本文介绍如何在Android中实现ListView的异步加载图片功能，通过使用软引用和回调函数，提高用户体验并减少等待时间。
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：
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：
ViewCache是辅助获取adapter的子元素布局
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：
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package cn.wangmeng.test;
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import java.io.IOException;
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import java.io.InputStream;
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import java.lang.ref.SoftReference;
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import java.net.MalformedURLException;
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import java.net.URL;
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import java.util.HashMap;
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import android.graphics.drawable.Drawable;
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import android.os.Handler;
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import android.os.Message;
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public class AsyncImageLoader {
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private HashMap<String, SoftReference<Drawable>> imageCache;
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public AsyncImageLoader() {
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imageCache = new HashMap<String, SoftReference<Drawable>>();
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}
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public Drawable loadDrawable(final String imageUrl, final ImageCallback imageCallback) {
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if (imageCache.containsKey(imageUrl)) {
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SoftReference<Drawable> softReference = imageCache.get(imageUrl);
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Drawable drawable = softReference.get();
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if (drawable != null) {
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return drawable;
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}
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}
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final Handler handler = new Handler() {
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public void handleMessage(Message message) {
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imageCallback.imageLoaded((Drawable) message.obj, imageUrl);
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}
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};
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new Thread() {
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@Override
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public void run() {
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Drawable drawable = loadImageFromUrl(imageUrl);
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imageCache.put(imageUrl, new SoftReference<Drawable>(drawable));
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Message message = handler.obtainMessage(0, drawable);
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handler.sendMessage(message);
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}
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}.start();
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return null;
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}
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public static Drawable loadImageFromUrl(String url) {
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URL m;
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InputStream i = null;
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try {
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m = new URL(url);
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i = (InputStream) m.getContent();
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} catch (MalformedURLException e1) {
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e1.printStackTrace();
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} catch (IOException e) {
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e.printStackTrace();
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}
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Drawable d = Drawable.createFromStream(i, "src");
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return d;
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}
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public interface ImageCallback {
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public void imageLoaded(Drawable imageDrawable, String imageUrl);
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}
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}
几个辅助类文件：
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package cn.wangmeng.test;
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public class ImageAndText {
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private String imageUrl;
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private String text;
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public ImageAndText(String imageUrl, String text) {
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this.imageUrl = imageUrl;
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this.text = text;
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}
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public String getImageUrl() {
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return imageUrl;
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}
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public String getText() {
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return text;
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}
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}
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package cn.wangmeng.test;
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import android.view.View;
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import android.widget.ImageView;
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import android.widget.TextView;
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public class ViewCache {
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private View baseView;
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private TextView textView;
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private ImageView imageView;
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public ViewCache(View baseView) {
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this.baseView = baseView;
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}
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public TextView getTextView() {
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if (textView == null) {
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textView = (TextView) baseView.findViewById(R.id.text);
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}
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return textView;
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}
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public ImageView getImageView() {
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if (imageView == null) {
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imageView = (ImageView) baseView.findViewById(R.id.image);
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}
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return imageView;
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}
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}
ViewCache是辅助获取adapter的子元素布局
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package cn.wangmeng.test;
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import java.util.List;
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import cn.wangmeng.test.AsyncImageLoader.ImageCallback;
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import android.app.Activity;
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import android.graphics.drawable.Drawable;
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import android.view.LayoutInflater;
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import android.view.View;
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import android.view.ViewGroup;
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import android.widget.ArrayAdapter;
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import android.widget.ImageView;
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import android.widget.ListView;
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import android.widget.TextView;
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public class ImageAndTextListAdapter extends ArrayAdapter<ImageAndText> {
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private ListView listView;
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private AsyncImageLoader asyncImageLoader;
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public ImageAndTextListAdapter(Activity activity, List<ImageAndText> imageAndTexts, ListView listView) {
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super(activity, 0, imageAndTexts);
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this.listView = listView;
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asyncImageLoader = new AsyncImageLoader();
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}
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public View getView(int position, View convertView, ViewGroup parent) {
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Activity activity = (Activity) getContext();
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// Inflate the views from XML
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View rowView = convertView;
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ViewCache viewCache;
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if (rowView == null) {
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LayoutInflater inflater = activity.getLayoutInflater();
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rowView = inflater.inflate(R.layout.image_and_text_row, null);
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viewCache = new ViewCache(rowView);
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rowView.setTag(viewCache);
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} else {
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viewCache = (ViewCache) rowView.getTag();
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}
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ImageAndText imageAndText = getItem(position);
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// Load the image and set it on the ImageView
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String imageUrl = imageAndText.getImageUrl();
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ImageView imageView = viewCache.getImageView();
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imageView.setTag(imageUrl);
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Drawable cachedImage = asyncImageLoader.loadDrawable(imageUrl, new ImageCallback() {
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public void imageLoaded(Drawable imageDrawable, String imageUrl) {
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ImageView imageViewByTag = (ImageView) listView.findViewWithTag(imageUrl);
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if (imageViewByTag != null) {
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imageViewByTag.setImageDrawable(imageDrawable);
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}
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}
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});
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if (cachedImage == null) {
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imageView.setImageResource(R.drawable.default_image);
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}else{
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imageView.setImageDrawable(cachedImage);
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}
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// Set the text on the TextView
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TextView textView = viewCache.getTextView();
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textView.setText(imageAndText.getText());
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return rowView;
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}
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}
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：
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<?xml version="1.0" encoding="utf-8"?>
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<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
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android:orientation="horizontal"
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android:layout_width="fill_parent"
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android:layout_height="wrap_content">
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<ImageView android:id="@+id/image"
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android:layout_width="wrap_content"
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android:layout_height="wrap_content"
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/>
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<TextView android:id="@+id/text"
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android:layout_width="wrap_content"
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android:layout_height="wrap_content"/>
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</LinearLayout>
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