原理很簡單,主要是用到了回調方法,下面是異步加載圖片的類
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;
/**
* 圖片異步加載
*
*/
public class AsyncImageLoader {
private HashMap<String, SoftReference<Drawable>> imageCache;
public AsyncImageLoader() {
imageCache = new HashMap<String, SoftReference<Drawable>>();
}
/**
* 加載drawable,讀取到drawable後用hashmap存起來,以圖片地址爲key,這樣在重複加載的時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 {
/** 這個接口用來實現回調
* @param imageDrawable
* @param imageUrl
*/
public void imageLoaded(Drawable imageDrawable, String imageUrl);
}
}
這個類用到了SoftReference軟引用,在內存不足的情況下自動釋放緩存,不至於在加載圖片過多的時候產生內存泄露。
然後自定義的Adapter的getView方法中這樣加載圖片:
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); //在圖片沒有讀取出來的情況下預先放一張圖
} else {
holder.imageView.setImageDrawable(cachedImage);
}