本文主要是介绍图片的异步加载（双缓存）+ViewHolder，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
图片的异步加载（双缓存）+ViewHolder
上次只是提到了本地图片的异步加载，最后还出现了OOM，不过最后解决了（BaseApapter解决OOM问题）。这次就给大家带来这个网络图片的异步加载+双缓存+ViewHolder。我先叙述一下异步加载的原理，说的通俗一点就是UI主线程继续做与用户交互的响应监听和操作，而加载图片的任务交到其他线程中去做，当图片加载完成之后，再跟据某种机制（比如回调）绘制到要显示的控件中。
首先，贴出AsyncImageLoader.java，这个类是关键，主要做的就是当加载图片的时候，去缓冲区查找，如果有的话，则立刻返回Bitmap对象，省掉再去网络服务器下载的时间和流量
package com.tz.asyncimageload;import android.annotation.SuppressLint;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Environment;
import android.os.Handler;
import android.os.Message;
import android.widget.ImageView;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.lang.ref.SoftReference;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.HashMap;/*** Created 2015/9/9 Adminstrator tianzhao*/
public class AsyncImageLoader {/*** 内存图片软引用缓存*/private HashMap<String, SoftReference<Bitmap>> imageCache = null;private final static String CACHE_PATH = Environment.getExternalStorageDirectory().getPath() + "/AsyncLoad";public AsyncImageLoader() {System.out.println("path-->" + CACHE_PATH);imageCache = new HashMap<String, SoftReference<Bitmap>>();}@SuppressLint("HandlerLeak")public Bitmap loadBitmap(final ImageView imageView, final String imageURL,final ImageCcllBack imageCcllBack) {/*** 在内存缓存中，则返回Bitmap对象*/if (imageCache.containsKey(imageURL)) {SoftReference<Bitmap> reference = imageCache.get(imageURL);Bitmap bitmap = reference.get();if (null != bitmap) {return bitmap;}} else {/*** 在本地缓存中查找*/String bitmapName = imageURL.substring(imageURL.lastIndexOf("/") + 1);File cacheDir = new File(CACHE_PATH);File[] cacheFiles = cacheDir.listFiles();if (cacheFiles != null) {int i = 0;for (; i < cacheFiles.length; i++) {if (bitmapName.equals(cacheFiles[i].getName())) {break;}}// 找到，则返回该图片的Bitmapif (i < cacheFiles.length) {Bitmap bitmap = BitmapFactory.decodeFile(CACHE_PATH + "/"+ bitmapName);imageCcllBack.imageLoadSuccess(imageView, bitmap);return bitmap;}}}/*** 两个缓存中都没有的话则开启新线程进行下载*/final Handler handler = new Handler() {@Overridepublic void handleMessage(Message msg) {super.handleMessage(msg);if (0 == msg.what) {imageCcllBack.imageLoadSuccess(imageView, (Bitmap) msg.obj);} else if (1 == msg.what) {imageCcllBack.imageLoadFaluire(imageView);}}};new Thread() {@Overridepublic void run() {Bitmap bitmap = null;try {URL url = new URL(imageURL);HttpURLConnection conn = (HttpURLConnection) url.openConnection();conn.setDoInput(true);conn.connect();InputStream bitmapIs = conn.getInputStream();bitmap = BitmapFactory.decodeStream(bitmapIs);bitmapIs.close();imageCache.put(imageURL, new SoftReference<Bitmap>(bitmap));Message msg = Message.obtain();msg.obj = bitmap;msg.what = 0;handler.sendMessage(msg);} catch (MalformedURLException e) {e.printStackTrace();handler.sendEmptyMessage(1);} catch (IOException e) {e.printStackTrace();handler.sendEmptyMessage(1);}if (null != bitmap) {/*** 将图片保存到本地缓存*/File dir = new File(CACHE_PATH);if (!dir.exists()) {dir.mkdirs();}File bitmapFile = new File(CACHE_PATH + "/"+ imageURL.substring(imageURL.lastIndexOf("/") + 1));if (!bitmapFile.exists()) {try {bitmapFile.createNewFile();} catch (IOException e) {e.printStackTrace();}}FileOutputStream fos;try {fos = new FileOutputStream(bitmapFile);bitmap.compress(Bitmap.CompressFormat.JPEG, 100, fos);fos.close();} catch (FileNotFoundException e) {e.printStackTrace();} catch (IOException e) {e.printStackTrace();}} else {handler.sendEmptyMessage(1);}}}.start();return null;}public interface ImageCcllBack {public void imageLoadSuccess(ImageView imageView, Bitmap bitmap);public void imageLoadFaluire(ImageView imageView);}
}
接下来就是Adapter类了，这里继承BaseAdapter类MyAdapter.java
package com.tz.asyncimageload;import android.content.Context;
