淺談Android O Touch聲音播放流程

前言

當我們點擊屏幕按鍵時，就會聽到touch音，那麼touch音是如何播放起來的呢，由於最近項目需求順便熟悉下了touch音的邏輯。

正文

ViewRootImpl.java位於frameworks/base/core/java/android/view下，ViewRootImpl的主要功能：
A：鏈接WindowManager和DecorView的紐帶，更廣一點可以說是Window和View之間的紐帶。
B：完成View的繪製過程，包括measure、layout、draw過程。
C：向DecorView分發收到的用戶發起的event事件，如按鍵，觸屏等事件。
關於ViewRootImpl的具體邏輯可參照博客ViewRootImpl類源碼解析，這裏touch邏輯

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

當我們點擊某個控件時，會先觸發performFocusNavigation()這個方法，然後當控件獲取到focus後便會調用playSoundEffect()方法，我只截取了performFocusNavigation()中關鍵代碼playSoundEffect()部分，來看下playSoundEffect()這個方法

    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();
        }
    }

發現調用了audioManager的playSoundEffect()方法，audiomanager就不說了，接觸android audio最先接觸的可能就是AudioManager了，音量控制，聲音焦點申請等。接着看

    public void  playSoundEffect(int effectType) {
        if (effectType < 0 || effectType >= NUM_SOUND_EFFECTS) {
            return;
        }
        //查詢是否開啓透徹音，如果settings中關閉了，則直接返回
        if (!querySoundEffectsEnabled(Process.myUserHandle().getIdentifier())) {
            return;
        }

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

查詢touch音是否可播放，因爲畢竟在android的setting中有個touch音的開關，如果可播放則調用到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);
    }

其實AudioService初始化的時候會創建一個子線HandlerThread,HandlerThread主要處理一些相對耗時的操作，這裏將播放touch音的功能放在了這個子線程中去執行，這樣避免了主線程的阻塞，其實大家在做mediaplayer播放時也建議放在子線程去播放，接下來看看handler裏對消息的處理，關鍵代碼如下

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;

直接調用onPlaySoundEffect()的方法

       private void onPlaySoundEffect(int effectType, int volume) {
            synchronized (mSoundEffectsLock) {
                //初始化mSoundPool和要播放的資源文件
                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);
                    }
                }
            }
        }

最終通過soundPool來播放指定的資源文件實現了touch音的播放，因此大家在工作中如果有什麼需要對應touch音的邏輯，可參照AudioService的onPlaySoundEffect()中的邏輯。
比如指定touch音的AudioAttributes使touch音輸出到指定的device上等。

總結

touch音的流程就簡單分析到這裏，歡迎大家交流指正。
努力學習ing~