我正在使用谷歌的ImageDownloader，如他们的多线程性能博客文章所示：http://android-developers.blogspot.com/2010/07/multithreading-for-performance.html
我希望能够在从互联网上下载图像时指定默认占位符图像。我怎么能这样做？
以下是代码：
/**
* This helper class download images from the Internet and binds those with the provided ImageView.
*
* <p>It requires the INTERNET permission, which should be added to your application's manifest
* file.</p>
*
* A local cache of downloaded images is maintained internally to improve performance.
*/
public class ImageDownloader {
private static final String LOG_TAG = "ImageDownloader";
public enum Mode { NO_ASYNC_TASK, NO_DOWNLOADED_DRAWABLE, CORRECT }
private Mode mode = Mode.CORRECT; //Mode.NO_ASYNC_TASK;
/**
* Download the specified image from the Internet and binds it to the provided ImageView. The
* binding is immediate if the image is found in the cache and will be done asynchronously
* otherwise. A null bitmap will be associated to the ImageView if an error occurs.
*
* @param url The URL of the image to download.
* @param imageView The ImageView to bind the downloaded image to.
*/
public void download(String url, ImageView imageView) {
resetPurgeTimer();
Bitmap bitmap = getBitmapFromCache(url);
if (bitmap == null) {
forceDownload(url, imageView);
} else {
cancelPotentialDownload(url, imageView);
imageView.setImageBitmap(bitmap);
}
}
/*
* Same as download but the image is always downloaded and the cache is not used.
* Kept private at the moment as its interest is not clear.
private void forceDownload(String url, ImageView view) {
forceDownload(url, view, null);
}
*/
/**
* Same as download but the image is always downloaded and the cache is not used.
* Kept private at the moment as its interest is not clear.
*/
private void forceDownload(String url, ImageView imageView) {
// State sanity: url is guaranteed to never be null in DownloadedDrawable and cache keys.
if (url == null) {
imageView.setImageDrawable(null);
return;
}
if (cancelPotentialDownload(url, imageView)) {
switch (mode) {
case NO_ASYNC_TASK:
Bitmap bitmap = downloadBitmap(url);
addBitmapToCache(url, bitmap);
imageView.setImageBitmap(bitmap);
break;
case NO_DOWNLOADED_DRAWABLE:
imageView.setMinimumHeight(156);
BitmapDownloaderTask task = new BitmapDownloaderTask(imageView);
task.execute(url);
break;
case CORRECT:
task = new BitmapDownloaderTask(imageView);
DownloadedDrawable downloadedDrawable = new DownloadedDrawable(task);
imageView.setImageDrawable(downloadedDrawable);
imageView.setMinimumHeight(156);
task.execute(url);
break;
}
}
}
/**
* Returns true if the current download has been canceled or if there was no download in
* progress on this image view.
* Returns false if the download in progress deals with the same url. The download is not
* stopped in that case.
*/
private static boolean cancelPotentialDownload(String url, ImageView imageView) {
BitmapDownloaderTask bitmapDownloaderTask = getBitmapDownloaderTask(imageView);
if (bitmapDownloaderTask != null) {
String bitmapUrl = bitmapDownloaderTask.url;
if ((bitmapUrl == null) || (!bitmapUrl.equals(url))) {
bitmapDownloaderTask.cancel(true);
} else {
// The same URL is already being downloaded.
return false;
}
}
return true;
}
/**
* @param imageView Any imageView
* @return Retrieve the currently active download task (if any) associated with this imageView.
* null if there is no such task.
*/
private static BitmapDownloaderTask getBitmapDownloaderTask(ImageView imageView) {
if (imageView != null) {
Drawable drawable = imageView.getDrawable();
if (drawable instanceof DownloadedDrawable) {
DownloadedDrawable downloadedDrawable = (DownloadedDrawable)drawable;
return downloadedDrawable.getBitmapDownloaderTask();
}
}
return null;
}
Bitmap downloadBitmap(String url) {
//final int IO_BUFFER_SIZE = 4 * 1024;
// AndroidHttpClient is not allowed to be used from the main thread
final HttpClient client = (mode == Mode.NO_ASYNC_TASK) ? new DefaultHttpClient() :
AndroidHttpClient.newInstance("Android");
final HttpGet getRequest = new HttpGet(url);
try {
HttpResponse response = client.execute(getRequest);
final int statusCode = response.getStatusLine().getStatusCode();
if (statusCode != HttpStatus.SC_OK) {
Log.w("ImageDownloader", "Error " + statusCode +
" while retrieving bitmap from " + url);
return null;
}
final HttpEntity entity = response.getEntity();
if (entity != null) {
InputStream inputStream = null;
try {
inputStream = entity.getContent();
// return BitmapFactory.decodeStream(inputStream);
// Bug on slow connections, fixed in future release.
