Android实现ListView异步加载图片

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;    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>

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