主要通过学习AVAudioPlayer和AVAudioRecorder类来实现应用程序中添加音频播放及音频录制的功能。
AV Foundation定义了7中分类来描述应用程序所使用的音频行为

AVAudioSessionCategory 7大类别

AVAudioSessionCategoryOptions 类别的子选项

AVAudioSessionMode

配置音频会话

    AVAudioSession *session = [AVAudioSession sharedInstance];
    NSError *error;
    if (![session setCategory:AVAudioSessionCategoryPlayback error:&error]) {
        NSLog(@"Category Error: %@", [error localizedDescription]);
    }
    // 配置激活
    if (![session setActive:YES error:&error]) {
        NSLog(@"Activation Error: %@", [error localizedDescription]);
    }

/* returns singleton instance */
+ (AVAudioSession *)sharedInstance;

/* Set the session active or inactive. 
*/
- (BOOL)setActive:(BOOL)active error:(NSError **)outError;
- (BOOL)setActive:(BOOL)active withOptions:(AVAudioSessionSetActiveOptions)options error:(NSError **)outError;

/* Asynchronously activate the session. 异步激活会话，handler是回调
 */
- (void)activateWithOptions:(AVAudioSessionActivationOptions)options completionHandler:(void (^)(BOOL activated, NSError * _Nullable error))handler;
// 获取设备可用的SessionCategory
@property (readonly) NSArray<AVAudioSessionCategory> *availableCategories;

/* 设置session的category */
- (BOOL)setCategory:(AVAudioSessionCategory)category error:(NSError **)outError ;
/* set session category with options */
- (BOOL)setCategory:(AVAudioSessionCategory)category withOptions:(AVAudioSessionCategoryOptions)options error:(NSError **)outError;
/* set session category and mode with options */
- (BOOL)setCategory:(AVAudioSessionCategory)category mode:(AVAudioSessionMode)mode options:(AVAudioSessionCategoryOptions)options error:(NSError **)outError ;
- (BOOL)setCategory:(AVAudioSessionCategory)category mode:(AVAudioSessionMode)mode routeSharingPolicy:(AVAudioSessionRouteSharingPolicy)policy options:(AVAudioSessionCategoryOptions)options error:(NSError **)outError;
@property (readonly) AVAudioSessionCategory category;
@property (readonly) AVAudioSessionCategoryOptions categoryOptions;
@property (readonly) AVAudioSessionRouteSharingPolicy routeSharingPolicy;
@property (readonly) NSArray<AVAudioSessionMode> *availableModes;
- (BOOL)setMode:(AVAudioSessionMode)mode error:(NSError **)outError; /* set session mode */
@property (readonly) AVAudioSessionMode mode; /* get session mode */
/* 返回权限*/
@property (readonly) AVAudioSessionRecordPermission recordPermission;

typedef void (^PermissionBlock)(BOOL granted);
- (void)requestRecordPermission:(PermissionBlock)response;
- (BOOL)overrideOutputAudioPort:(AVAudioSessionPortOverride)portOverride error:(NSError **)outError;
@property (readonly, getter=isOtherAudioPlaying) BOOL otherAudioPlaying;
@property (readonly) BOOL secondaryAudioShouldBeSilencedHint;
@property (readonly) AVAudioSessionRouteDescription *currentRoute;
- (BOOL)setPreferredInput:(nullable AVAudioSessionPortDescription *)inPort error:(NSError **)outError API_AVAILABLE(ios(7.0), tvos(9.0)) API_UNAVAILABLE(watchos, macos);
@property (readonly, nullable) AVAudioSessionPortDescription *preferredInput API_AVAILABLE(ios(7.0), tvos(9.0)) API_UNAVAILABLE(watchos, macos); /* Get the preferred input port.  Will be nil if no preference has been set */

@property (readonly, nullable) NSArray<AVAudioSessionPortDescription *> *availableInputs ;

创建AVAudioPlayer

AVAudioPlayer构建与CoreAudio中的C-based Audio Queue Services的最顶层。它可以提供播放，循环，音频计量等简单友好的Objective-C接口。

- (AVAudioPlayer *)playerForFile:(NSString *)name {

    NSURL *fileURL = [[NSBundle mainBundle] URLForResource:name
                                             withExtension:@"caf"];

    NSError *error;
    AVAudioPlayer *player = [[AVAudioPlayer alloc] initWithContentsOfURL:fileURL
                                                                   error:&error];
    if (player) {
        player.numberOfLoops = -1; // 无限循环
        player.enableRate = YES;
        [player prepareToPlay];  //相当于预加载Audio Queue的缓冲区
    } else {
        NSLog(@"Error creating player: %@", [error localizedDescription]);
    }
    return player;
}

播放控制

• 修改播放器的音量
• 修改播放器的pan值 允许使用立体声播放声音，-1.0 到 1.0 之间
• 调整播放率rate 0.5表示半速，1表示正常，2表示2倍速播放
• numberOfLoops 实现无缝循环 -1表示无限循环，n > 0表示循环n次

