【移动开发】ListView的setSelection()和setSelectionFromTop()联系

1. 【移动开发】ListView的setSelection()和setSelectionFromTop()联系
2. 通常，app中的数据都是以ListView的形式展示的。默认地，把“新”数据添加到数据列表的尾部。
3. 但是，如果是IM类型的app，比如查看历史消息这个模块。新数据并不是插到数据列表的尾部，而是插到数据列表的头部。
4. 要实现比较好的用户体验，需要保持当前的ListView的位置。换句话说，如果我们能够随心所欲地指定ListView滚动的位置，那么这个问题就迎刃而解。
5. 在ListView中，有一个setSelectionFromTop()方法，下面是一个使用范例。代码如下

6. java] view plain copy
   @Override
   publicvoid loaded(Long loadTime,int thisPageNumber,boolean isFromZero,boolean isHasMoreToLoad,List data){
   refreshComplete();
   checkIfHasMoreToLoad(isHasMoreToLoad);
   if(thisPageNumber !=1){// 不是第一页
   mListView.setSelectionFromTop(5+2, mIMPullToRefreshListView.getHeaderHeight());
   mIMPullToRefreshListView.getHeaderView().setVisibility(View.GONE);
   }
   }
   看一下setSelectionFromTop()的具体实现，代码如下：
   [java] view plain copy
   /**
   * Sets the selected item and positions the selection y pixels from the top edge
   * of the ListView. (If in touch mode, the item will not be selected but it will
   * still be positioned appropriately.)
   *
   * @param position Index (starting at 0) of the data item to be selected.
   * @param y The distance from the top edge of the ListView (plus padding) that the
   * item will be positioned.
   */
   publicvoid setSelectionFromTop(int position,int y){
   if(mAdapter ==null){
   return;
   }
   if(!isInTouchMode()){
   position = lookForSelectablePosition(position,true);
   if(position >=0){
   setNextSelectedPositionInt(position);
   }
   }else{
   mResurrectToPosition = position;
   }
   if(position >=0){
   mLayoutMode = LAYOUT_SPECIFIC;
   mSpecificTop = mListPadding.top + y;
   if(mNeedSync){
   mSyncPosition = position;
   mSyncRowId = mAdapter.getItemId(position);
   }
   requestLayout();
   }
   }
   从上面的代码可以得知，setSelectionFromTop()的作用是设置ListView选中的位置，同时在Y轴设置一个偏移量（padding值）。
   ListView还有一个方法叫setSelection()，传入一个index整型数值，就可以让ListView定位到指定Item的位置。
   这两个方法有什么区别呢？看一下setSelection()的具体实现，代码如下：
   [java] view plain copy
   /**
   * Sets the currently selected item. If in touch mode, the item will not be selected
   * but it will still be positioned appropriately. If the specified selection position
   * is less than 0, then the item at position 0 will be selected.
   *
   * @param position Index (starting at 0) of the data item to be selected.
   */
   @Override
   publicvoid setSelection(int position){
   setSelectionFromTop(position,0);
   }
   原来，setSelection()内部就是调用了setSelectionFromTop()，只不过是Y轴的偏移量是0而已。
   现在应该对setSelection()和setSelectionFromTop()有了更深刻的认识了。
   

    

8. 参考资料
9. http://developer.android.com/reference/android/widget/ListView.html#setSelection%28int%29
10. http://www.cnblogs.com/over140/archive/2013/05/20/2948239.html
11. http://blog.csdn.net/jdsjlzx/article/details/17794209
12. http://blog.csdn.net/a859522265/article/details/8154103
14. 来源： http://blog.csdn.net/manoel/article/details/39183025