Adapter(16)关于Adapter中getView与ViewHolder的更正

更正：

之前一直喜欢在getview里不用holder，而是直接用contentView.findViewById(xxx)； 这和种写法不好，每次都要findview。 

下面是正确的写法：

        /**
         * 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 avoid unneccessary calls
            // to findViewById() on each row.
            ViewHolder holder;

            // When convertView is not null, we can reuse it directly, there is no need
            // to reinflate it. 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;
        }

        static class ViewHolder {
            TextView text;
            ImageView icon;
        }

 

原理

1，数据过多怎么办？

　　数据过多时，不用给每条数据都new一个view，可利用convertView，如果它不空，就可以直接更新它显示的数据为第position条就可，空再构造它。

2，View的setTag方法是什么回事？

　　setTag方法是干什么的，SDK解释为

Tags
Unlike IDs, tags are not used to identify views. Tags are essentially an extra piece of information that can be associated with a view. They are most often used as a convenience to store data related to views in the views themselves rather than by putting them in a separate structure.

　　Tag不像ID是用标示view的。Tag从本质上来讲是就是相关联的view的额外的信息。它们经常用来存储一些view的数据，这样做非常方便而不用存入另外的单独结构。

3，ViewHolder怎么回事？ 

　　如果convertView不空，就可以直接更新数据了，那么就要TextView t = convertView.findViewById(xxx)，然后再更新t，

　　如果把convertView构造时已经find到的的TextView 以额外数据的形式保存在convertView的tag中，下次就不用findViewById了。