原理很简单，主要是用到了回调方法，下面是异步加载图片的类
<span style="font-size:16px;">package com.xxx.xxx;
import java.io.InputStream; import java.lang.ref.SoftReference; import java.net.URL; import java.util.HashMap;
import android.graphics.drawable.Drawable; import android.os.Handler; import android.os.Message;
/** * 图片异步加载 * */ public class AsyncImageLoader {
private HashMap<String, SoftReference<Drawable>> imageCache;
public AsyncImageLoader() { imageCache = new HashMap<String, SoftReference<Drawable>>(); }
/** * 加载drawable，读取到drawable后用hashmap存起来，以图片地址为key，这样在重复加载的时候就可以直接取值了 * * @param imageUrl * @param imageCallback * @return 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; }
/** * @param strUrl * @return drawable */ private static Drawable loadImageFromUrl(String strUrl) { URL url; InputStream i = null; try { url = new URL(strUrl); i = (InputStream) url.getContent(); } catch (Exception e) { e.printStackTrace(); } Drawable d = Drawable.createFromStream(i, "src"); return d; }
public interface ImageCallback {
/** 这个接口用来实现回调 * @param imageDrawable * @param imageUrl */ public void imageLoaded(Drawable imageDrawable, String imageUrl); } }</span>
这个类用到了SoftReference软引用，在内存不足的情况下自动释放缓存，不至于在加载图片过多的时候产生内存泄露。
然后自定义的Adapter的getView方法中这样加载图片：
<span style="font-size:16px;"></span><span style="font-size:16px;"> Drawable cachedImage = imageLoader.loadDrawable(AppMessageUrl.IMAGEURL + iconUrl, new ImageCallback() { public void imageLoaded(Drawable imageDrawable, String imageUrl) { holder.imageView.setImageDrawable(imageDrawable); } }); if (cachedImage == null) { holder.imageView.setImageResource(R.drawable.no_apk); //在图片没有读取出来的情况下预先放一张图 } else { holder.imageView.setImageDrawable(cachedImage); }</span>
ListView与GridView异步加载图片