Support delayed AudioUnit initialization.

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

 /*
  *  Copyright 2016 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #import "webrtc/modules/audio_device/ios/objc/RTCAudioSession.h"
 
 #include <vector>
 
 NS_ASSUME_NONNULL_BEGIN
 
+@class RTCAudioSessionConfiguration;
+
 @interface RTCAudioSession ()
 
 /** Number of times setActive:YES has succeeded without a balanced call to
  *  setActive:NO.
  */
-@property(nonatomic, readonly) NSInteger activationCount;
+@property(nonatomic, readonly) int activationCount;
+
+/** The number of times |beginWebRTCSession| was called without a balanced call
+ *  to |endWebRTCSession|.
+ */
+@property(nonatomic, readonly) int webRTCSessionCount;
+
+/** The configuration of the audio session before configureWebRTCSession
+ *  was first called.
+ */
+@property(nonatomic, strong, nullable)
+    RTCAudioSessionConfiguration *savedConfiguration;
 
 - (BOOL)checkLock:(NSError **)outError;
 
 /** Adds the delegate to the list of delegates, and places it at the front of
  *  the list. This delegate will be notified before other delegates of
  *  audio events.
  */
 - (void)pushDelegate:(id<RTCAudioSessionDelegate>)delegate;
 
+/** Signals RTCAudioSession that a WebRTC session is about to begin and
+ *  audio configuration is needed. Will configure the audio session for WebRTC
+ *  if not already configured and if configuration is not delayed.
+ *  Successful calls must be balanced by a call to endWebRTCSession.
+ */
+- (BOOL)beginWebRTCSession:(NSError **)outError;
+
+/** Signals RTCAudioSession that a WebRTC session is about to end and audio
+ *  unconfiguration is needed. Will unconfigure the audio session for WebRTC
+ *  if this is the last unmatched call and if configuration is not delayed.
+ */
+- (BOOL)endWebRTCSession:(NSError **)outError;
+
+/** Returns a configuration error with the given description. */
+- (NSError *)configurationErrorWithDescription:(NSString *)description;
+
 // Properties and methods for tests.
 @property(nonatomic, readonly)
     std::vector<__weak id<RTCAudioSessionDelegate> > delegates;
+
 - (void)notifyDidBeginInterruption;
 - (void)notifyDidEndInterruptionWithShouldResumeSession:
     (BOOL)shouldResumeSession;
 - (void)notifyDidChangeRouteWithReason:(AVAudioSessionRouteChangeReason)reason
     previousRoute:(AVAudioSessionRouteDescription *)previousRoute;
 - (void)notifyMediaServicesWereLost;
 - (void)notifyMediaServicesWereReset;
+- (void)notifyShouldConfigure;
+- (void)notifyShouldUnconfigure;
+- (void)notifyDidConfigure;
+- (void)notifyDidUnconfigure;
 
 @end
 
 NS_ASSUME_NONNULL_END