ListView为什么要使用convertView和ViewHolder

原因就是：

1. 使用convertView可以避免重复地调用inflate

2. 使用ViewHolder可以避免重复地调用findViewById

下面的getView代码来自于

 development/samples/ApiDemos/src/com/example/android/apis/view/List14.java

 /**
          * Make a view to hold each row.
          *
          * @see android.widget.ListAdapter#getView(int, android.view.View,
          *      android.view.ViewGroup)
          */
         public View getView(int position, View convertView, ViewGroup parent) {
             // A ViewHolder keeps references to children views to<span style="color:#ff0000;"> avoid unneccessary calls
             // to findViewById()</span> on each row.
             ViewHolder holder;

             // When convertView is not null, we can reuse it directly, <span style="color:#ff0000;">there is no need
             // to reinflate it.</span> We only inflate a new View when the convertView supplied
             // by ListView is null.
             if (convertView == null) {
                 convertView = mInflater.inflate(R.layout.list_item_icon_text, null);

                 // Creates a ViewHolder and store references to the two children views
                 // we want to bind data to.
                 holder = new ViewHolder();
                 holder.text = (TextView) convertView.findViewById(R.id.text);
                 holder.icon = (ImageView) convertView.findViewById(R.id.icon);

                 convertView.setTag(holder);
             } else {
                 // Get the ViewHolder back to get fast access to the TextView
                 // and the ImageView.
                 holder = (ViewHolder) convertView.getTag();
             }

             // Bind the data efficiently with the holder.
             holder.text.setText(DATA[position]);
             holder.icon.setImageBitmap((position & 1) == 1 ? mIcon1 : mIcon2);

             return convertView;
         }