return BitmapFactory.decodeStream(new FlushedInputStream(inputStream));
} finally {
if (inputStream != null) {
inputStream.close();
}
entity.consumeContent();
}
}
} catch (IOException e) {
getRequest.abort();
Log.w(LOG_TAG, "I/O error while retrieving bitmap from " + url, e);
} catch (IllegalStateException e) {
getRequest.abort();
Log.w(LOG_TAG, "Incorrect URL: " + url);
} catch (Exception e) {
getRequest.abort();
Log.w(LOG_TAG, "Error while retrieving bitmap from " + url, e);
} finally {
if ((client instanceof AndroidHttpClient)) {
((AndroidHttpClient) client).close();
}
}
return null;
}
/*
* An InputStream that skips the exact number of bytes provided, unless it reaches EOF.
*/
static class FlushedInputStream extends FilterInputStream {
public FlushedInputStream(InputStream inputStream) {
super(inputStream);
}
@Override
public long skip(long n) throws IOException {
long totalBytesSkipped = 0L;
while (totalBytesSkipped < n) {
long bytesSkipped = in.skip(n - totalBytesSkipped);
if (bytesSkipped == 0L) {
int b = read();
if (b < 0) {
break; // we reached EOF
} else {
bytesSkipped = 1; // we read one byte
}
}
totalBytesSkipped += bytesSkipped;
}
return totalBytesSkipped;
}
}
/**
* The actual AsyncTask that will asynchronously download the image.
*/
class BitmapDownloaderTask extends AsyncTask<String, Void, Bitmap> {
private String url;
private final WeakReference<ImageView> imageViewReference;
public BitmapDownloaderTask(ImageView imageView) {
imageViewReference = new WeakReference<ImageView>(imageView);
}
/**
* Actual download method.
*/
@Override
protected Bitmap doInBackground(String... params) {
url = params[0];
return downloadBitmap(url);
}
/**
* Once the image is downloaded, associates it to the imageView
*/
@Override
protected void onPostExecute(Bitmap bitmap) {
if (isCancelled()) {
bitmap = null;
}
addBitmapToCache(url, bitmap);
if (imageViewReference != null) {
ImageView imageView = imageViewReference.get();
BitmapDownloaderTask bitmapDownloaderTask = getBitmapDownloaderTask(imageView);
// Change bitmap only if this process is still associated with it
// Or if we don't use any bitmap to task association (NO_DOWNLOADED_DRAWABLE mode)
if ((this == bitmapDownloaderTask) || (mode != Mode.CORRECT)) {
imageView.setImageBitmap(bitmap);
}
}
}
}
/**
* A fake Drawable that will be attached to the imageView while the download is in progress.
*
* <p>Contains a reference to the actual download task, so that a download task can be stopped
* if a new binding is required, and makes sure that only the last started download process can
* bind its result, independently of the download finish order.</p>
*/
static class DownloadedDrawable extends ColorDrawable {
private final WeakReference<BitmapDownloaderTask> bitmapDownloaderTaskReference;
public DownloadedDrawable(BitmapDownloaderTask bitmapDownloaderTask) {
super(Color.BLACK);
bitmapDownloaderTaskReference =
new WeakReference<BitmapDownloaderTask>(bitmapDownloaderTask);
}
public BitmapDownloaderTask getBitmapDownloaderTask() {
return bitmapDownloaderTaskReference.get();
}
}
public void setMode(Mode mode) {
this.mode = mode;
clearCache();
}
/*
* Cache-related fields and methods.
*
* We use a hard and a soft cache. A soft reference cache is too aggressively cleared by the
* Garbage Collector.
