Android实现ListView异步加载图片

版权声明：ThomasKwan https://blog.csdn.net/thomassamul/article/details/86650179

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

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);   
         }   
  
} 

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：

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;   
        }   
} 

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;
     }
}

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

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;   
        }   
  
} 

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。

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>