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;
}
}
To work efficiently the adapter implemented here uses two techniques:
-It reuses the convertView passed to getView() to avoid inflating View when it is not necessary
（译:重用缓存convertView传递给getView()方法来避免填充不必要的视图）
-It uses the ViewHolder pattern to avoid calling findViewById() when it is not necessary
（译：使用ViewHolder模式来避免没有必要的调用findViewById()：因为太多的findViewById也会影响性能）
ViewHolder类的作用
-The ViewHolder pattern consists in storing a data structure in the tag of the view
returned by getView().This data structures contains references to the views we want to bind data to,
thus avoiding calling to findViewById() every time getView() is invoked
public class Demo17Activity extends Activity {
private ListView lv;
private List<Map<String, Object>> data;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
lv = (ListView)findViewById(R.id.lv);
//获取将要绑定的数据设置到data中
data = getData();
MyAdapter adapter = new MyAdapter(this);
lv.setAdapter(adapter);
}
private List<Map<String, Object>> getData()
{
List<Map<String, Object>> list = new ArrayList<Map<String, Object>>();
Map<String, Object> map;
for(int i=0;i<10;i++)
{
map = new HashMap<String, Object>();
map.put("img", R.drawable.ic_launcher);
map.put("title", "跆拳道");
map.put("info", "快乐源于生活...");
list.add(map);
}
return list;
}
//ViewHolder静态类
static class ViewHolder
{
public ImageView img;
public TextView title;
public TextView info;
}
public class MyAdapter extends BaseAdapter
{
private LayoutInflater mInflater = null;
private MyAdapter(Context context)
{
//根据context上下文加载布局，这里的是Demo17Activity本身，即this
this.mInflater = LayoutInflater.from(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 data.size();
}
@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 position;
}
@Override
public long getItemId(int position) {
//Get the row id associated with the specified position in the list.
//获取在列表中与指定索引对应的行id
return position;
}
//Get a View that displays the data at the specified position in the data set.
//获取一个在数据集中指定索引的视图来显示数据
@Override
public View getView(int position, View convertView, ViewGroup parent) {
ViewHolder holder = null;
//如果缓存convertView为空，则需要创建View
if(convertView == null)
{
holder = new ViewHolder();
//根据自定义的Item布局加载布局
convertView = mInflater.inflate(R.layout.list_item, null);
holder.img = (ImageView)convertView.findViewById(R.id.img);
holder.title = (TextView)convertView.findViewById(R.id.tv);
holder.info = (TextView)convertView.findViewById(R.id.info);
//将设置好的布局保存到缓存中，并将其设置在Tag里，以便后面方便取出Tag
convertView.setTag(holder);
}else
{
holder = (ViewHolder)convertView.getTag();
}
holder.img.setBackgroundResource((Integer)data.get(position).get("img"));
holder.title.setText((String)data.get(position).get("title"));
holder.info.setText((String)data.get(position).get("info"));
return convertView;
}
}
}
目的
减少findViewById()使用，如果不用viewholder的话，So if you have 1000 rows in List and 990 rows will be out of View then 990 times will be called findViewById() again.
Holder design pattern is used for View caching - Holder (arbitrary) object holds child widgets of each row and when row is out of View then findViewById() won’t be called but View will be recycled and widgets will be obtained from Holder.
public class ViewHolder {
// I added a generic return type to reduce the casting noise in client code
@SuppressWarnings("unchecked")
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;
}
}