Android开发中的常见任务是从Web加载图像并在Activity中与ImageView绑定。在我看过的代码中，程序员经常使用AsyncTask在后台运行加载并在上下文中发布结果。 这种方法使我们在某些情况下开始使用，因为方向改变。原因是背景。因为活动可能被销毁并再次创建，我们的上下文也将丢失，但在运行AsyncTask时，上下文不会更新。因此，当调用postExectute时，AsyncTask会尝试将结果发布到不再存在的。
我想知道您使用什么方法将数据加载为图像，然后发布到Activity？
答案 0 :(得分：4)
您应该尝试使用所谓的AsyncTaskLoader。它就像一个AsycnTask，除了整个“需要在活动死亡时删除AsyncTask”位时才会为你处理。一般来说，加载程序几乎专门用于改进异步加载listviews数据的任务。事实上，他们很快成为我最喜欢的新Android类之一。有关加载器的更多信息，请查看此documentation。
作为最后一点，在API级别10之前不会引入加载器。也就是说，您仍然可以使用android Support Package从较小的api级别访问它们。
答案 1 :(得分：0)
以下是在后台加载图像并将其存储到sdcard的完整代码。如果图像已经存在，则它不会向服务器发出请求。 包com.packsmooch.utils;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.URL;
import java.util.HashMap;
import java.util.Stack;
import android.app.Activity;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.widget.ImageView;
public class Albumartloader
{
private HashMap<String, Bitmap> cache = new HashMap<String, Bitmap>();
private File cacheDir;
private Bitmap useThisBitmap;
public Albumartloader(Context context)
{
photoLoaderThread.setPriority(Thread.NORM_PRIORITY - 1);
if (android.os.Environment.getExternalStorageState().equals(android.os.Environment.MEDIA_MOUNTED))
cacheDir = new File(android.os.Environment.getExternalStorageDirectory(),"Streaming_Service/AlbumArt/");
else
cacheDir = context.getCacheDir();
if (!cacheDir.exists())
cacheDir.mkdirs();
}
public void DisplayImage(String url, Activity activity, ImageView imageView)
{
if(!url.equals(""))
{
if (cache.containsKey(url))
{
imageView.setImageBitmap(cache.get(url));
}
else
{
queuePhoto(url, activity, imageView);
}
}
}
private void queuePhoto(String url, Activity activity, ImageView imageView)
{
photosQueue.Clean(imageView);
PhotoToLoad p = new PhotoToLoad(url, imageView);
synchronized (photosQueue.photosToLoad)
{
photosQueue.photosToLoad.push(p);
photosQueue.photosToLoad.notifyAll();
}
// start thread if it's not started yet
if (photoLoaderThread.getState() == Thread.State.NEW)
photoLoaderThread.start();
}
public Bitmap getBitmap(String url)
{
try
{
// I identify images by hashcode. Not a perfect solution, good for the
// demo.
String filename = String.valueOf(url.hashCode());
File f = new File(cacheDir, filename);
// from SD cache
Bitmap b = decodeFile(f);
if (b != null)
return b;
// from web
try {
Bitmap bitmap = null;
if(!url.equals("")){
InputStream is = new URL(url).openStream();
OutputStream os = new FileOutputStream(f);
Utils.CopyStream(is, os);
os.close();
bitmap = decodeFile(f);
}
return bitmap;
} catch (Exception ex) {
ex.printStackTrace();
return null;
}
}
catch(Exception e)
{
return null;
}
}
/*decodes image and scales it to reduce memory consumption
* @param file path
* @throws FileNotFoundException
* @return bitmap
* */
private Bitmap decodeFile(File f){
Bitmap b = null;
try {
useThisBitmap = null;
//Decode image size
BitmapFactory.Options o = new BitmapFactory.Options();
o.inJustDecodeBounds = true;
final int IMAGE_MAX_SIZE =50;
BitmapFactory.decodeStream(new FileInputStream(f), null, o);
int scale = 2;
if (o.outHeight > IMAGE_MAX_SIZE || o.outWidth > IMAGE_MAX_SIZE) {
scale = 2 ^ (int) Math.ceil(Math.log(IMAGE_MAX_SIZE / (double) Math.max(o.outHeight, o.outWidth)) / Math.log(0.5));
}
//Decode with inSampleSize
BitmapFactory.Options o2 = new BitmapFactory.Options();
o2.inSampleSize = scale;
b = BitmapFactory.decodeStream(new FileInputStream(f), null, o2);
useThisBitmap = b;
} catch (FileNotFoundException e) {
}
catch(Exception e)
{
}
finally{
System.gc();
}
return useThisBitmap;
}
// Task for the queue
private class PhotoToLoad
{
public String url;
public ImageView imageView;
public PhotoToLoad(String u, ImageView i) {
url = u;
imageView = i;
}
}
PhotosQueue photosQueue = new PhotosQueue();
public void stopThread()
{
photoLoaderThread.interrupt();
}
// stores list of photos to download
class PhotosQueue {
private Stack<PhotoToLoad> photosToLoad = new Stack<PhotoToLoad>();
// removes all instances of this ImageView
public void Clean(ImageView image) {
for (int j = 0; j < photosToLoad.size();) {
if (photosToLoad.get(j).imageView == image)
photosToLoad.remove(j);
else
++j;
}
}
}
class PhotosLoader extends Thread {
public void run() {
try {
while (true) {
// thread waits until there are any images to load in the
// queue
if (photosQueue.photosToLoad.size() == 0)
synchronized (photosQueue.photosToLoad) {
photosQueue.photosToLoad.wait();
}
if (photosQueue.photosToLoad.size() != 0) {
PhotoToLoad photoToLoad;
synchronized (photosQueue.photosToLoad) {
photoToLoad = photosQueue.photosToLoad.pop();
}
Bitmap bmp = getBitmap(photoToLoad.url);
cache.put(photoToLoad.url, bmp);
if (((String) photoToLoad.imageView.getTag())
.equals(photoToLoad.url)) {
BitmapDisplayer bd = new BitmapDisplayer(bmp,
photoToLoad.imageView);
Activity a = (Activity) photoToLoad.imageView
.getContext();
a.runOnUiThread(bd);
}
}
if (Thread.interrupted())
break;
}
} catch (InterruptedException e) {
// allow thread to exit
}
}
}
PhotosLoader photoLoaderThread = new PhotosLoader();
// Used to display bitmap in the UI thread
class BitmapDisplayer implements Runnable {
Bitmap bitmap;
ImageView imageView;
public BitmapDisplayer(Bitmap b, ImageView i) {
bitmap = b;
imageView = i;
}
public void run()
{
if (bitmap != null)
imageView.setImageBitmap(bitmap);
}
}
public void clearCache()
{
// clear memory cache
cache.clear();
// clear SD cache
File[] files = cacheDir.listFiles();
for (File f : files)
f.delete();
}
}
以下是用法：
Albumartloader albumartloader=new Albumartloader(this);
imageview.settag(urlofimage);
albumartloader.DisplayImage(url of image,activity context,imageview);
最佳用法： - 当您需要显示来自服务器的图像数量并且不想等待所有图像下载时，您可以将其用于列表视图。