安卓音频焦点机制学习

1.API使用：请求焦点，requestAudioFocus参数有多种，这里只列举一种

AudioManager mAudioManager = (AudioManager) getSystemService(Context.AUDIO_SERVICE);

AudioAttributes audioAttributes = builder.setLegacyStreamType(streamType).build();
AudioFocusRequest afr = new AudioFocusRequest.Builder(AudioManager.AUDIOFOCUS_GAIN）

.setOnAudioFocusChangeListener(mAFCListener)
.setAudioAttributes(audioAttributes).build();
int retFocus = mAudioManager.requestAudioFocus(afr);

 AudioManager.OnAudioFocusChangeListener mAFCListener = new AudioManager.OnAudioFocusChangeListener() {

        @Override
        public void onAudioFocusChange(int focusChange) {
            switch (focusChange) {
                case AudioManager.AUDIOFOCUS_GAIN:
                case AudioManager.AUDIOFOCUS_GAIN_TRANSIENT:
                case AudioManager.AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK:
                    break;
                case AudioManager.AUDIOFOCUS_LOSS:
                case AudioManager.AUDIOFOCUS_LOSS_TRANSIENT:
                case AudioManager.AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK:
                    break;
            }
        }
    };

第一个参数：OnAudioFocusChangeListener ，此为一个监听控制器，通过这个监听器可以知道自己获取到焦点或者失去焦点。
第二个参数：streamType音频流类型，焦点获得之后的数据传输类型，这个参数不会影响焦点机制，不同的音频流类型同样遵守一个焦点机制。
第三个参数：durationHint，获得焦点的时间长短，定义了四种类型
a、AUDIOFOCUS_GAIN //长时间获得焦点，此参数会触发其他监听器的AudioManager.AUDIOFOCUS_LOSS
b、AUDIOFOCUS_GAIN_TRANSIENT //短暂性获得焦点，用完应立即释放，此参数会触发其他监听器的AudioManager.AUDIOFOCUS_LOSS_TRANSIENT
c、AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK //短暂性获得焦点并降音，可混音播放，此参数会触发其他监听器的AudioManager.AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK
d、AUDIOFOCUS_GAIN_TRANSIENT_EXCLUSIVE //短暂性获得焦点，录音或者语音识别，此参数会触发其他监听器的AudioManager.AUDIOFOCUS_LOSS_TRANSIENT
我们通常使用前面三种类型就可以了。

 

1.AudioService注册到service_manager进程流程

　　由system_server进程启动，然后通过ServiceManager#addService注册到service_manager进程。

1.SystemServer.java
/**
 * The main entry point from zygote.
 */
public static void main(String[] args) {
    new SystemServer().run();
}

run()>>
try {
    traceBeginAndSlog("StartServices");
    startBootstrapServices();
    startCoreServices();
    startOtherServices();
    SystemServerInitThreadPool.shutdown();
} catch (Throwable ex) {
    Slog.e("System", "******************************************");
    Slog.e("System", "************ Failure starting system services", ex);
    throw ex;
} finally {
    traceEnd();
}

startOtherServices()>>
traceBeginAndSlog("StartAudioService");
mSystemServiceManager.startService(AudioService.Lifecycle.class);
traceEnd();

2.AudioService.java
startService(AudioService.Lifecycle.class)>>
public static final class Lifecycle extends SystemService {

    private AudioService mService;

    public Lifecycle(Context context) {
        super(context);
        mService = new AudioService(context);
    }

    @Override
    public void onStart() {
        publishBinderService(Context.AUDIO_SERVICE, mService);
    }

    @Override
    public void onBootPhase(int phase) {
        if (phase == SystemService.PHASE_ACTIVITY_MANAGER_READY) {
            mService.systemReady();
        }
    }
}