转载: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:

 

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:

 

代码
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;
        }
}

 

 

代码
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

 

代码
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:

 

代码
<?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>

 

 

posted @ 2010-08-31 19:45  白羽雕弓  阅读(4902)  评论(1编辑  收藏  举报