android view holder 优化

android 一般都用viewholder来优化contentView，采用sparseArray能够进一步优化

/**
   * 用法： ImageView bananaView = ViewHolder.get(convertView, R.id.banana);
   * 
   * @param convertView
   * @param id
   * @return
   */
  @SuppressWarnings("unchecked")
  public static <T extends View> T getView(View convertView, int id) {
    SparseArray<View> viewHolder = (SparseArray<View>) convertView.getTag();
    if (viewHolder == null) {
      viewHolder = new SparseArray<View>();
      convertView.setTag(viewHolder);
    }
    View childView = viewHolder.get(id);
    if (childView == null) {
      childView = convertView.findViewById(id);
      viewHolder.put(id, childView);
    }
    return (T) childView;
  }