零基础入门多媒体音频（2）-音频焦点2

说实话，android的代码是越来越难以阅读。业务函数里面狗皮膏药似的补丁与日俱增。继上篇简要介绍音频焦点的文章，这篇文章的主要内容是分析audiofocus的实现。看了一下午的相关代码都没找到做audiofocus策略的核心逻辑。目前能看懂的大概包含下面两个逻辑。欢迎评论区沟通。

audiofocus实现的核心代码是在project_dir/frameworks/base/services/core/java/com/android/server/audio/MediaFocusControl.java
project_dir/frameworks/base/services/core/java/com/android/server/audio/FocusRequester.java
project_dir在这里指代android源码的根目录。

1.MediaFocusControl会维护所有audiofocus申请信息的堆栈mFocusStack。
2.android当系统进入铃声或者通话状态，会静音住媒体播放器，代码片段如下，mRingOrCallActive变量会在应用申请焦点时进行判断刷新。

                    if (mRingOrCallActive) {
                        mFocusEnforcer.mutePlayersForCall(USAGES_TO_MUTE_IN_RING_OR_CALL);
                    } else {
                        mFocusEnforcer.unmutePlayersForCall();
                    }
                   

3.audiofocus的监听器都是在AudioManager的队列里维护。代码如下：

    public void registerAudioFocusRequest(@NonNull AudioFocusRequest afr) {
        final Handler h = afr.getOnAudioFocusChangeListenerHandler();
        final FocusRequestInfo fri = new FocusRequestInfo(afr, (h == null) ? null :
            new ServiceEventHandlerDelegate(h).getHandler());
        final String key = getIdForAudioFocusListener(afr.getOnAudioFocusChangeListener());
        mAudioFocusIdListenerMap.put(key, fri);
    }

4.audiomanager进行焦点变化通知的核心类IAudioFocusDispatcher会调用listener。IAudioFocusDispatcher
自身会被注册给audioservice。

                status = service.requestAudioFocus(afr.getAudioAttributes(),
                        afr.getFocusGain(), mICallBack,
                        mAudioFocusDispatcher,
                        clientId,
                        getContext().getOpPackageName() /* package name */, afr.getFlags(),
                        ap != null ? ap.cb() : null,
                        sdk);

阅读代码期间出现重大错误，阅读函数handleFocusLossFromGain的时候，认为gain是增益的含义，没仔细阅读实现。导致增加阅读理解代码的时间周期。gain在这里应该是对应焦点的第一种类型，永久性焦点。犯了惯性思维的错误。遇到这种与认知相悖的地方应该更加小心。gain在这块代码里面的含义是获得，与之对应的是loss。

androidaudio进行audiofocus控制决策的核心代码是下面这一段

            if (mMultiAudioFocusEnabled
                    && (focusChangeHint == AudioManager.AUDIOFOCUS_GAIN)) {
                if (enteringRingOrCall) {
                    if (!mMultiAudioFocusList.isEmpty()) {
                        for (FocusRequester multifr : mMultiAudioFocusList) {
                            multifr.handleFocusLossFromGain(focusChangeHint, nfr, forceDuck);
                        }
                    }
                } else {
                    boolean needAdd = true;
                    if (!mMultiAudioFocusList.isEmpty()) {
                        for (FocusRequester multifr : mMultiAudioFocusList) {
                            if (multifr.getClientUid() == Binder.getCallingUid()) {
                                needAdd = false;
                                break;
                            }
                        }
                    }
                    if (needAdd) {
                        mMultiAudioFocusList.add(nfr);
                    }
                    nfr.handleFocusGainFromRequest(AudioManager.AUDIOFOCUS_REQUEST_GRANTED);
                    notifyExtPolicyFocusGrant_syncAf(nfr.toAudioFocusInfo(),
                            AudioManager.AUDIOFOCUS_REQUEST_GRANTED);
                    return AudioManager.AUDIOFOCUS_REQUEST_GRANTED;
                }
            }