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：

Java代码  

1. package cn.wangmeng.test;   
2.   
3. import java.io.IOException;   
4. import java.io.InputStream;   
5. import java.lang.ref.SoftReference;   
6. import java.net.MalformedURLException;   
7. import java.net.URL;   
8. import java.util.HashMap;   
9.   
10. import android.graphics.drawable.Drawable;   
11. import android.os.Handler;   
12. import android.os.Message;   
13.   
14. public class AsyncImageLoader {   
15.   
16.      private HashMap<String, SoftReference<Drawable>> p_w_picpathCache;   
17.          
18.          public AsyncImageLoader() {   
19.              p_w_picpathCache = new HashMap<String, SoftReference<Drawable>>();   
20.          }   
21.          
22.          public Drawable loadDrawable(final String p_w_picpathUrl, final ImageCallback p_w_picpathCallback) {   
23.              if (p_w_picpathCache.containsKey(p_w_picpathUrl)) {   
24.                  SoftReference<Drawable> softReference = p_w_picpathCache.get(p_w_picpathUrl);   
25.                  Drawable drawable = softReference.get();   
26.                  if (drawable != null) {   
27.                      return drawable;   
28.                  }   
29.              }   
30.              final Handler handler = new Handler() {   
31.                  public void handleMessage(Message message) {   
32.                      p_w_picpathCallback.p_w_picpathLoaded((Drawable) message.obj, p_w_picpathUrl);   
33.                  }   
34.              };   
35.              new Thread() {   
36.                  @Override  
37.                  public void run() {   
38.                      Drawable drawable = loadImageFromUrl(p_w_picpathUrl);   
39.                      p_w_picpathCache.put(p_w_picpathUrl, new SoftReference<Drawable>(drawable));   
40.                      Message message = handler.obtainMessage(0, drawable);   
41.                      handler.sendMessage(message);   
42.                  }   
43.              }.start();   
44.              return null;   
45.          }   
46.          
47.         public static Drawable loadImageFromUrl(String url) {   
48.             URL m;   
49.             InputStream i = null;   
50.             try {   
51.                 m = new URL(url);   
52.                 i = (InputStream) m.getContent();   
53.             } catch (MalformedURLException e1) {   
54.                 e1.printStackTrace();   
55.             } catch (IOException e) {   
56.                 e.printStackTrace();   
57.             }   
58.             Drawable d = Drawable.createFromStream(i, "src");   
59.             return d;   
60.         }   
61.          
62.          public interface ImageCallback {   
63.              public void p_w_picpathLoaded(Drawable p_w_picpathDrawable, String p_w_picpathUrl);   
64.          }   
65.   
66. }  

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>> p_w_picpathCache;
	  
	     public AsyncImageLoader() {
	    	 p_w_picpathCache = new HashMap<String, SoftReference<Drawable>>();
	     }
	  
	     public Drawable loadDrawable(final String p_w_picpathUrl, final ImageCallback p_w_picpathCallback) {
	         if (p_w_picpathCache.containsKey(p_w_picpathUrl)) {
	             SoftReference<Drawable> softReference = p_w_picpathCache.get(p_w_picpathUrl);
	             Drawable drawable = softReference.get();
	             if (drawable != null) {
	                 return drawable;
	             }
	         }
	         final Handler handler = new Handler() {
	             public void handleMessage(Message message) {
	                 p_w_picpathCallback.p_w_picpathLoaded((Drawable) message.obj, p_w_picpathUrl);
	             }
	         };
	         new Thread() {
	             @Override
	             public void run() {
	                 Drawable drawable = loadImageFromUrl(p_w_picpathUrl);
	                 p_w_picpathCache.put(p_w_picpathUrl, 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 p_w_picpathLoaded(Drawable p_w_picpathDrawable, String p_w_picpathUrl);
	     }

}

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：

Java代码  

1. package cn.wangmeng.test;   
2.   
3. public class ImageAndText {   
4.         private String p_w_picpathUrl;   
5.         private String text;   
6.   
7.         public ImageAndText(String p_w_picpathUrl, String text) {   
8.             this.p_w_picpathUrl = p_w_picpathUrl;   
9.             this.text = text;   
10.         }   
11.         public String getImageUrl() {   
12.             return p_w_picpathUrl;   
13.         }   
14.         public String getText() {   
15.             return text;   
16.         }   
17. }  

package cn.wangmeng.test;

public class ImageAndText {
	    private String p_w_picpathUrl;
	    private String text;

	    public ImageAndText(String p_w_picpathUrl, String text) {
	        this.p_w_picpathUrl = p_w_picpathUrl;
	        this.text = text;
	    }
	    public String getImageUrl() {
	        return p_w_picpathUrl;
	    }
	    public String getText() {
	        return text;
	    }
}

Java代码  

1. package cn.wangmeng.test;   
2.   
3. import android.view.View;   
4. import android.widget.ImageView;   
5. import android.widget.TextView;   
6.   
7. public class ViewCache {   
8.   
9.         private View baseView;   
10.         private TextView textView;   
11.         private ImageView p_w_picpathView;   
12.   
13.         public ViewCache(View baseView) {   
14.             this.baseView = baseView;   
15.         }   
16.   
17.         public TextView getTextView() {   
18.             if (textView == null) {   
19.                 textView = (TextView) baseView.findViewById(R.id.text);   
20.             }   
21.             return textView;   
22.         }   
23.   
24.         public ImageView getImageView() {   
25.             if (p_w_picpathView == null) {   
26.                 p_w_picpathView = (ImageView) baseView.findViewById(R.id.p_w_picpath);   
27.             }   
28.             return p_w_picpathView;   
29.         }   
30.   
31. }  

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 p_w_picpathView;

