ListView的优化

***ListView优化
*优化一：因为getView()方法中每次都将布局重新加载了一遍，所以当ListView快速滚动的时候会影响效率
解决方法：可利用convertView参数将之前加载好的布局缓存
*优化二：在getView()方法中每次都要调用View的findViewById()方法获取一次控件的实例
解决方法：可通过新增一个类，用于对控件实例的缓存

public class MainActivity extends Activity {
	private ListView lv;
	private List<String> list;

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		lv = (ListView) findViewById(R.id.lv);
		
		list = new ArrayList<String>();
		for (int i = 0; i < 10; i++) {
			list.add("item"+i);
		}
		
		MyAdapter adapter = new MyAdapter();
		lv.setAdapter(adapter);
		
	}
	
	class MyAdapter extends BaseAdapter{
		@Override
		public int getCount() {
			// TODO Auto-generated method stub
			return list.size();
		}

		@Override
		public Object getItem(int position) {
			// TODO Auto-generated method stub
			return list.get(position);
		}

		@Override
		public long getItemId(int position) {
			// TODO Auto-generated method stub
			return position;
		}

		@Override
		public View getView(int position, View convertView, ViewGroup parent) {
			ViewHolder holder = null;
			if(convertView==null){
				LayoutInflater inflater = (LayoutInflater) getSystemService(Context.LAYOUT_INFLATER_SERVICE);//获得布局填充器对象
				convertView  = inflater.inflate(R.layout.item_test, null);	//将item_test这个xml布局文件实例化成对应的view对象
				holder  = new ViewHolder();
				holder.tv = (TextView) convertView.findViewById(R.id.tv);
				convertView.setTag(holder);//利用setTag()方法对可复用的holder添加标记方便获取
			}else{
				holder = (ViewHolder) convertView.getTag();
			}
			holder.tv.setText(list.get(position));		
			return convertView;
		}	
	}
	//新定义一个内部类，用于对控件的实例进行缓存
	static class ViewHolder{
		TextView tv;
	}
}