Android实现ListView异步加载图片的方法

2020-04-11 11:23:18

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本文实例讲述了Android实现ListView异步加载图片的方法。分享给大家供大家参考。具体如下：

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>> 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>

运行效果截图如下：