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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
我要评论
<?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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