Android 原生安全音量逻辑设计

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<!-- Whether safe headphone volume is enabled or not (country specific). -->
<bool name="config_safe_media_volume_enabled">true</bool>

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<!-- Safe headphone volume index. When music stream volume is below this index
the SPL on headphone output is compliant to EN 60950 requirements for portable music
players. -->
<integer name="config_safe_media_volume_index">10</integer>

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public void onSystemReady() {
    ...
    sendMsg(mAudioHandler,
    MSG_CONFIGURE_SAFE_MEDIA_VOLUME_FORCED,
    SENDMSG_REPLACE,
    0,
    0,
    TAG,
    SystemProperties.getBoolean("audio.safemedia.bypass", false) ?
        0 : SAFE_VOLUME_CONFIGURE_TIMEOUT_MS);
    ...
}

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    private void onConfigureSafeVolume(boolean force, String caller) {
        synchronized (mSafeMediaVolumeStateLock) {
            //Mobile contry code，国家代码，主要用来区分不同国家，部分国家策略可能会不一致
            int mcc = mContext.getResources().getConfiguration().mcc;
            if ((mMcc != mcc) || ((mMcc == 0) && force)) {
                //从config_safe_media_volume_index中获取回来的安全音量触发阈值
                mSafeMediaVolumeIndex = mContext.getResources().getInteger(
                        com.android.internal.R.integer.config_safe_media_volume_index) * 10;

                mSafeUsbMediaVolumeIndex = getSafeUsbMediaVolumeIndex();

                //根据audio.safemedia.force属性值或者value配置的值来决定是否使能安全音量
                boolean safeMediaVolumeEnabled =
                        SystemProperties.getBoolean("audio.safemedia.force", false)
                        || mContext.getResources().getBoolean(
                                com.android.internal.R.bool.config_safe_media_volume_enabled);

                //确认是否需要bypass掉安全音量功能
                boolean safeMediaVolumeBypass =
                        SystemProperties.getBoolean("audio.safemedia.bypass", false);

                // The persisted state is either "disabled" or "active": this is the state applied
                // next time we boot and cannot be "inactive"
                int persistedState;
                if (safeMediaVolumeEnabled && !safeMediaVolumeBypass) {
                    persistedState = SAFE_MEDIA_VOLUME_ACTIVE; //这个值只能是disable或者active，不能是inactive，主要用于下次启动。
                    // The state can already be "inactive" here if the user has forced it before
                    // the 30 seconds timeout for forced configuration. In this case we don't reset
                    // it to "active".
                    if (mSafeMediaVolumeState != SAFE_MEDIA_VOLUME_INACTIVE) {
                        if (mMusicActiveMs == 0) { //mMusicActiveMs主要用于计数，当安全音量弹框弹出时，如果按了确定，这个值便开始递增，当其达到UNSAFE_VOLUME_MUSIC_ACTIVE_MS_MAX时，则重新使能安全音量
                            mSafeMediaVolumeState = SAFE_MEDIA_VOLUME_ACTIVE;
                            enforceSafeMediaVolume(caller);
                        } else {
                            //跑到这里则表示已经弹过安全音量警示了，并且按了确定，所以把值设置为inactive
                            // We have existing playback time recorded, already confirmed.
                            mSafeMediaVolumeState = SAFE_MEDIA_VOLUME_INACTIVE;
                        }
                    }
                } else {
                    persistedState = SAFE_MEDIA_VOLUME_DISABLED;
                    mSafeMediaVolumeState = SAFE_MEDIA_VOLUME_DISABLED;
                }
                mMcc = mcc;
                //持久化当前安全音量的状态
                sendMsg(mAudioHandler,
                        MSG_PERSIST_SAFE_VOLUME_STATE,
                        SENDMSG_QUEUE,
                        persistedState,
                        0,
                        null,
                        0);
            }
        }
    }

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case MSG_PERSIST_SAFE_VOLUME_STATE:
    onPersistSafeVolumeState(msg.arg1);
    break;
....
....
private void onPersistSafeVolumeState(int state) {
    Settings.Global.putInt(mContentResolver,
            Settings.Global.AUDIO_SAFE_VOLUME_STATE,
            state);
}

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protected void adjustStreamVolume(int streamType, int direction, int flags,
        String callingPackage, String caller, int uid) {
    ...
    ...
    ...
    } else if ((direction == AudioManager.ADJUST_RAISE) &&
            !checkSafeMediaVolume(streamTypeAlias, aliasIndex + step, device)) {
        Log.e(TAG, "adjustStreamVolume() safe volume index = " + oldIndex);
        mVolumeController.postDisplaySafeVolumeWarning(flags);
    ....
    ...
    

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private void setStreamVolume(int streamType, int index, int flags, String callingPackage,
            String caller, int uid) {
    ....
    ....
        if (!checkSafeMediaVolume(streamTypeAlias, index, device)) {
                mVolumeController.postDisplaySafeVolumeWarning(flags);
                mPendingVolumeCommand = new StreamVolumeCommand(
                                                    streamType, index, flags, device);
            } else {
                onSetStreamVolume(streamType, index, flags, device, caller);
                index = mStreamStates[streamType].getIndex(device);
            }
    ....
    ....

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private boolean checkSafeMediaVolume(int streamType, int index, int device) {
        synchronized (mSafeMediaVolumeStateLock) {
            if ((mSafeMediaVolumeState == SAFE_MEDIA_VOLUME_ACTIVE) &&
                    (mStreamVolumeAlias[streamType] == AudioSystem.STREAM_MUSIC) &&
                    ((device & mSafeMediaVolumeDevices) != 0) &&
                    (index > safeMediaVolumeIndex(device))) {
                return false;
            }
            return true;
        }
    }

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    /*package*/ final int mSafeMediaVolumeDevices = AudioSystem.DEVICE_OUT_WIRED_HEADSET
            | AudioSystem.DEVICE_OUT_WIRED_HEADPHONE
            | AudioSystem.DEVICE_OUT_USB_HEADSET;

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public class VolumeDialog {
    ...
    private void showSafetyWarningH(int flags) {
        if ((flags & (AudioManager.FLAG_SHOW_UI | AudioManager.FLAG_SHOW_UI_WARNINGS)) != 0
                || mShowing) {
            synchronized (mSafetyWarningLock) {
                if (mSafetyWarning != null) {
                    return;
                }
                mSafetyWarning = new SafetyWarningDialog(mContext, mController.getAudioManager()) {
                    @Override
                    protected void cleanUp() {
                        synchronized (mSafetyWarningLock) {
                            mSafetyWarning = null;
                        }
                        recheckH(null);
                    }
                };
                mSafetyWarning.show();
            }
            recheckH(null);
        }
        rescheduleTimeoutH();
    }
    ...
}

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public void disableSafeMediaVolume(String callingPackage) {
        enforceVolumeController("disable the safe media volume");
        synchronized (mSafeMediaVolumeStateLock) {
            setSafeMediaVolumeEnabled(false, callingPackage);
            if (mPendingVolumeCommand != null) {
                onSetStreamVolume(mPendingVolumeCommand.mStreamType,
                                  mPendingVolumeCommand.mIndex,
                                  mPendingVolumeCommand.mFlags,
                                  mPendingVolumeCommand.mDevice,
                                  callingPackage);
                mPendingVolumeCommand = null;
            }
        }
    }