浅谈Android O Touch声音播放流程

前言

当我们点击屏幕按键时，就会听到touch音，那么touch音是如何播放起来的呢，由于最近项目需求顺便熟悉下了touch音的逻辑。

正文

谈touch逻辑首先要说下这个类ViewRootImpl.java，位于frameworks/base/core/java/android/view下，ViewRootImpl的主要功能：
A：链接WindowManager和DecorView的纽带，更广一点可以说是Window和View之间的纽带。
B：完成View的绘制过程，包括measure、layout、draw过程。
C：向DecorView分发收到的用户发起的event事件，如按键，触屏等事件。
关于ViewRootImpl的源码可参照博客ViewRootImpl类源码解析，我们从performFocusNavigation()入手

private boolean performFocusNavigation(KeyEvent event) {
    //略
    if (v.requestFocus(direction, mTempRect)) {
        playSoundEffect(SoundEffectConstants.getContantForFocusDirection(direction));
        return true;
    }
    //略
    return false;
}

当我们点击某个控件时，会先触发performFocusNavigation()这个方法，然后当控件获取到focus后便会调用playSoundEffect()方法，我只截取了performFocusNavigation()中关键代码playSoundEffect()部分，来看下playSoundEffect()这个方法

    public void playSoundEffect(int effectId) {
        checkThread();

        try {
            final AudioManager audioManager = getAudioManager();

            switch (effectId) {
                case SoundEffectConstants.CLICK:
                    audioManager.playSoundEffect(AudioManager.FX_KEY_CLICK);
                    return;
                case SoundEffectConstants.NAVIGATION_DOWN:
                    audioManager.playSoundEffect(AudioManager.FX_FOCUS_NAVIGATION_DOWN);
                    return;
                case SoundEffectConstants.NAVIGATION_LEFT:
                    audioManager.playSoundEffect(AudioManager.FX_FOCUS_NAVIGATION_LEFT);
                    return;
                case SoundEffectConstants.NAVIGATION_RIGHT:
                    audioManager.playSoundEffect(AudioManager.FX_FOCUS_NAVIGATION_RIGHT);
                    return;
                case SoundEffectConstants.NAVIGATION_UP:
                    audioManager.playSoundEffect(AudioManager.FX_FOCUS_NAVIGATION_UP);
                    return;
                default:
                    throw new IllegalArgumentException("unknown effect id " + effectId +
                            " not defined in " + SoundEffectConstants.class.getCanonicalName());
            }
        } catch (IllegalStateException e) {
            // Exception thrown by getAudioManager() when mView is null
            Log.e(mTag, "FATAL EXCEPTION when attempting to play sound effect: " + e);
            e.printStackTrace();
        }
    }

发现调用了audioManager的playSoundEffect()方法，audiomanager就不说了，接触android audio最先接触的可能就是AudioManager了，音量控制，声音焦点申请等。接着看

    public void  playSoundEffect(int effectType) {
        if (effectType < 0 || effectType >= NUM_SOUND_EFFECTS) {
            return;
        }
        //查询是否开启touch音，如果settings中关闭了，则直接返回
        if (!querySoundEffectsEnabled(Process.myUserHandle().getIdentifier())) {
            return;
        }

        final IAudioService service = getService();
        try {
            //调用到AudioService的playSoundEffect()
            service.playSoundEffect(effectType);
        } catch (RemoteException e) {
            throw e.rethrowFromSystemServer();
        }
    }

查询touch音是否可播放，因为毕竟在android的setting中有个touch音的开关，如果可播放则调用到AudioService的playSoundEffect()

    public void playSoundEffect(int effectType) {
        playSoundEffectVolume(effectType, -1.0f);
    }
    
    public void playSoundEffectVolume(int effectType, float volume) {
        if (effectType >= AudioManager.NUM_SOUND_EFFECTS || effectType < 0) {
            Log.w(TAG, "AudioService effectType value " + effectType + " out of range");
            return;
        }

        sendMsg(mAudioHandler, MSG_PLAY_SOUND_EFFECT, SENDMSG_QUEUE,
                effectType, (int) (volume * 1000), null, 0);
    }

其实AudioService初始化的时候会创建一个子线HandlerThread,HandlerThread主要处理一些相对耗时的操作，这里将播放touch音的功能放在了这个子线程中去执行，这样避免了主线程的阻塞，其实大家在做mediaplayer播放时也建议放在子线程去播放，接下来看看handler里对消息的处理，关键代码如下

case MSG_PLAY_SOUND_EFFECT:
     if (msg.obj == null) {
         onPlaySoundEffect(msg.arg1, msg.arg2, 0);
    } else {
        onPlaySoundEffect(msg.arg1, msg.arg2, (int) msg.obj);
     }
     break;

直接调用onPlaySoundEffect()的方法

       private void onPlaySoundEffect(int effectType, int volume) {
            synchronized (mSoundEffectsLock) {
                //初始化mSoundPool和要播放的资源文件
                onLoadSoundEffects();

                if (mSoundPool == null) {
                    return;
                }
                float volFloat;
                // use default if volume is not specified by caller
                if (volume < 0) {
                    volFloat = (float)Math.pow(10, (float)sSoundEffectVolumeDb/20);
                } else {
                    volFloat = volume / 1000.0f;
                }

                if (SOUND_EFFECT_FILES_MAP[effectType][1] > 0) {
                    //播放touch音
                    mSoundPool.play(SOUND_EFFECT_FILES_MAP[effectType][1],
                                        volFloat, volFloat, 0, 0, 1.0f);
                } else {
                    MediaPlayer mediaPlayer = new MediaPlayer();
                    try {
                        String filePath = Environment.getRootDirectory() + SOUND_EFFECTS_PATH +
                                    SOUND_EFFECT_FILES.get(SOUND_EFFECT_FILES_MAP[effectType][0]);
                        mediaPlayer.setDataSource(filePath);
                        mediaPlayer.setAudioStreamType(AudioSystem.STREAM_SYSTEM);
                        mediaPlayer.prepare();
                        mediaPlayer.setVolume(volFloat);
                        mediaPlayer.setOnCompletionListener(new OnCompletionListener() {
                            public void onCompletion(MediaPlayer mp) {
                                cleanupPlayer(mp);
                            }
                        });
                        mediaPlayer.setOnErrorListener(new OnErrorListener() {
                            public boolean onError(MediaPlayer mp, int what, int extra) {
                                cleanupPlayer(mp);
                                return true;
                            }
                        });
                        mediaPlayer.start();
                    } catch (IOException ex) {
                        Log.w(TAG, "MediaPlayer IOException: "+ex);
                    } catch (IllegalArgumentException ex) {
                        Log.w(TAG, "MediaPlayer IllegalArgumentException: "+ex);
                    } catch (IllegalStateException ex) {
                        Log.w(TAG, "MediaPlayer IllegalStateException: "+ex);
                    }
                }
            }
        }

最终通过soundPool来播放指定的资源文件实现了touch音的播放，因此大家在工作中如果有什么需要对应touch音的逻辑，可参照AudioService的onPlaySoundEffect()中的逻辑。
比如指定touch音的AudioAttributes使touch音输出到指定的device上等。

总结

touch音的流程就简单分析到这里，欢迎大家交流指正。
努力学习ing~