import android.graphics.Bitmap;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.BaseAdapter;
import android.widget.ImageView;
import android.widget.TextView;
import java.util.List;
import com.example.asyncimageload.R;/*** Created 2015/9/9 Adminstrator tianzhao*/
public class MyAdapter extends BaseAdapter {private Context context;private List<String> imgList;private AsyncImageLoader asyncImageLoader;public MyAdapter(Context context, List<String> imgList) {this.context = context;this.imgList = imgList;asyncImageLoader = new AsyncImageLoader();}@Overridepublic int getCount() {return null == imgList ? 0 : imgList.size();}@Overridepublic Object getItem(int position) {return imgList.get(position);}@Overridepublic long getItemId(int position) {return position;}@Overridepublic View getView(int position, View convertView, ViewGroup parent) {ViewHolder viewHolder = null;if (null == convertView) {viewHolder = new ViewHolder();LayoutInflater mInflater = LayoutInflater.from(context);convertView = mInflater.inflate(R.layout.item_layout, null);viewHolder.textView = (TextView) convertView.findViewById(R.id.tv);viewHolder.imageView = (ImageView) convertView.findViewById(R.id.iv);convertView.setTag(viewHolder);} else {viewHolder = (ViewHolder) convertView.getTag();}viewHolder.textView.setText(getItem(position).toString());viewHolder.imageView.setBackgroundResource(R.drawable.no_data);asyncImageLoader.loadBitmap(viewHolder.imageView, getItem(position).toString(), new AsyncImageLoader.ImageCcllBack() {@Overridepublic void imageLoadSuccess(ImageView imageView, Bitmap bitmap) {imageView.setImageBitmap(bitmap);}@Overridepublic void imageLoadFaluire(ImageView imageView) {imageView.setImageResource(R.drawable.no_data);}});return convertView;}private static class ViewHolder {ImageView imageView;TextView textView;}
}asyncImageLoader是我们定义的异步加载类的对象，通过这个类的public Bitmap loadBitmap(final ImageView imageView, final String imageURL, final ImageCallBack imageCallBack)加载图片，传递参数也与参考博客有些不同，我觉得这样更好理解一下，就是要显示图片的URL链接，和图片要显示对应的控件，当然最重要的还有这个接口实现的回调对象，是在线程中下载完图片之后，用以加载图片的回调对象。而这个回调对象在传递的时候已经实现了接口方法，即将下载好的图片绘制在对应的控件之中
接下来就是MainActivity.java了
package com.tz.asyncimageload;import java.util.ArrayList;
import java.util.List;
import com.example.asyncimageload.R;
import android.app.Activity;
import android.os.Bundle;
import android.widget.ListView;public class MainActivity extends Activity {private ListView mListView;private List<String> imgList = new ArrayList<String>();@Overrideprotected void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);imgList.add("http://img0.imgtn.bdimg.com/it/u=3292852797,3921168757&fm=21&gp=0.jpg");imgList.add("http://img4.imgtn.bdimg.com/it/u=1268041620,2779298692&fm=21&gp=0.jpg");imgList.add("http://img0.imgtn.bdimg.com/it/u=2818850098,881187710&fm=21&gp=0.jpg");imgList.add("http://img3.yxlady.com/mr/UploadFiles_9207/20150715/20150715163557826.jpg");imgList.add("http://img0.imgtn.bdimg.com/it/u=2949470383,236653565&fm=21&gp=0.jpg");imgList.add("http://d03.res.meilishuo.net/pic/r/36/48/488a12972a38065cd9d98495ca12_800_800.jpeg");imgList.add("http://pic.wenwen.soso.com/p/20101227/20101227153650-1873348623.jpg");imgList.add("http://img3.imgtn.bdimg.com/it/u=1054064343,3338205640&fm=21&gp=0.jpg");imgList.add("http://img3.imgtn.bdimg.com/it/u=1025044414,887723369&fm=21&gp=0.jpg");imgList.add("http://file.azg168.cn/file/mianxiangdaquan/xiangshu/20141006/bffcb52bd45827e6f05621dd259c7ff2.jpg");imgList.add("http://img0.imgtn.bdimg.com/it/u=3427470176,4008266000&fm=21&gp=0.jpg");mListView = (ListView) findViewById(R.id.lv);MyAdapter myAdapter = new MyAdapter(MainActivity.this, imgList);mListView.setAdapter(myAdapter);}}
OK！网络图片的异步加载+双缓存+ViewHolder到这里接结束了。
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