public class MainActivity extends Activity implements OnScrollListener{
    private ProgressBar loadInfo;
    private ListView listView;
    private LinearLayout loadLayout;
    private ArrayList<string> items;
    private DatabaseService service;
    private int currentPage = 1; //默认在第一页
    private static final int lineSize =10;    //每次显示数
    private int allRecorders = 0;  //全部记录数
    private int pageSize = 1;  //默认共一页
    private Aleph0 baseAdapter;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        listView = (ListView) findViewById(R.id.listview);
        service = new DatabaseService(this);
        //创建一个角标线性布局用来显示正在加载
        loadLayout = new LinearLayout(this);
        loadLayout.setGravity(Gravity.CENTER);
        //定义一个ProgressBar表示“正在加载”
        loadInfo = new ProgressBar(this,null,android.R.attr.progressBarStyleSmall);
        //增加组件
        loadLayout.addView(loadInfo, new LayoutParams(LinearLayout.LayoutParams.MATCH_PARENT, LinearLayout.LayoutParams.WRAP_CONTENT));
        //增加到listView头部
        listView.addHeaderView(loadLayout);
        listView.setOnScrollListener(this);
         
        showAllData();
         
         
    }
     
    /**
     * 读取全部数据
     */
    public void showAllData(){
         
        allRecorders = service.getCount();
        //计算总页数
        pageSize = (allRecorders + lineSize -1) / lineSize; 
        items = service.getAllItems(currentPage, lineSize);
        Collections.reverse(items);  //倒序
        baseAdapter = new Aleph0();
        listView.setAdapter(baseAdapter);
        listView.setSelection(items.size());//直接定位到最底部
    }
    int firstItem=-1;
 
    @Override
    public void onScroll(AbsListView absView, int firstVisibleItem,
            int visibleItemCount, int totalItemCount) {
        firstItem=firstVisibleItem;
    }
    @Override
    public void onScrollStateChanged(AbsListView view, int scorllState) {
        if (firstItem==0&& currentPage < pageSize&& scorllState == OnScrollListener.SCROLL_STATE_IDLE) {// 不再滚动
            Log.e(log, 滑到第一位);
            currentPage++;
            // 增加数据
            appendDate();
        }
    }
     
    /**
     * 增加数据
     */
    private void appendDate(){
        final ArrayList<string> additems = service.getAllItems(currentPage, lineSize);
        Collections.reverse(additems);
        baseAdapter.setCount(baseAdapter.getCount() + additems.size());
        //判断，如果到了最末尾则去掉“正在加载”
        if(allRecorders == baseAdapter.getCount()){
            listView.removeHeaderView(loadLayout);
        }
        items.addAll(0, additems);
         
        baseAdapter.notifyDataSetChanged();
        listView.setSelection(additems.size());
         
    }
     
    class Aleph0 extends BaseAdapter {
        int count = lineSize;
 
        public int getCount() {
            return count;
        }
         
        public void setCount(int count){
            this.count = count;
        }
 
        public Object getItem(int pos) {
            return pos;
        }
 
        public long getItemId(int pos) {
            return pos;
        }
 
        public View getView(int pos, View v, ViewGroup p) {
            TextView view = new TextView(MainActivity.this);
            view.setTextSize(60);
            if(items != null){
                view.setText(items.get(pos));
            }else{
                view.setText(pos);
            }
            return view;
        }
    }
     
     
 
}

    /**
     * 刷新视图 *
     */
    public void setList(List<ChatMode.RecentLetter> bcData) {
        Collections.reverse(bcData);
        this.listMsg = bcData;
        notifyDataSetInvalidated();
        lv.setSelection(bcData.size());

    }

    /**
     * 加载更多 *
     */
    public void addList(final List<ChatMode.RecentLetter> newMsgData) {
        if (this.listMsg != null) {
            Collections.reverse(newMsgData);
            this.listMsg.addAll(0, newMsgData);
            notifyDataSetChanged();
            lv.setSelection(newMsgData.size() + 1);
        } else {
            setList(newMsgData);
        }
    }