Android ListView之EfficientAdapte的使用詳解
Android ListView之EfficientAdapte的使用詳解
在做Android手機(jī)應(yīng)用開發(fā)時(shí), ListView是一個(gè)非常常用的控件。如何更新的使用它呢?其實(shí)SDK中的例子已經(jīng)非常的完整了,并且能滿足大多數(shù)的需要。
如果大家剛開始學(xué)習(xí)ListView,我建議大家還是直接先看官方的例子好了,這樣大家會(huì)學(xué)到更好的寫法以及養(yǎng)成更好的習(xí)慣。
下面就以EfficientAdapter為例,看看官網(wǎng)例子是如何使用ListView的:
請(qǐng)大家格外注意getView的書寫方法,大家可能從網(wǎng)上也能找到過一些其它的例子,但是網(wǎng)上的寫法和官網(wǎng)不同,建議大家采用官網(wǎng)例子的寫法。
簡(jiǎn)要說明:要實(shí)現(xiàn)高效的Adapter,需要做兩件事:
1. 重用getView()中的convertView,避免在不必要的時(shí)候inflating View。
2. 使用ViewHolder模式,避免在不必要的時(shí)候調(diào)用findViewById()。
順便再提一句:若繼承的是ListActivity,如果在layout xml里定義了ListView,那么該ListView的ID必須是"@id/android:list",最好再包含一個(gè)ID是"@id/android:empty"的TextView,供ListView中沒有數(shù)據(jù)時(shí),顯示提示文字用。如下所示:
Xml代碼
<?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>
官網(wǎng)EfficientAdapter例子如下:
Java代碼
/**
* 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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