iOS音频开发之Audio Session

系统通过Audio Session来管理App，App之间的和设备级别的音频行为。通过设置AVAudioSession的category和mode,可以告知系统如何在你的App中使用音频；可以监听音频中断（例如来电话）和音频路由（例如插上耳机）改变的通知；可以使用设置音频的采样率，I/O缓存持续时间和音频频道数。

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系统默认的AudioSession有以下默认行为：1.只支持音频播放，不支持录音 2.设置响铃静音开关为静音模式下不会播放声音 3.设备锁屏不会播放声音 4.当你的App音频播放时，其他在后台播放的音频不会播放声音。如果需要与系统默认不同的音频播放行为，需要设置AudioSession，可以通过它的category和mode属性来实现。

// Access the shared, singleton audio session instance
let session = AVAudioSession.sharedInstance()
do {
    // Configure the audio session for movie playback
    try session.setCategory(AVAudioSessionCategoryPlayback,
                            mode: AVAudioSessionModeMoviePlayback,
                            options: [])
} catch let error as NSError {
    print("Failed to set the audio session category and mode: \(error.localizedDescription)")
}

multiroute category 不同与其它category，它可以同时支持多个外接设备，例如同时插入了耳机和HDMI，那么音频流会同时流向这两个音频接口，也可以控制，不同的音频流流向不同的设备。

只有如下的category支持AirPlay的镜像和非镜像版本,

AVAudioSessionCategorySoloAmbient
AVAudioSessionCategoryAmbient
AVAudioSessionCategoryPlayback

AVAudioSessionCategoryPlayAndRecord category 下 modes 只支持 AirPlay的镜像版本:

AVAudioSessionModeDefault
AVAudioSessionModeVideoChat
AVAudioSessionModeGameChat

对于不同App之间的AVAudioSession竞争，由系统来管理。默认情况下，当你的App在前台运行并播放音频时，系统会默认激活AVAudioSession,你也可以通过session.setActive(true)来显式激活音频会话，但是当你的App进入后台时，如果不需要再使用AVAudioSession，那么需要显式释放它，让系统可以回收音频资源，供其他App使用。

可以在applicationDidBecomeActive:中通过secondaryAudioShouldBeSilencedHint来检测,也可以通过订阅AVAudioSessionSilenceSecondaryAudioHintNotification通知来实现

func setupNotifications() {
    NotificationCenter.default.addObserver(self,
                                           selector: #selector(handleSecondaryAudio),
                                           name: .AVAudioSessionSilenceSecondaryAudioHint,
                                           object: AVAudioSession.sharedInstance())
}
 
func handleSecondaryAudio(notification: Notification) {
    // Determine hint type
    guard let userInfo = notification.userInfo,
        let typeValue = userInfo[AVAudioSessionSilenceSecondaryAudioHintTypeKey] as? UInt,
        let type = AVAudioSessionSilenceSecondaryAudioHintType(rawValue: typeValue) else {
            return
    }
 
    if type == .begin {
        // Other app audio started playing - mute secondary audio
    } else {
        // Other app audio stopped playing - restart secondary audio
    }
}

当来电话，或者闹铃响了，或者是日历提醒发生时，你App的音频会话会被系统中断，需要注册监听系统通知来处理该类事件。

Figure 3-1 An audio session is interrupted A timeline representation of an application's audio session getting interrupted by a phone call.

func registerForNotifications() {
    NotificationCenter.default.addObserver(self,
                                           selector: #selector(handleInterruption),
                                           name: .AVAudioSessionInterruption,
                                           object: AVAudioSession.sharedInstance())
}
 
func handleInterruption(_ notification: Notification) {
    guard let info = notification.userInfo,
        let typeValue = info[AVAudioSessionInterruptionTypeKey] as? UInt,
        let type = AVAudioSessionInterruptionType(rawValue: typeValue) else {
            return
    }
    if type == .began {
        // Interruption began, take appropriate actions (save state, update user interface)
    }
    else if type == .ended {
        guard let optionsValue =
            userInfo[AVAudioSessionInterruptionOptionKey] as? UInt else {
                return
        }
        let options = AVAudioSessionInterruptionOptions(rawValue: optionsValue)
        if options.contains(.shouldResume) {
            // Interruption Ended - playback should resume
        }
    }
}

当系统的音频服务被重置的时候，如果你需要对这种情况作出处理，需要监听AVAudioSessionMediaServicesWereResetNotification通知来处理，需要是实现以下操作

处理孤立的音频对象（如播放器，录音机，转换器或音频队列）并创建新的音频对象
重置正在跟踪的所有内部音频状态，包括所有属性 AVAudioSession
适当时，AVAudioSession使用该setActive:error:方法重新激活实例

