在 Android中有很多时候会选择用ListView加载数据，有的是分批加载，比如每次加载20条，100条就要加载5次，如果一次性加载100 条，ListView加载就会变慢，如果是有图片的话，一是浪费流量，二是item中带图片会出现错位的问题l，ListView加载数据目前好多App 的做法
1. 分批加载，滑动到底部自动更新
2. 滑动到底部，手动的点击加载更多
3. 下拉刷新 + 底部加载更多
今天实现ListView滑动到底部自动更新的效果。
原理：监听ListView的滑动事件,判断ListView是否滑动到底部,然后去加载数据。
    public class MainActivity extends Activity { 
        private ListView listview; 
        private List<String> datas; 
        private LayoutInflater inflater; 
        private MyAdapter adapter; 
        private List<String>  contents; 
        private int count = 0; 
        private View footView; 
        private Handler handler = new Handler(); 
        int lastItem; 
        private RelativeLayout loading; 
        @Override 
        protected void onCreate(Bundle savedInstanceState) { 
            super.onCreate(savedInstanceState); 
            setContentView(R.layout.activity_main); 
            initData(); 
            inflater = LayoutInflater.from(this); 
            listview = (ListView) findViewById(R.id.listview); 
            adapter = new MyAdapter(); 
            footView = inflater.inflate(R.layout.footer, null); 
            loading = (RelativeLayout) footView.findViewById(R.id.loading); 
            //listview的addFooterView()添加view到listview底部一定要加在listview.setAdapter(adapter);这代码前面 
            listview.addFooterView(footView); 
            listview.setAdapter(adapter); 
            //添加listview滚动监听 
            listview.setOnScrollListener(new OnScrollListener() { 
                //AbsListView view 这个view对象就是listview 
                @Override 
                public void onScrollStateChanged(AbsListView view, int scrollState) { 
                    if (scrollState == OnScrollListener.SCROLL_STATE_IDLE) { 
                        if (view.getLastVisiblePosition() == view.getCount() - 1) { 
                            loadData(); 
                        } 
                    } 
                } 
                @Override 
                public void onScroll(AbsListView view, int firstVisibleItem, 
                        int visibleItemCount, int totalItemCount) { 
                       lastItem = firstVisibleItem + visibleItemCount - 1 ; 
                } 
            }); 
        } 
        protected void loadData() { 
            loading.setVisibility(View.VISIBLE); 
            handler.postDelayed(new Runnable() { 
                @Override 
                public void run() { 
                     load(); 
                     loading.setVisibility(View.GONE); 
                     adapter.notifyDataSetChanged(); 
                } 
            }, 5000); 
        } 
        protected void load() { 
            int count = adapter.getCount() + 1; 
            for(int i = count; i < count + 20; i++) {  //每次加载20条数据
                 contents.add("加载数据:::" + i); 
            } 
        } 
        private void initData() { 
            contents = new ArrayList<String>(); 
            for(int i = 1; i < 20; i++){ 
                contents.add("加载数据:::" + i); 
            } 
        } 
         
        class MyAdapter extends BaseAdapter{ 
            @Override 
            public int getCount() { 
                return contents.size(); 
            } 
            @Override 
            public Object getItem(int position) { 
                return contents.get(position); 
            } 
     
            @Override 
            public long getItemId(int position) { 
                return position; 
            } 
     
            @Override 
            public View getView(int position, View convertView, ViewGroup parent) { 
                ViewHolder holder; 
                if(convertView == null){ 
                    convertView = inflater.inflate(R.layout.item, parent, false); 
                    holder = new ViewHolder(); 
                    holder.tvContent = (TextView) convertView.findViewById(R.id.tvContent); 
                    convertView.setTag(holder); 
                }else{ 
                    holder = (ViewHolder) convertView.getTag(); 
                } 
                holder.tvContent.setText(contents.get(position)); 
                return convertView; 
            } 
            class ViewHolder{ 
                TextView tvContent; 
            } 
        } 
    }