Android ListView之EfficientAdapte的使用详解
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2023-11-29 19:49:58
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?...
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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