Android实现ListView异步加载图片

一般情况下使用Xutil，Imageloader等第三方框架

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，下法的代码：

 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>