listview優化，convertview重複使用，並使用viewholder

listview中的優化，不只包含convertview的重用，並且要使用viewholder讓convertview中的各個組件也均可以重用，若是每次使用findViewById，效率會下降不少。下面是代碼：

若是在getView中存在按鈕的點擊事件，也應該再getview以外定義好，而後再getView中重用，而不是在getView中每次new一個。

public View getView(int postion, View convertView, ViewGroup parent) {
 ViewHolder holder = null;
 if(convertView == null){
 convertView = LayoutInflater.from(getActivity().getApplicationContext()).inflate(R.layout.kedan_yixiadan_listitem, null);
 holder = new ViewHolder();
 holder.caiming = (TextView)convertView.findViewById(R.id.caiming);
 holder.jiage = (TextView)convertView.findViewById(R.id.jiage);
 holder.shuliang = (TextView)convertView.findViewById(R.id.yidian_cai_shuliang);
 holder.beizhuBtn = (Button)convertView.findViewById(R.id.beizhu_item_btn);
 holder.zhangdanPinpan = (LinearLayout)convertView.findViewById(R.id.zhangdan_pinpan);
 holder.zhangdanTaocan = (LinearLayout)convertView.findViewById(R.id.zhangdan_taocan);
 holder.zhangdanZengcai = (LinearLayout)convertView.findViewById(R.id.zhangdan_zengcai);
 convertView.setTag(holder);
 }else{
 holder = (ViewHolder)convertView.getTag();
 }
 
 YidianCais cai = (YidianCais)yixiadanList.get(postion);
 holder.caiming.setText(cai.getCnName());
 holder.shuliang.setText(cai.getNum() + " 份");
 holder.jiage.setText(cai.getPrice() + " 元/" + cai.getUnit());
 
 holder.beizhuBtn.setTag(cai.getCnName());
 holder.beizhuBtn.setTag(R.id.tag_id, cai.getId());
 holder.beizhuBtn.setTag(R.id.tag_name, cai.getCnName());  return convertView;
 }

private class ViewHolder{
    	private TextView caiming;
    	private TextView jiage;
    	private TextView shuliang;
    	private Button beizhuBtn;
    	private TextView jiageUnit;
    	private LinearLayout zhangdanPinpan;
    	private LinearLayout zhangdanTaocan;
    	private LinearLayout zhangdanZengcai;
    	private Button zhangdanJianBtn;
    	private Button zhangdanJiaBtn;
    }