ViewHolder的另一種寫法

最標準的寫法，就是爲每一個AdapterView的子View新建一個對應的ViewHolder，同時聲明爲prtivate static。

@Override
public View getView(int position, View convertView, ViewGroup parent) {
    ViewHolder holder;
    if (convertView == null) {
        convertView = inflater.inflate(R.layout.list_item, null);
        holder = new ViewHolder();
        ……
        convertView.setTag(holder);
    } else {
        holder = (ViewHolder)convertView.getTag();
    }
}

private static class ViewHolder{
    TextView name;
    TextView age;
    TextView sex;
}

這樣的話，每次新建BaseAdapter都得新增加ViewHolder，略顯麻煩。下面介紹一種愜意的書寫方法，寫一個工具類，等價於官方推薦的做法。

public class ViewHolder{
    //I added a generic return type to reduce the casting noise in client code
    public static <T extends View>T get(View view,int id){
        SparseArray<View> viewHolder = (SparseArray<View>)view.getTag();
        if(viewHolder == null){
            viewHolder = new SparseArray<View>();
            view.setTag(viewHolder);
        }
        View childView = viewHolder.get(id);
        if(childView == null){
            childView = view.findViewById(id);
            viewHolder.put(id,childView);
        }
        return (T)childView;
    }
}

然後就是在BaseAdapter的getView中使用。

@Override
public View getView(int position,View convertView,ViewGroup parent){
    if(convertView == null){
        convertView = mInflater.inflate(R.layout.item_hotel_list,parent,false);
    }
    ImageView img = ViewHolder.get(convertView,R.id.item_hotel_list_img);
    return convertView;
}