android开发小技巧：实现listview异步加载图片

针对listview异步加载图片这个问题，麦子学院android开发老师讲了一种非常实用的方法，麦子学院android开发老师说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>