Adapter ：适配器

链接后端数据与前端显示适配接口，是数据（data）与UI（View）之间一个重要纽带，常见的View（ListView，GridView）等地方都会使用到Adapter，下图直观表达了Data、Adapter、View三者关系：

比较常用Adapter ： BaseAdapter

使用BaseAdapter需要重写如下方法，

class MyAdapter extends BaseAdapter
    {
        private Context context;
        public MyAdapter(Context context)
        {
            this.context = context;
        }
        @Override
        public int getCount() {
            // How many items are in the data set represented by this Adapter.(在此适配器中所代表的数据集中的条目数)
            return 0;
        }
        @Override
        public Object getItem(int position) {
            // Get the data item associated with the specified position in the data set.(获取数据集中与指定索引对应的数据项)
            return null;
        }
        @Override
        public long getItemId(int position) {
            // Get the row id associated with the specified position in the list.(取在列表中与指定索引对应的行id)
            return 0;
        }
        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            // Get a View that displays the data at the specified position in the data set.
            return null;
        }
                                                                                                                            
    }

其中getView（）方法很重要，涉及到内存优化与回收，需要引入缓冲contentView（缓存contentView的方式可以判断如果缓存中不存在View才创建View，如果已经存在可以利用缓存中的View，提升了性能）与静态类ViewHolder

//在外面先定义，ViewHolder静态类
static class ViewHolder
{
    public ImageView img;
    public TextView title;
    public TextView info;
}
//然后重写getView
@Override
public View getView(int position, View convertView, ViewGroup parent) {
    ViewHolder holder;
    if(convertView == null)
    {
        holder = new ViewHolder();
        convertView = mInflater.inflate(R.layout.list_item, null);
        holder.img = (ImageView)item.findViewById(R.id.img)
        holder.title = (TextView)item.findViewById(R.id.title);
        holder.info = (TextView)item.findViewById(R.id.info);
        convertView.setTag(holder);
    }else
    {
        holder = (ViewHolder)convertView.getTag();
    }
        holder.img.setImageResource(R.drawable.ic_launcher);
        holder.title.setText("Hello");
        holder.info.setText("World");
    }
                                                                                                
    return convertView;
}