StorageNotification继承于SystemUI,主要处理Storage事件，比如Storage插拔等。

在StorageNotfication的start函数，会去注册StorageEventListener。

 @Override
    public void start() {
        mNotificationManager = mContext.getSystemService(NotificationManager.class);

        mStorageManager = mContext.getSystemService(StorageManager.class);
        mStorageManager.registerListener(mListener);

对于U盘或SD卡插拔事件，我们常用的是通过广播的方式去接收，具体可以参考：

http://blog.csdn.net/zhudaozhuan/article/details/7199799

StorageEventListener的实现如下：

    private final StorageEventListener mListener = new StorageEventListener() {
        @Override
        public void onVolumeStateChanged(VolumeInfo vol, int oldState, int newState) {
            onVolumeStateChangedInternal(vol);
        }

        @Override
        public void onVolumeRecordChanged(VolumeRecord rec) {
            // Avoid kicking notifications when getting early metadata before
            // mounted. If already mounted, we're being kicked because of a
            // nickname or init'ed change.
            final VolumeInfo vol = mStorageManager.findVolumeByUuid(rec.getFsUuid());
            if (vol != null && vol.isMountedReadable()) {
                onVolumeStateChangedInternal(vol);
            }
        }

        @Override
        public void onVolumeForgotten(String fsUuid) {
            // Stop annoying the user
            mNotificationManager.cancelAsUser(fsUuid, PRIVATE_ID, UserHandle.ALL);
        }

        @Override
        public void onDiskScanned(DiskInfo disk, int volumeCount) {
            onDiskScannedInternal(disk, volumeCount);
        }
    };

StorageEventListener中还有其他的call back函数，对于外置存储设备插拔，我觉得这两个函数应该都可以拿到对应的事件：

public void onStorageStateChanged(String path, String oldState, String newState)
public void onVolumeStateChanged(VolumeInfo vol, int oldState, int newState)

在StorageNotification使用的是onVolumeStateChanged和onVolumeRecordChanged

  private void onVolumeStateChangedInternal(VolumeInfo vol) {
        switch (vol.getType()) {
            case VolumeInfo.TYPE_PRIVATE:
                onPrivateVolumeStateChangedInternal(vol);
                break;
            case VolumeInfo.TYPE_PUBLIC:
                onPublicVolumeStateChangedInternal(vol);
                break;
        }
    }

TYPE_PRIVATE和TYPE_PUBLIC没有详细的解释，对于U盘TYPE_PUBLIC。

 private void onPublicVolumeStateChangedInternal(VolumeInfo vol) {
        Log.d(TAG, "Notifying about public volume: " + vol.toString());

        final Notification notif;
        switch (vol.getState()) {
            case VolumeInfo.STATE_UNMOUNTED:
                notif = onVolumeUnmounted(vol);
                break;
            case VolumeInfo.STATE_CHECKING:
                notif = onVolumeChecking(vol);
                break;
            case VolumeInfo.STATE_MOUNTED:
            case VolumeInfo.STATE_MOUNTED_READ_ONLY:
                notif = onVolumeMounted(vol);
                showMMDialog();
                break;
            case VolumeInfo.STATE_FORMATTING:
                notif = onVolumeFormatting(vol);
                break;
            case VolumeInfo.STATE_EJECTING:
                notif = onVolumeEjecting(vol);
                break;
            case VolumeInfo.STATE_UNMOUNTABLE:
                notif = onVolumeUnmountable(vol);
                break;
            case VolumeInfo.STATE_REMOVED:
                notif = onVolumeRemoved(vol);
                break;
            case VolumeInfo.STATE_BAD_REMOVAL:
                notif = onVolumeBadRemoval(vol);
                break;
            default:
                notif = null;
                break;
        }

VolumeInfo记录了外置存储设备的状态，根据不同的状态，进行相应的处理。