SectionIndexer中的getSectionForPosition()与getPositionForSection()解惑

转自http://blog.csdn.net/jack_l1/article/details/14165291

大家在做字母索引的时候常常会用到SectionIndexer这个类，里面有2个重要的方法

1. getSectionForPosition（）通过该项的位置，获得所在分类组的索引号

2. getPositionForSection() 根据分类列的索引号获得该序列的首个位置

getSectionForPosition(0) 返回 0
getSectionForPosition(1) 返回 0
getSectionForPosition(2) 返回 0
getSectionForPosition(3) 返回 1
这下，明白这两个方法的区别了把

所以一般有如下代码，来确定是否滑动到了分类的首字母位置

public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount,    
                  int totalItemCount) {    
              int section = indexer.getSectionForPosition(firstVisibleItem);  //获得索引号 
              int nextSecPosition = indexer.getPositionForSection(section + 1);  //根据索引号获得下一个索引的初始位置 
              if (firstVisibleItem != lastFirstVisibleItem) {    
                  MarginLayoutParams params = (MarginLayoutParams) titleLayout.getLayoutParams();    
                  params.topMargin = 0;    
                  titleLayout.setLayoutParams(params);    
                  title.setText(String.valueOf(alphabet.charAt(section)));    
              }    
              if (nextSecPosition == firstVisibleItem + 1) {  //若是下一个索引号的位置与当前所见的项+1，则进行位移 
                  View childView = view.getChildAt(0);    
                  if (childView != null) {    
                      int titleHeight = titleLayout.getHeight();    
                      int bottom = childView.getBottom();    
                      MarginLayoutParams params = (MarginLayoutParams) titleLayout    
                              .getLayoutParams();    
                      if (bottom < titleHeight) {    
                          float pushedDistance = bottom - titleHeight;    
                          params.topMargin = (int) pushedDistance;    
                          titleLayout.setLayoutParams(params);    
                      } else {    
                          if (params.topMargin != 0) {    
                              params.topMargin = 0;    
                              titleLayout.setLayoutParams(params);    
                          }    
                      }    
                  }    
              }    
              lastFirstVisibleItem = firstVisibleItem;    
          }    
      });