[原创] Android - ListView - 高效Adapter - EfficientAdapter - ListActivity

在做Android手机应用开发时， ListView是一个非常常用的控件。如何更新的使用它呢？其实SDK中的例子已经非常的完整了，并且能满足大多数的需要。

如果大家刚开始学习ListView，我建议大家还是直接先看官方的例子好了，这样大家会学到更好的写法以及养成更好的习惯。

下面就以EfficientAdapter为例，看看官网例子是如何使用ListView的：

请大家格外注意getView的书写方法，大家可能从网上也能找到过一些其它的例子，但是网上的写法和官网不同，建议大家采用官网例子的写法。

简要说明：要实现高效的Adapter，需要做两件事：

1. 重用getView()中的convertView，避免在不必要的时候inflating View。
2. 使用ViewHolder模式，避免在不必要的时候调用findViewById()。

顺便再提一句：若继承的是ListActivity，如果在layout xml里定义了ListView，那么该ListView的ID必须是”@id/android:list”，最好再包含一个ID是”@id/android:empty”的TextView，供ListView中没有数据时，显示提示文字用。如下所示：

<?xml version="1.0" encoding="utf-8"?>
 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
         android:orientation="vertical"
         android:layout_width="match_parent"
         android:layout_height="match_parent"
         android:paddingLeft="8dp"
         android:paddingRight="8dp">
     <ListView android:id="@id/android:list"
               android:layout_width="match_parent"
               android:layout_height="match_parent"
               android:background="#00FF00"
               android:layout_weight="1"
               android:drawSelectorOnTop="false"/>
     <TextView android:id="@id/android:empty"
               android:layout_width="match_parent"
               android:layout_height="match_parent"
               android:background="#FF0000"
               android:text="No data"/>
 </LinearLayout>

官网EfficientAdapter例子如下：

/**
* Demonstrates how to write an efficient list adapter. The adapter used in this example binds
* to an ImageView and to a TextView for each row in the list.
*
* 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
* - It uses the ViewHolder pattern to avoid calling findViewById() when it is not necessary
*
* 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 calls to findViewById() every time getView() is invoked.
*/
public class List14 extends ListActivity {
    private static class EfficientAdapter extends BaseAdapter {
        private LayoutInflater mInflater;
        private Bitmap mIcon1;
        private Bitmap mIcon2;
        public EfficientAdapter(Context context) {
// Cache the LayoutInflate to avoid asking for a new one each time.
            mInflater = LayoutInflater.from(context);
// Icons bound to the rows.
            mIcon1 = BitmapFactory.decodeResource(context.getResources(), R.drawable.icon48x48_1);
            mIcon2 = BitmapFactory.decodeResource(context.getResources(), R.drawable.icon48x48_2);
        }
/**
* The number of items in the list is determined by the number of speeches
* in our array.
*
* @see android.widget.ListAdapter#getCount()
*/
        public int getCount() {
            return DATA.length;
        }
/**
* Since the data comes from an array, just returning the index is
* sufficent to get at the data. If we were using a more complex data
* structure, we would return whatever object represents one row in the
* list.
*
* @see android.widget.ListAdapter#getItem(int)
*/
        public Object getItem(int position) {
            return position;
        }
/**
* Use the array index as a unique id.
*
* @see android.widget.ListAdapter#getItemId(int)
*/
        public long getItemId(int position) {
            return position;
        }
/**
* 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;
        }
    }
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setListAdapter(new EfficientAdapter(this));
    }
    private static final String[] DATA = {
"Abbaye de Belloc", "Abbaye du Mont des Cats", "Abertam"};
}