Android实现ListView异步加载图片的方法

本文实例讲述了android实现listview异步加载图片的方法。分享给大家供大家参考。具体如下：

listview异步加载图片是非常实用的方法，凡是是要通过网络获取图片资源一般使用这种方法比较好，用户体验好，不用让用户等待下去，下面就说实现方法，先贴上主方法的代码：

package cn.wangmeng.test;
import java.io.ioexception;
import java.io.inputstream;
import java.lang.ref.softreference;
import java.net.malformedurlexception;
import java.net.url;
import java.util.hashmap;
import android.graphics.drawable.drawable;
import android.os.handler;
import android.os.message;
public class asyncimageloader {
 private hashmap<string, softreference<drawable>> imagecache;
 public asyncimageloader() {
  imagecache = new hashmap<string, softreference<drawable>>();
  }
 public drawable loaddrawable(final string imageurl, final imagecallback imagecallback) {
  if (imagecache.containskey(imageurl)) {
   softreference<drawable> softreference = imagecache.get(imageurl);
   drawable drawable = softreference.get();
   if (drawable != null) {
   return drawable;
   }
  }
  final handler handler = new handler() {
   public void handlemessage(message message) {
   imagecallback.imageloaded((drawable) message.obj, imageurl);
   }
  };
  new thread() {
   @override
   public void run() {
   drawable drawable = loadimagefromurl(imageurl);
   imagecache.put(imageurl, new softreference<drawable>(drawable));
   message message = handler.obtainmessage(0, drawable);
   handler.sendmessage(message);
   }
  }.start();
  return null;
  }
 public static drawable loadimagefromurl(string url) {
  url m;
  inputstream i = null;
  try {
  m = new url(url);
  i = (inputstream) m.getcontent();
  } catch (malformedurlexception e1) {
  e1.printstacktrace();
  } catch (ioexception e) {
  e.printstacktrace();
  }
  drawable d = drawable.createfromstream(i, "src");
  return d;
 }
 public interface imagecallback {
  public void imageloaded(drawable imagedrawable, string imageurl);
  }
}

以上代码是实现异步获取图片的主方法，softreference是软引用，是为了更好的为了系统回收变量，重复的url直接返回已有的资源，实现回调函数，让数据成功后，更新到ui线程。

几个辅助类文件：

package cn.wangmeng.test;
public class imageandtext {
 private string imageurl;
 private string text;
 public imageandtext(string imageurl, string text) {
  this.imageurl = imageurl;
  this.text = text;
 }
 public string getimageurl() {
  return imageurl;
 }
 public string gettext() {
  return text;
 }
}

package cn.wangmeng.test;
import android.view.view;
import android.widget.imageview;
import android.widget.textview;
public class viewcache {
 private view baseview;
 private textview textview;
 private imageview imageview;
 public viewcache(view baseview) {
  this.baseview = baseview;
 }
 public textview gettextview() {
  if (textview == null) {
  textview = (textview) baseview.findviewbyid(r.id.text);
  }
  return textview;
 }
 public imageview getimageview() {
  if (imageview == null) {
  imageview = (imageview) baseview.findviewbyid(r.id.image);
  }
  return imageview;
 }
}

viewcache是辅助获取adapter的子元素布局：

package cn.wangmeng.test;
import java.util.list;
import cn.wangmeng.test.asyncimageloader.imagecallback;
import android.app.activity;
import android.graphics.drawable.drawable;
import android.view.layoutinflater;
import android.view.view;
import android.view.viewgroup;
import android.widget.arrayadapter;
import android.widget.imageview;
import android.widget.listview;
import android.widget.textview;
public class imageandtextlistadapter extends arrayadapter<imageandtext> {
 private listview listview;
 private asyncimageloader asyncimageloader;
 public imageandtextlistadapter(activity activity, list<imageandtext> imageandtexts, listview listview) {
  super(activity, 0, imageandtexts);
  this.listview = listview;
  asyncimageloader = new asyncimageloader();
 }
 public view getview(int position, view convertview, viewgroup parent) {
  activity activity = (activity) getcontext();
  // inflate the views from xml
  view rowview = convertview;
  viewcache viewcache;
  if (rowview == null) {
  layoutinflater inflater = activity.getlayoutinflater();
  rowview = inflater.inflate(r.layout.image_and_text_row, null);
  viewcache = new viewcache(rowview);
  rowview.settag(viewcache);
  } else {
  viewcache = (viewcache) rowview.gettag();
  }
  imageandtext imageandtext = getitem(position);
  // load the image and set it on the imageview
  string imageurl = imageandtext.getimageurl();
  imageview imageview = viewcache.getimageview();
  imageview.settag(imageurl);
  drawable cachedimage = asyncimageloader.loaddrawable(imageurl, new imagecallback() {
  public void imageloaded(drawable imagedrawable, string imageurl) {
   imageview imageviewbytag = (imageview) listview.findviewwithtag(imageurl);
   if (imageviewbytag != null) {
   imageviewbytag.setimagedrawable(imagedrawable);
   }
  }
  });
  if (cachedimage == null) {
  imageview.setimageresource(r.drawable.default_image);
  }else{
  imageview.setimagedrawable(cachedimage);
  }
  // set the text on the textview
  textview textview = viewcache.gettextview();
  textview.settext(imageandtext.gettext());
  return rowview;
 }
}

imageandtextlistadapter是实现listview的adapter，里面有个技巧就是imageview.settag(imageurl)，settag是存储数据的，这样是为了保证在回调函数时，listview去更新自己对应item,大家仔细阅读就知道了。

最后贴出布局文件：

<?xml version="1.0" encoding="utf-8"?>
<linearlayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:orientation="horizontal"
  android:layout_width="fill_parent"
  android:layout_height="wrap_content">
 <imageview android:id="@+id/image"
   android:layout_width="wrap_content"
   android:layout_height="wrap_content"
   />
 <textview android:id="@+id/text"
   android:layout_width="wrap_content"
   android:layout_height="wrap_content"/>
</linearlayout>

运行效果截图如下：

希望本文所述对大家的c#程序设计有所帮助。