	    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 (p_w_picpathView == null) {
	            p_w_picpathView = (ImageView) baseView.findViewById(R.id.p_w_picpath);
	        }
	        return p_w_picpathView;
	    }

}

ViewCache是辅助获取adapter的子元素布局
 

Java代码  

1. package cn.wangmeng.test;   
2.   
3. import java.util.List;   
4.   
5. import cn.wangmeng.test.AsyncImageLoader.ImageCallback;   
6.   
7. import android.app.Activity;   
8. import android.graphics.drawable.Drawable;   
9. import android.view.LayoutInflater;   
10. import android.view.View;   
11. import android.view.ViewGroup;   
12. import android.widget.ArrayAdapter;   
13. import android.widget.ImageView;   
14. import android.widget.ListView;   
15. import android.widget.TextView;   
16.   
17. public class ImageAndTextListAdapter extends ArrayAdapter<ImageAndText> {   
18.   
19.         private ListView listView;   
20.         private AsyncImageLoader asyncImageLoader;   
21.   
22.         public ImageAndTextListAdapter(Activity activity, List<ImageAndText> p_w_picpathAndTexts, ListView listView) {   
23.             super(activity, 0, p_w_picpathAndTexts);   
24.             this.listView = listView;   
25.             asyncImageLoader = new AsyncImageLoader();   
26.         }   
27.   
28.         public View getView(int position, View convertView, ViewGroup parent) {   
29.             Activity activity = (Activity) getContext();   
30.   
31.             // Inflate the views from XML   
32.             View rowView = convertView;   
33.             ViewCache viewCache;   
34.             if (rowView == null) {   
35.                 LayoutInflater inflater = activity.getLayoutInflater();   
36.                 rowView = inflater.inflate(R.layout.p_w_picpath_and_text_row, null);   
37.                 viewCache = new ViewCache(rowView);   
38.                 rowView.setTag(viewCache);   
39.             } else {   
40.                 viewCache = (ViewCache) rowView.getTag();   
41.             }   
42.             ImageAndText p_w_picpathAndText = getItem(position);   
43.   
44.             // Load the p_w_picpath and set it on the ImageView   
45.             String p_w_picpathUrl = p_w_picpathAndText.getImageUrl();   
46.             ImageView p_w_picpathView = viewCache.getImageView();   
47.             p_w_picpathView.setTag(p_w_picpathUrl);   
48.             Drawable cachedImage = asyncImageLoader.loadDrawable(p_w_picpathUrl, new ImageCallback() {   
49.                 public void p_w_picpathLoaded(Drawable p_w_picpathDrawable, String p_w_picpathUrl) {   
50.                     ImageView p_w_picpathViewByTag = (ImageView) listView.findViewWithTag(p_w_picpathUrl);   
51.                     if (p_w_picpathViewByTag != null) {   
52.                         p_w_picpathViewByTag.setImageDrawable(p_w_picpathDrawable);   
53.                     }   
54.                 }   
55.             });   
56.             if (cachedImage == null) {   
57.                 p_w_picpathView.setImageResource(R.drawable.default_p_w_picpath);   
58.             }else{   
59.                 p_w_picpathView.setImageDrawable(cachedImage);   
60.             }   
61.             // Set the text on the TextView   
62.             TextView textView = viewCache.getTextView();   
63.             textView.setText(p_w_picpathAndText.getText());   
64.   
65.             return rowView;   
66.         }   
67.   
68. }  

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> p_w_picpathAndTexts, ListView listView) {
	        super(activity, 0, p_w_picpathAndTexts);
	        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.p_w_picpath_and_text_row, null);
	            viewCache = new ViewCache(rowView);
	            rowView.setTag(viewCache);
	        } else {
	            viewCache = (ViewCache) rowView.getTag();
	        }
	        ImageAndText p_w_picpathAndText = getItem(position);

	        // Load the p_w_picpath and set it on the ImageView
	        String p_w_picpathUrl = p_w_picpathAndText.getImageUrl();
	        ImageView p_w_picpathView = viewCache.getImageView();
	        p_w_picpathView.setTag(p_w_picpathUrl);
	        Drawable cachedImage = asyncImageLoader.loadDrawable(p_w_picpathUrl, new ImageCallback() {
	            public void p_w_picpathLoaded(Drawable p_w_picpathDrawable, String p_w_picpathUrl) {
	                ImageView p_w_picpathViewByTag = (ImageView) listView.findViewWithTag(p_w_picpathUrl);
	                if (p_w_picpathViewByTag != null) {
	                    p_w_picpathViewByTag.setImageDrawable(p_w_picpathDrawable);
	                }
	            }
	        });
			if (cachedImage == null) {
				p_w_picpathView.setImageResource(R.drawable.default_p_w_picpath);
			}else{
				p_w_picpathView.setImageDrawable(cachedImage);
			}
	        // Set the text on the TextView
	        TextView textView = viewCache.getTextView();
	        textView.setText(p_w_picpathAndText.getText());

	        return rowView;
	    }

}

ImageAndTextListAdapter是实现ListView的Adapter，里面有个技巧就是p_w_picpathView.setTag(p_w_picpathUrl)，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代码  

1. <?xml version="1.0" encoding="utf-8"?>  
2. <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"  
3.               android:orientation="horizontal"  
4.               android:layout_width="fill_parent"  
5.               android:layout_height="wrap_content">  
6.   
7.         <ImageView android:id="@+id/p_w_picpath"  
8.                    android:layout_width="wrap_content"  
9.                    android:layout_height="wrap_content"  
10.                    />  
11.   
12.         <TextView android:id="@+id/text"  
13.                   android:layout_width="wrap_content"  
14.                   android:layout_height="wrap_content"/>  
15.   
16. </LinearLayout>  

<?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/p_w_picpath"
                   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>

http://www.iteye.com/topic/685986

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