THPlayerController.h
@interface THPlayerController : NSObject
@property (nonatomic, getter = isPlaying) BOOL playing; //是否正在播放
// 播放
- (void)play;
//停止
- (void)stop;
//设置速率
- (void)adjustRate:(float)rate;
// 设置pan值
- (void)adjustPan:(float)pan forPlayerAtIndex:(NSUInteger)index;
//设置音量
- (void)adjustVolume:(float)volume forPlayerAtIndex:(NSUInteger)index;

@end

#import "THPlayerController.h"
#import <AVFoundation/AVFoundation.h>

@interface THPlayerController () <AVAudioPlayerDelegate>
@property (strong, nonatomic) NSArray *players;
@end

@implementation THPlayerController

#pragma mark - Initialization

- (id)init {
    self = [super init];
    if (self) {
        AVAudioPlayer *guitarPlayer = [self playerForFile:@"guitar"];
        AVAudioPlayer *bassPlayer = [self playerForFile:@"bass"];
        AVAudioPlayer *drumsPlayer = [self playerForFile:@"drums"];

        guitarPlayer.delegate = self;

        _players = @[guitarPlayer, bassPlayer, drumsPlayer];
    }
    return self;
}

- (AVAudioPlayer *)playerForFile:(NSString *)name {

    NSURL *fileURL = [[NSBundle mainBundle] URLForResource:name
                                             withExtension:@"caf"];

    NSError *error;
    AVAudioPlayer *player = [[AVAudioPlayer alloc] initWithContentsOfURL:fileURL
                                                                   error:&error];
    if (player) {
        player.numberOfLoops = -1; // loop indefinitely
        player.enableRate = YES;
        [player prepareToPlay];
    } else {
        NSLog(@"Error creating player: %@", [error localizedDescription]);
    }

    return player;
}


#pragma mark - Global playback control methods

- (void)play {
    if (!self.playing) {
        NSTimeInterval delayTime = [self.players[0] deviceCurrentTime] + 0.01;
        for (AVAudioPlayer *player in self.players) {
            [player playAtTime:delayTime];
        }
        self.playing = YES;
    }
}

- (void)stop {
    if (self.playing) {
        for (AVAudioPlayer *player in self.players) {
            [player stop];
            player.currentTime = 0.0f; //player.currentTime回到原点
        }
        self.playing = NO;
    }
}

- (void)adjustRate:(float)rate {
    for (AVAudioPlayer *player in self.players) {
        player.rate = rate;
    }
}
- (void)adjustPan:(float)pan forPlayerAtIndex:(NSUInteger)index {
    if ([self isValidIndex:index]) {
        AVAudioPlayer *player = self.players[index];
        player.pan = pan;
    }
}
- (void)adjustVolume:(float)volume forPlayerAtIndex:(NSUInteger)index {
    if ([self isValidIndex:index]) {
        AVAudioPlayer *player = self.players[index];
        player.volume = volume;
    }
}
- (BOOL)isValidIndex:(NSUInteger)index {
    return index == 0 || index < self.players.count;
}
@end

处理中断事件

音频播放过程中，遇到电话呼入、闹钟等事件，会中断音频的播放
设置监听中断事件

初始化AVAudioPlayer之后设置中断事件的监听
NSNotificationCenter *nsnc = [NSNotificationCenter defaultCenter];
[nsnc addObserver:self selector:@selector(handleInterruption:) name:AVAudioSessionInterruptionNotification object:[AVAudioSession sharedInstance]];
//相应回调
-(void)handleInterruption:(NSNotification *)notification{
    NSDictionary *info = notification.userInfo;
    AVAudioSessionInterruptionType type = [info[AVAudioSessionInterruptionTypeKey] unsignedIntegerValue];
    if (type == AVAudioSessionInterruptionTypeBegan) {  //中断开始
        [self stop]；  
    }else{
        AVAudioSessionInterruptionOptions options = [info[AVAudioSessionInterruptionTypeKey] unsignedIntegerValue];
        if (options == AVAudioSessionInterruptionOptionShouldResume) {  //恢复
            [self play];
            }
        }
    }
}

对线路改变的响应

线路是指输出线路从扬声器模式变为耳机，或者耳机变为扬声器模式。

线路改变原因的枚举
AVAudioSessionRouteChangeReasonUnknown = 0,
AVAudioSessionRouteChangeReasonNewDeviceAvailable = 1,
AVAudioSessionRouteChangeReasonOldDeviceUnavailable = 2,
AVAudioSessionRouteChangeReasonCategoryChange = 3,
AVAudioSessionRouteChangeReasonOverride = 4,
AVAudioSessionRouteChangeReasonWakeFromSleep = 6,
AVAudioSessionRouteChangeReasonNoSuitableRouteForCategory = 7,
AVAudioSessionRouteChangeReasonRouteConfigurationChange = 8 

