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，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。

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。