Android ListView 适配器（Adapter）优化

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;
         }
     }