Android音频焦点详解（下）——源码详解

转载请注明出处：http://blog.youkuaiyun.com/wangjihuanghun/article/details/56069283

---

耽搁了几天，最近一直在忙找工作的事情，今天把这篇文章补上。

---

本文基于Android7.1.1版本进行分析，主要涉及以下几个文件:
1 AudioManager –> /frameworks/base/media/java/android/media/
2 AudioService –> /frameworks/base/services/core/java/com/android/server/audio/
3 MediaFocusControl –>/frameworks/base/services/core/java/com/android/server/audio/
4 FocusRequester –> /frameworks/base/services/core/java/com/android/server/audio/
5 AudioAttributes –> /frameworks/base/media/java/android/media/
6 AudioFocusInfo –> /frameworks/base/media/java/android/media/
7 AudioSystem –> /frameworks/base/media/java/android/media/

之前一直用的是4.4的源码，这两天看了下7.1的源码发现这块内容改动还是挺大的，主要是新增了几个文件，并且对MediaFocusControl类进行了瘦身，代码从2700多行减到了500多行。
我们从入口方法requestAudioFocus开始，还记得我们是怎么使用该方法的么？
通过Audio Manager的对象来调用

mAudioManager.requestAudioFocus(mAudioFocusChange, AudioManager.STREAM_MUSIC, AudioManager.AUDIOFOCUS_GAIN);

具体请参考我的上篇博客点击这里

---

从AudioManager开始（有些方法里代码较多，只贴出来部分关键代码，下同）

public int requestAudioFocus(OnAudioFocusChangeListener l, int streamType, int durationHint) {
    int status = AUDIOFOCUS_REQUEST_FAILED;
    try {
        //调用内部重载后的方法
        status = requestAudioFocus(l,
                new AudioAttributes.Builder()
                        .setInternalLegacyStreamType(streamType).build(),
                durationHint,
                0 /* flags, legacy behavior */);
    } catch (IllegalArgumentException e) {
        Log.e(TAG, "Audio focus request denied due to ", e);
    }
    return status;
}

根据传进来的streamType，构造了一个AudioAttributes对象向下传递，这个AudioAttributes主要是存储了一些音频流信息的属性，后面会用到。接着看

@SystemApi
public int requestAudioFocus(OnAudioFocusChangeListener l,
                             @NonNull AudioAttributes requestAttributes,
                             int durationHint,
                             int flags) throws IllegalArgumentException {
    return requestAudioFocus(l, requestAttributes, durationHint,
            flags & AUDIOFOCUS_FLAGS_APPS,
            null /* no AudioPolicy*/);
}

这里面对falgs进行了与操作，由于之前传进来的是0，所以转换后的结果还是0。接着看

@SystemApi
public int requestAudioFocus(OnAudioFocusChangeListener l,
                             @NonNull AudioAttributes requestAttributes,
                             int durationHint,
                             int flags,
                             AudioPolicy ap) throws IllegalArgumentException {
    // 参数检查
    //...

    int status = AUDIOFOCUS_REQUEST_FAILED;
    registerAudioFocusListener(l);
    //获取AudioService实例，这里采用了binder通信，我们只需要知道从此处开始将会进入AudioServie的requestAudioFocus方法
    IAudioService service = getService();
    try {
        status = service.requestAudioFocus(requestAttributes, durationHint, mICallBack,
                mAudioFocusDispatcher, getIdForAudioFocusListener(l),
                getContext().getOpPackageName() /* package name */, flags,
                ap != null ? ap.cb() : null);
    } catch (RemoteException e) {
        throw e.rethrowFromSystemServer();
    }
    return status;
}

这里面重点看一下 registerAudioFocusListener(l);

private final HashMap<String, OnAudioFocusChangeListener> mAudioFocusIdListenerMap =
            new HashMap<String, OnAudioFocusChangeListener>();