android ListView 异步加载图片
异步加载图片的目的只有一个：让用户体验更好
因为异步加载不利用主线程（UI线程）
一、源码结构：
二、异步图片加载器（实现异步加载的核心）
package com.ning.lazytest;
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);
}
}
三、ListView适配器
package com.ning.lazytest;
import java.util.List;
import com.ning.lazytest.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.ic_launcher);
}else{
imageView.setImageDrawable(cachedImage);
}
// Set the text on the TextView
TextView textView = viewCache.getTextView();
textView.setText(imageAndText.getText());
return rowView;
}
}
四、ViewCache 即ListView的每一个Item
package com.ning.lazytest;
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;
}
}
五、Java Bean -> ImageAndText
package com.ning.lazytest;
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;
}
}
六、Activity
package com.ning.lazytest;
import java.util.ArrayList;
import java.util.List;
import android.app.Activity;
import android.os.Bundle;
import android.widget.ListView;
public class LazyLoadActivity extends Activity {
private String[] myPicUrl = new String[]{
"http://imguxv.penshow.cn/uploadfile/2010/04/06/thumb/thumb_250_0_20100406094922546.jpg",
"http://imguxv.penshow.cn/uploadfile/2010/04/06/thumb/thumb_250_0_20100406095559309.jpg",
"http://imguxv.penshow.cn/uploadfile/2010/04/06/thumb/thumb_250_0_20100406094916270.jpg",
"http://imguxv.penshow.cn/uploadfile/2010/04/06/thumb/thumb_250_0_20100406085150824.jpg",
"http://imguxv.penshow.cn/uploadfile/2010/04/18/thumb/thumb_250_0_20100418061054239.jpg",
"http://imguxv.penshow.cn/uploadfile/2010/04/18/thumb/thumb_250_0_20100418052701708.jpg",
"http://imguxv.penshow.cn/uploadfile/2010/04/18/thumb/thumb_250_0_20100418050521586.jpg"
};
private ImageAndText imageAndText = null;
private List<ImageAndText> imageAndTexts = null;
private ListView listView = null;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
imageAndTexts = new ArrayList<ImageAndText>();
for(int i = 0;i < myPicUrl.length; i++){
imageAndText = new ImageAndText(myPicUrl[i], "Pic " + i);
imageAndTexts.add(imageAndText);
imageAndText = null;
}
listView = (ListView)findViewById(R.id.list);
listView.setAdapter(new ImageAndTextListAdapter(LazyLoadActivity.this, imageAndTexts, listView));
}
}