Support delayed AudioUnit initialization.

Applications can choose to decide when to give up control of the
AVAudioSession to WebRTC. Otherwise, behavior should be
unchanged.

Adds a toggle to AppRTCDemo so developers can see the different
paths.

BUG=
R=haysc@webrtc.org

Review URL: https://codereview.webrtc.org/1822543002 .

Cr-Commit-Position: refs/heads/master@{#12080}

webrtc/examples/objc/AppRTCDemo/ios/ARDMainViewController.m

@@ -10,32 +10,44 @@
 
 #import "ARDMainViewController.h"
 
+#import <AVFoundation/AVFoundation.h>
+
+#import "webrtc/base/objc/RTCDispatcher.h"
+#import "webrtc/base/objc/RTCLogging.h"
+#import "webrtc/modules/audio_device/ios/objc/RTCAudioSession.h"
+#import "webrtc/modules/audio_device/ios/objc/RTCAudioSessionConfiguration.h"
+
 #import "ARDAppClient.h"
 #import "ARDMainView.h"
 #import "ARDVideoCallViewController.h"
 
-@interface ARDMainViewController () <ARDMainViewDelegate>
+@interface ARDMainViewController () <
+    ARDMainViewDelegate,
+    RTCAudioSessionDelegate>
 @end
 
-@implementation ARDMainViewController
-
-- (void)loadView {
-  ARDMainView *mainView = [[ARDMainView alloc] initWithFrame:CGRectZero];
-  mainView.delegate = self;
-  self.view = mainView;
+@implementation ARDMainViewController {
+  ARDMainView *_mainView;
+  AVAudioPlayer *_audioPlayer;
+  BOOL _shouldDelayAudioConfig;
 }
 
-- (void)applicationWillResignActive:(UIApplication *)application {
-  // Terminate any calls when we aren't active.
-  [self dismissViewControllerAnimated:NO completion:nil];
+- (void)loadView {
+  _mainView = [[ARDMainView alloc] initWithFrame:CGRectZero];
+  _mainView.delegate = self;
+  self.view = _mainView;
+
+  [self setupAudioSession];
+  [self setupAudioPlayer];
 }
 
 #pragma mark - ARDMainViewDelegate
 
 - (void)mainView:(ARDMainView *)mainView
-    didInputRoom:(NSString *)room
-      isLoopback:(BOOL)isLoopback
-     isAudioOnly:(BOOL)isAudioOnly {
+              didInputRoom:(NSString *)room
+                isLoopback:(BOOL)isLoopback
+               isAudioOnly:(BOOL)isAudioOnly
+    shouldDelayAudioConfig:(BOOL)shouldDelayAudioConfig {
   if (!room.length) {
     [self showAlertWithMessage:@"Missing room name."];
     return;
@@ -65,6 +77,10 @@
     return;
   }
 
+  _shouldDelayAudioConfig = shouldDelayAudioConfig;
+  RTCAudioSession *session = [RTCAudioSession sharedInstance];
+  session.shouldDelayAudioConfiguration = _shouldDelayAudioConfig;
+
   // Kick off the video call.
   ARDVideoCallViewController *videoCallViewController =
       [[ARDVideoCallViewController alloc] initForRoom:trimmedRoom
@@ -77,8 +93,82 @@
                    completion:nil];
 }
 
+- (void)mainViewDidToggleAudioLoop:(ARDMainView *)mainView {
+  if (mainView.isAudioLoopPlaying) {
+    [_audioPlayer stop];
+  } else {
+    [_audioPlayer play];
+  }
+  mainView.isAudioLoopPlaying = _audioPlayer.playing;
+}
+
+#pragma mark - RTCAudioSessionDelegate
+
+- (void)audioSessionShouldConfigure:(RTCAudioSession *)session {
+  // Won't get called unless audio config is delayed.
+  // Stop playback on main queue and then configure WebRTC.
+  [RTCDispatcher dispatchAsyncOnType:RTCDispatcherTypeMain
+                               block:^{
+    if (_mainView.isAudioLoopPlaying) {
+      RTCLog(@"Stopping audio loop due to WebRTC start.");
+      [_audioPlayer stop];
+    }
+    // TODO(tkchin): Shouldn't lock on main queue. Figure out better way to
+    // check audio loop state.
+    [session lockForConfiguration];
+    [session configureWebRTCSession:nil];
+    [session unlockForConfiguration];
+  }];
+}
+
+- (void)audioSessionShouldUnconfigure:(RTCAudioSession *)session {
+  // Won't get called unless audio config is delayed.
+  [session lockForConfiguration];
+  [session unconfigureWebRTCSession:nil];
+  [session unlockForConfiguration];
+}
+
+- (void)audioSessionDidUnconfigure:(RTCAudioSession *)session {
+  // WebRTC is done with the audio session. Restart playback.
+  [RTCDispatcher dispatchAsyncOnType:RTCDispatcherTypeMain
+                               block:^{
+    if (_mainView.isAudioLoopPlaying) {
+      RTCLog(@"Starting audio loop due to WebRTC end.");
+      [_audioPlayer play];
+    }
+  }];
+}
+
 #pragma mark - Private
 
+- (void)setupAudioSession {
+  RTCAudioSessionConfiguration *configuration =
+      [[RTCAudioSessionConfiguration alloc] init];
+  configuration.category = AVAudioSessionCategoryAmbient;
+  configuration.categoryOptions = AVAudioSessionCategoryOptionDuckOthers;
+  configuration.mode = AVAudioSessionModeDefault;
+
+  RTCAudioSession *session = [RTCAudioSession sharedInstance];
+  [session addDelegate:self];
+  [session lockForConfiguration];
+  NSError *error = nil;
+  if (![session setConfiguration:configuration active:YES error:&error]) {
+    RTCLogError(@"Error setting configuration: %@", error.localizedDescription);
+  }
+  [session unlockForConfiguration];
+}
+
+- (void)setupAudioPlayer {
+  NSString *audioFilePath =
+      [[NSBundle mainBundle] pathForResource:@"mozart" ofType:@"mp3"];
+  NSURL *audioFileURL = [NSURL URLWithString:audioFilePath];
+  _audioPlayer = [[AVAudioPlayer alloc] initWithContentsOfURL:audioFileURL
+                                                        error:nil];
+  _audioPlayer.numberOfLoops = -1;
+  _audioPlayer.volume = 1.0;
+  [_audioPlayer prepareToPlay];
+}
+
 - (void)showAlertWithMessage:(NSString*)message {
   UIAlertView* alertView = [[UIAlertView alloc] initWithTitle:nil
                                                       message:message