Android实现异步加载图片

转载

AlexMichel 2011-08-02 16:44:15

文章标签 职场移动开发休闲 android 文章分类 Android 移动开发

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 >> 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。

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 > 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());