ListView异步加载图片是非常实用的方法

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;

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
>

原文地址：http://blog.jteam.nl/2009/09/17/exploring-the-world-of-android-part-2

• AsyncListImage.zip (50.5 KB)