viewholder的使用

viewholder的使用

博客分类：  android 开发

 

Adapter的作用就是ListView界面与数据之间的桥梁，当列表里的每一项显示到页面时，都会调用Adapter的getView方法返回一个View。 

优化的思路两种： 

1. View的重用 
    View的每次创建是比较耗时的，因此对于getview方法传入的convertView应充分利用 != null的判断 

2.ViewHolder的应用 

View的findViewById()方法也是比较耗时的，因此需要考虑只调用一次，之后就用View.getTag()方法来获得ViewHolder对象。 


下面是优化后的代码供参考： 

 public class MyAdapter extends BaseAdapter {
     private Context mContext;
     private LayoutInflater mInflater;

     public MyAdapter(Context context) {
         this.mContext = context;
         mInflater = LayoutInflater.from(mContext);
     }

     public int getCount() {
         return mmsList.size();
     }

     public Object getItem(int arg0) {
         return arg0;
     }

     public long getItemId(int arg0) {
         return arg0;
     }

     public View getView(int position, View convertView, ViewGroup parent) {
         ViewHolder holder = null;
         if (convertView == null) {
             holder = new ViewHolder();
             convertView = mInflater.inflate(R.layout.mmslistitem, null);
             holder.img = (ImageView) convertView.findViewById(R.id.mmsflag);
             holder.subject = (TextView) convertView
                     .findViewById(R.id.mmssubject);
             holder.date = (TextView) convertView.findViewById(R.id.mmsdate);
             convertView.setTag(holder);
         } else {
             holder = (ViewHolder) convertView.getTag();
         }
         MMSDBItem mmsItem = mmsList.get(position);
         holder.subject.setText(mmsItem.getSubject());
         String today = DateFormat.format("yyyy-MM-dd",
                 System.currentTimeMillis()).toString();
         String dateStr = DateFormat.format("yyyy-MM-dd kk:mm",
                 mmsItem.getDate()).toString();
         holder.date.setText(dateStr);
         if (mmsItem.getFlag() == 0) {
             holder.img.setBackgroundResource(R.drawable.new_message_32);
         } else {
             holder.img
                     .setBackgroundResource(R.drawable.message_already_read_32);
         }
         return convertView;
     }

     public final class ViewHolder {
         public ImageView img;
         public TextView subject;
         public TextView date;
     }
 }