Android ListView之EfficientAdapte的使用详解

Android ListView之EfficientAdapte的使用详解

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代码

<?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：

Java代码

/**
 * 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"};
}

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