord:(RTCAudioSession *)session;
 
+/** Called when the AVAudioSession output volume value changes. */
+- (void)audioSession:(RTCAudioSession *)audioSession
+    didChangeOutputVolume:(float)outputVolume;
+
 @end
 
 /** This is a protocol used to inform RTCAudioSession when the audio session

webrtc/modules/audio_device/ios/objc/RTCAudioSession.mm

@@ -24,6 +24,7 @@
 NSString * const kRTCAudioSessionErrorDomain = @"org.webrtc.RTCAudioSession";
 NSInteger const kRTCAudioSessionErrorLockRequired = -1;
 NSInteger const kRTCAudioSessionErrorConfiguration = -2;
+NSString * const kRTCAudioSessionOutputVolumeSelector = @"outputVolume";
 
 // This class needs to be thread-safe because it is accessed from many threads.
 // TODO(tkchin): Consider more granular locking. We're not expecting a lot of
@@ -86,6 +87,11 @@ NSInteger const kRTCAudioSessionErrorConfiguration = -2;
                selector:@selector(handleApplicationDidBecomeActive:)
                    name:UIApplicationDidBecomeActiveNotification
                  object:nil];
+    [_session addObserver:self
+               forKeyPath:kRTCAudioSessionOutputVolumeSelector
+                  options:NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld
+                  context:nil];
+
     RTCLog(@"RTCAudioSession (%p): init.", self);
   }
   return self;
@@ -93,6 +99,7 @@ NSInteger const kRTCAudioSessionErrorConfiguration = -2;
 
 - (void)dealloc {
   [[NSNotificationCenter defaultCenter] removeObserver:self];
+  [_session removeObserver:self forKeyPath:kRTCAudioSessionOutputVolumeSelector context:nil];
   RTCLog(@"RTCAudioSession (%p): dealloc.", self);
 }
 
@@ -803,6 +810,22 @@ NSInteger const kRTCAudioSessionErrorConfiguration = -2;
   [self decrementActivationCount];
 }
 
+- (void)observeValueForKeyPath:(NSString *)keyPath
+                      ofObject:(id)object
+                        change:(NSDictionary *)change
+                       context:(void *)context {
+  if (object == _session) {
+    NSNumber *newVolume = change[NSKeyValueChangeNewKey];
+    RTCLog(@"OutputVolumeDidChange to %f", newVolume.floatValue);
+    [self notifyDidChangeOutputVolume:newVolume.floatValue];
+  } else {
+    [super observeValueForKeyPath:keyPath
+                         ofObject:object
+                           change:change
+                          context:context];
+  }
+}
+
 - (void)notifyDidBeginInterruption {
   for (auto delegate : self.delegates) {
     SEL sel = @selector(audioSessionDidBeginInterruption:);
@@ -880,4 +903,13 @@ NSInteger const kRTCAudioSessionErrorConfiguration = -2;
   }
 }
 
+- (void)notifyDidChangeOutputVolume:(float)volume {
+  for (auto delegate : self.delegates) {
+    SEL sel = @selector(audioSession:didChangeOutputVolume:);
+    if ([delegate respondsToSelector:sel]) {
+      [delegate audioSession:self didChangeOutputVolume:volume];
+    }
+  }
+}
+
 @end

webrtc/modules/audio_device/ios/objc/RTCAudioSessionDelegateAdapter.mm

@@ -81,4 +81,8 @@
 - (void)audioSessionDidStopPlayOrRecord:(RTCAudioSession *)session {
 }
 
+- (void)audioSession:(RTCAudioSession *)audioSession
+    didChangeOutputVolume:(float)outputVolume {
+}
+
 @end

webrtc/modules/audio_device/ios/objc/RTCAudioSessionTest.mm

@@ -18,10 +18,22 @@
 #import "webrtc/modules/audio_device/ios/objc/RTCAudioSessionConfiguration.h"
 
 @interface RTCAudioSessionTestDelegate : NSObject <RTCAudioSessionDelegate>
+
+@property (nonatomic, readonly) float outputVolume;
+
 @end
 
 @implementation RTCAudioSessionTestDelegate
 
+@synthesize outputVolume = _outputVolume;
+
+- (instancetype)init {
+  if (self = [super init]) {
+    _outputVolume = -1;
+  }
+  return self;
+}
+
 - (void)audioSessionDidBeginInterruption:(RTCAudioSession *)session {
 }
 
@@ -46,6 +58,11 @@
 - (void)audioSessionShouldUnconfigure:(RTCAudioSession *)session {
 }
 
+- (void)audioSession:(RTCAudioSession *)audioSession
+    didChangeOutputVolume:(float)outputVolume {
+  _outputVolume = outputVolume;
+}
+
 @end
 
 // A delegate that adds itself to the audio session on init and removes itself
@@ -246,6 +263,23 @@ OCMLocation *OCMMakeLocation(id testCase, const char *fileCString, int line){
   [mockAudioSession stopMocking];
 }
 
+- (void)testAudioVolumeDidNotify {
+  RTCAudioSession *session = [RTCAudioSession sharedInstance];
+  RTCAudioSessionTestDelegate *delegate =
+      [[RTCAudioSessionTestDelegate alloc] init];
+  [session addDelegate:delegate];
+
+  [session observeValueForKeyPath:@"outputVolume"
+                         ofObject:[AVAudioSession sharedInstance]
+                           change:
+        @{NSKeyValueChangeNewKey :
+            @([AVAudioSession sharedInstance].outputVolume) }
+                          context:nil];
+
+  EXPECT_NE(delegate.outputVolume, -1);
+  EXPECT_EQ([AVAudioSession sharedInstance].outputVolume, delegate.outputVolume);
+}
+
 @end
 
 namespace webrtc {
@@ -295,4 +329,9 @@ TEST_F(AudioSessionTest, ConfigureWebRTCSession) {
   [test testConfigureWebRTCSession];
 }
 
+TEST_F(AudioSessionTest, AudioVolumeDidNotify) {
+  RTCAudioSessionTest *test = [[RTCAudioSessionTest alloc] init];
+  [test testAudioVolumeDidNotify];
+}
+
 }  // namespace webrtc