@Override
  protected List<? extends BaseUser> doInBackground(Void... params) {
    if (adapter == null || microBlog == null) {
      return null;
    }

    List<? extends BaseUser> userList = null;
    boolean isFirstLoad = context.isFirstLoad();
    SocialGraphDao dao = new SocialGraphDao(context);
    try {
      paging = adapter.getPaging();
      if (!isFirstLoad && paging.moveToNext()) {
        if (relation == Relation.Followingship) {
          userList = dao.getFriends(user, paging);
        } else {
          userList = dao.getFollowers(user, paging);
        }
        if (paging.getPageIndex() == 1 && ListUtil.isEmpty(userList)) {
          isFirstLoad = true;
          context.setFirstLoad(isFirstLoad);
        }
      }

      SocialGraphCacheTask cacheTask = null;
      if (isFirstLoad) {
        // 第一次加载时,做一个缓冲task
        if (paging.getPageIndex() == 1 && adapter.getCount() == 0) {
          Paging<User> remotePaging = new Paging<User>();
          adapter.setPaging(remotePaging);
          // 缓冲远程数据;
          cacheTask = new SocialGraphCacheTask(context, account, relation);
        }

        paging = adapter.getPaging();
        if (paging.moveToNext()) {
          if (relation == Relation.Followingship) {
            userList = microBlog.getFriends(paging);
          } else {
            userList = microBlog.getFollowers(paging);
          }
        }
      } else if (paging.getPageIndex() == 1) {
        // 防止新关注,缓冲第一次。
        cacheTask = new SocialGraphCacheTask(context, account, relation);
        cacheTask.setCycleTime(1);
        cacheTask.setPageSize(20);
      }
      if (cacheTask != null) {
        cacheTask.execute();
      }
    } catch (LibException e) {
      if (Constants.DEBUG) Log.e(TAG, "Task", e);
      resultMsg = ResourceBook.getStatusCodeValue(e.getExceptionCode(), context);
      paging.moveToPrevious();
    }

    return userList;
  }
  @Override
  protected void onPostExecute(List<? extends BaseUser> result) {
    if (ListUtil.isNotEmpty(result)) {
      adapter.addCacheToDivider(null, (List<User>) result);
    } else {
      adapter.notifyDataSetChanged();

      if (resultMsg != null) {
        Toast.makeText(adapter.getContext(), resultMsg, Toast.LENGTH_LONG).show();
      }
    }

    if (paging.hasNext()) {
      ((UserQuickSelectorActivity) context).showMoreFooter();
    } else {
      ((UserQuickSelectorActivity) context).showNoMoreFooter();
    }
  }
 @Override
 protected void onPreExecute() {
   context.showLoadingFooter();
 }