另外当音频的路由变化时，也需要做相应的处理

Figure 4-1 Handling audio hardware route changes

func setupNotifications() {
    NotificationCenter.default.addObserver(self,
                                           selector: #selector(handleRouteChange),
                                           name: .AVAudioSessionRouteChange,
                                           object: AVAudioSession.sharedInstance())
}

func handleRouteChange(notification: NSNotification) {
    guard let userInfo = notification.userInfo,
        let reasonValue = userInfo[AVAudioSessionRouteChangeReasonKey] as? UInt,
        let reason = AVAudioSessionRouteChangeReason(rawValue:reasonValue) else {
            return
    }
    switch reason {
    case .newDeviceAvailable:
        let session = AVAudioSession.sharedInstance()
        for output in session.currentRoute.outputs where output.portType == AVAudioSessionPortHeadphones {
            headphonesConnected = true
        }
    case .oldDeviceUnavailable:
        if let previousRoute =
            userInfo[AVAudioSessionRouteChangePreviousRouteKey] as? AVAudioSessionRouteDescription {
            for output in previousRoute.outputs where output.portType == AVAudioSessionPortHeadphones {
                headphonesConnected = false
            }
        }
    default: ()
    }
}

A flowchart representation of how Core Audio, and your property listener callback function, interact to provide good user experience upon an audio hardware route change.

还可以设置硬件相关的属性：采样率，输入输出缓存等。

let session = AVAudioSession.sharedInstance()
 
// Configure category and mode
do {
    try session.setCategory(AVAudioSessionCategoryRecord, mode: AVAudioSessionModeDefault)
} catch let error as NSError {
    print("Unable to set category:  \(error.localizedDescription)")
}
 
// Set preferred sample rate
do {
    try session.setPreferredSampleRate(44_100)
} catch let error as NSError {
    print("Unable to set preferred sample rate:  \(error.localizedDescription)")
}
 
// Set preferred I/O buffer duration
do {
    try session.setPreferredIOBufferDuration(0.005)
} catch let error as NSError {
    print("Unable to set preferred I/O buffer duration:  \(error.localizedDescription)")
}
 
// Activate the audio session
do {
    try session.setActive(true)
} catch let error as NSError {
    print("Unable to activate session. \(error.localizedDescription)")
}
 
// Query the audio session's ioBufferDuration and sampleRate properties
// to determine if the preferred values were set
print("Audio Session ioBufferDuration: \(session.ioBufferDuration), sampleRate: \(session.sampleRate)")

设置麦克风

// Preferred Mic = Front, Preferred Polar Pattern = Cardioid
let preferredMicOrientation = AVAudioSessionOrientationFront
let preferredPolarPattern = AVAudioSessionPolarPatternCardioid
 
// Retrieve your configured and activated audio session
let session = AVAudioSession.sharedInstance()
 
// Get available inputs
guard let inputs = session.availableInputs else { return }
 
// Find built-in mic
guard let builtInMic = inputs.first(where: {
    $0.portType == AVAudioSessionPortBuiltInMic
}) else { return }
 
// Find the data source at the specified orientation
guard let dataSource = builtInMic.dataSources?.first (where: {
    $0.orientation == preferredMicOrientation
}) else { return }
 
// Set data source's polar pattern
do {
    try dataSource.setPreferredPolarPattern(preferredPolarPattern)
} catch let error as NSError {
    print("Unable to preferred polar pattern: \(error.localizedDescription)")
}
 
// Set the data source as the input's preferred data source
do {
    try builtInMic.setPreferredDataSource(dataSource)
} catch let error as NSError {
    print("Unable to preferred dataSource: \(error.localizedDescription)")
}
 
// Set the built-in mic as the preferred input
// This call will be a no-op if already selected
do {
    try session.setPreferredInput(builtInMic)
} catch let error as NSError {
    print("Unable to preferred input: \(error.localizedDescription)")
}
 
// Print Active Configuration
session.currentRoute.inputs.forEach { portDesc in
    print("Port: \(portDesc.portType)")
    if let ds = portDesc.selectedDataSource {
        print("Name: \(ds.dataSourceName)")
        print("Polar Pattern: \(ds.selectedPolarPattern ?? "[none]")")
    }
}

请求录音授权：iOS10以后需要在plist文件里加NSMicrophoneUsageDescription。

AVAudioSession.sharedInstance().requestRecordPermission { granted in
    if granted {
        // User granted access. Present recording interface.
    } else {
        // Present message to user indicating that recording
        // can't be performed until they change their preference
        // under Settings -> Privacy -> Microphone
    }
}

iOS音频开发之Audio Session

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