音频输入
AVAudioSessionPortUSBAudio,
 AVAudioSessionPortHeadsetMic, 
AVAudioSessionPortBuiltInMic.  
音频输出
AVAudioSessionPortUSBAudio,  usb接口
AVAudioSessionPortLineOut, 
AVAudioSessionPortHeadphones,  耳机输出
AVAudioSessionPortHDMI, 
AVAudioSessionPortBuiltInSpeaker.   内置扬声器

NSNotificationCenter *nsnc = [NSNotificationCenter defaultCenter];
[nsnc addObserver:self
         selector:@selector(handleRouteChange:)
             name:AVAudioSessionRouteChangeNotification
           object:[AVAudioSession sharedInstance]];

//线路的改变  比如耳机断开是停止播放
- (void)handleRouteChange:(NSNotification *)notification {
    NSDictionary *info = notification.userInfo;
    AVAudioSessionRouteChangeReason reason =
        [info[AVAudioSessionRouteChangeReasonKey] unsignedIntValue];
//旧设备断开链接
    if (reason == AVAudioSessionRouteChangeReasonOldDeviceUnavailable) {
        AVAudioSessionRouteDescription *previousRoute =
            info[AVAudioSessionRouteChangePreviousRouteKey];
//端口描述        
AVAudioSessionPortDescription *previousOutput = previousRoute.outputs[0];
        NSString *portType = previousOutput.portType;
        if ([portType isEqualToString:AVAudioSessionPortHeadphones]) {
            [self stop];
        }
    }
}

音频录制AVAudioRecorder

创建AVAudioRecorder实例时需要为其提供数据的一些信息

- (nullable instancetype)initWithURL:(NSURL *)url settings:(NSDictionary<NSString *, id> *)settings error:(NSError **)outError;

• 音频流写入文件的本地url地址
• 配置录音会话的设置信息
  在 #import <AVFAudio/AVAudioSettings.h> 文件里有音频的设置键信息。

AVFormatIDKey     音频格式
AVSampleRateKey;    采样率         
AVNumberOfChannelsKey;      通道数             

格式：kAudioFormatLinearPCM会将未压缩的音频流写入文件中。 这中格式保真度最高，kAudioFormatMPEG4AAC的压缩格式会显著缩小文件，还能保证高质量的音频内容。
采样率：对输入的音频信号每一秒内的采样数。一般标准的采样率为8000、16000、22050和44100赫兹。
通道数：定义记录音频内容的通道数。默认值为1意味着使用单声道录制，设置为2采用立体声录制。

-(NSURL *)getSavePath{
    NSString *urlStr = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) firstObject];
    urlStr = [urlStr stringByAppendingPathComponent:kRecordAudioFile];
    NSLog(@"file path:%@",urlStr);
    NSURL *url = [NSURL fileURLWithPath:urlStr];
    return url;
}

//录音参数设置
-(NSDictionary *)getAudioSetting{
    NSMutableDictionary *dicM = @{}.mutableCopy;
    [dicM setObject:@(kAudioFormatLinearPCM) forKey:AVFormatIDKey];
    [dicM setObject:@(44100) forKey:AVSampleRateKey];
    [dicM setObject:@(1) forKey:AVNumberOfChannelsKey];
    [dicM setObject:@(16) forKey:AVEncoderBitDepthHintKey];
    [dicM setObject:@(AVAudioQualityMedium) forKey:AVEncoderAudioQualityKey];
    return dicM;
}
//录音器
-(AVAudioRecorder *)audioRecorder{
    if (!_audioRecorder) {
        //保存路径
        NSURL *url = [self getSavePath];
        NSDictionary *setting = [self getAudioSetting];
        NSError *error;
        _audioRecorder = [[AVAudioRecorder alloc] initWithURL:url settings:setting error:&error];
        _audioRecorder.delegate = self;
        _audioRecorder.meteringEnabled = YES;
        [_audioRecorder prepareToRecord];
    }
    return _audioRecorder;
}

//开始录制
- (BOOL)record {
    return [self.audioRecorder record];
}
//中断
- (void)pause {
    [self.audioRecorder pause];
}
// 停止
- (void)stop {
    [self.audioRecorder stop];
}
//实现录制完成代理
- (void)audioRecorderDidFinishRecording:(AVAudioRecorder *)recorder successfully:(BOOL)success {
   
}
//实现代理方法 录制中断
- (void)audioRecorderBeginInterruption:(AVAudioRecorder *)recorder {

}

记录分贝等级

- (float)peakPowerForChannel:(NSUInteger)channelNumber; /* returns peak power in decibels for a given channel */
- (float)averagePowerForChannel:(NSUInteger)channelNumber; /* returns average power in decibels for a given channel */

返回用于表示分贝等级的浮点值，最大分贝的0dB到最小分贝的-160dB。 在调用这两个方法之前需要设置_audioRecorder.meteringEnabled = YES才能使用。

AV Foundation开发秘籍源码