项目背景 : 音乐或者视频App播放视频或者音乐
设计思想 ： 在播放时抢占音频焦点，同时监听音频焦点，在丢失音频焦点时暂停播放。

1.抢占音频焦点

//监听音频焦点
private val audioFocusListener = createOnAudioFocusChangeListener()
//继承AudioManager.OnAudioFocusChangeListener接口
private fun createOnAudioFocusChangeListener(): AudioManager.OnAudioFocusChangeListener {
        return object : AudioManager.OnAudioFocusChangeListener {
            private var lossTransientVolume = -1
            private var wasPlaying = false

            override fun onAudioFocusChange(focusChange: Int) {
                /*
             * Pause playback during alerts and notifications
             */
                when (focusChange) {
                    AudioManager.AUDIOFOCUS_LOSS -> {
                        Log.i(TAG, "AUDIOFOCUS_LOSS")
                        // Pause playback
                        changeAudioFocus(false)
                        lossTransient = false
                        service.pause()
                        LocalBroadcastManager.getInstance(service.applicationContext)
                            .sendBroadcast(Intent(Parameter.ACTION_AUDIO_LOSS))
                    }
                    AudioManager.AUDIOFOCUS_LOSS_TRANSIENT -> {
                        Log.i(TAG, "AUDIOFOCUS_LOSS_TRANSIENT")
                        // Pause playback
//                        pausePlayback()
                        lossTransient = true
                        wasPlaying = service.isPlaying()
                        Log.i(TAG, "wasPlaying = $wasPlaying")
                        if (wasPlaying) service.pause()
                    }
                    AudioManager.AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK -> {
                        Log.i(TAG, "AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK")
                        // Lower the volume
                        if (service.isPlaying()) {
                            val volume = service.getVolume()
                            lossTransientVolume = volume
                            service.setVolume(volume / 3)
                        }
                    }
                    AudioManager.AUDIOFOCUS_GAIN -> {
                        Log.i(TAG, "AUDIOFOCUS_GAIN: ")
                        // Resume playback
                        if (lossTransientVolume != -1) {
                            service.setVolume(lossTransientVolume)
                            lossTransientVolume = -1
                        }
//                        if (lossTransient) {
                        if (wasPlaying) service.play()
                        lossTransient = false
//                        }
                    }
                }
                service.audioFocusListener?.onAudioFocusChange(focusChange)
            }
        }
    }
 //请求焦点   
private fun requestAudioFocus() = audioManager.requestAudioFocus(
        audioFocusListener,
        AudioManager.STREAM_MUSIC, AudioManager.AUDIOFOCUS_GAIN
    )
audioManager.setParameters("bgm_state=true")
hasAudioFocus = true   


play(){
//在播放视频的时候调用请求焦点方法
} 
    

监听音频焦点

//禁止掉请求的音频焦点
private fun abandonAudioFocus() = audioManager.abandonAudioFocus(audioFocusListener)
stop(){
//调用下面方法
}
if (hasAudioFocus) {
            abandonAudioFocus()
            audioManager.setParameters("bgm_state=false")
            hasAudioFocus = false
//上面对音频焦点的监听中处理
 AudioManager.AUDIOFOCUS_LOSS -> {
 	//暂停视频
 	stop()
 }