*/
private static final int HARD_CACHE_CAPACITY = 10;
private static final int DELAY_BEFORE_PURGE = 10 * 1000; // in milliseconds
// Hard cache, with a fixed maximum capacity and a life duration
private final HashMap<String, Bitmap> sHardBitmapCache =
new LinkedHashMap<String, Bitmap>(HARD_CACHE_CAPACITY / 2, 0.75f, true) {
/**
*
*/
private static final long serialVersionUID = 1L;
@Override
protected boolean removeEldestEntry(LinkedHashMap.Entry<String, Bitmap> eldest) {
if (size() > HARD_CACHE_CAPACITY) {
// Entries push-out of hard reference cache are transferred to soft reference cache
sSoftBitmapCache.put(eldest.getKey(), new SoftReference<Bitmap>(eldest.getValue()));
return true;
} else
return false;
}
};
// Soft cache for bitmaps kicked out of hard cache
private final static ConcurrentHashMap<String, SoftReference<Bitmap>> sSoftBitmapCache =
new ConcurrentHashMap<String, SoftReference<Bitmap>>(HARD_CACHE_CAPACITY / 2);
private final Handler purgeHandler = new Handler();
private final Runnable purger = new Runnable() {
public void run() {
clearCache();
}
};
/**
* Adds this bitmap to the cache.
* @param bitmap The newly downloaded bitmap.
*/
private void addBitmapToCache(String url, Bitmap bitmap) {
if (bitmap != null) {
synchronized (sHardBitmapCache) {
sHardBitmapCache.put(url, bitmap);
}
}
}
/**
* @param url The URL of the image that will be retrieved from the cache.
* @return The cached bitmap or null if it was not found.
*/
public Bitmap getBitmapFromCache(String url) {
// First try the hard reference cache
synchronized (sHardBitmapCache) {
final Bitmap bitmap = sHardBitmapCache.get(url);
if (bitmap != null) {
// Bitmap found in hard cache
// Move element to first position, so that it is removed last
sHardBitmapCache.remove(url);
sHardBitmapCache.put(url, bitmap);
return bitmap;
}
}
// Then try the soft reference cache
SoftReference<Bitmap> bitmapReference = sSoftBitmapCache.get(url);
if (bitmapReference != null) {
final Bitmap bitmap = bitmapReference.get();
if (bitmap != null) {
// Bitmap found in soft cache
return bitmap;
} else {
// Soft reference has been Garbage Collected
sSoftBitmapCache.remove(url);
}
}
return null;
}
/**
* Clears the image cache used internally to improve performance. Note that for memory
* efficiency reasons, the cache will automatically be cleared after a certain inactivity delay.
*/
public void clearCache() {
sHardBitmapCache.clear();
sSoftBitmapCache.clear();
}
/**
* Allow a new delay before the automatic cache clear is done.
*/
private void resetPurgeTimer() {
purgeHandler.removeCallbacks(purger);
purgeHandler.postDelayed(purger, DELAY_BEFORE_PURGE);
}
}
答案 0 :(得分：4)
在致电ImageView之前，将download()设置为forceDownload()中的占位符。
答案 1 :(得分：3)
首先解码要放置的位图。将它传递给你的DownloadedDrawable，它扩展了BitmapDrawable。当我创建类时，我使用有效的上下文填充mContext。
有效下载：
Bitmap bitmap = BitmapFactory.decodeResource(mContext.getResources(), R.resource.placeholder);
imageView.setImageDrawable(new DownloadedDrawable(mContext, handler, bitmap));
已下载的可绘制类：
static class DownloadedDrawable extends BitmapDrawable {
private final SoftReference<BitmapDownloaderTask> bitmapDownloaderTaskReference;
public DownloadedDrawable(Context context, BitmapDownloaderTask bitmapDownloaderTask, Bitmap bitmap) {
super(context.getResources(), bitmap );
bitmapDownloaderTaskReference =
new SoftReference< BitmapDownloaderTask >(bitmapDownloaderTask);
}
public BinaryHttpResponseHandler get BitmapDownloaderTask() {
return bitmapDownloaderTaskReference.get();
}
}
希望它有所帮助：＆gt;
答案 2 :(得分：0)
我不确定我是否完全理解您的问题，但这里有一些代码，可能会对您有所帮助。
public class Downloader {
private ImageView someImageView;
public Downloader() {
someImageView = new ImageView(getApplicationContext());
new BitmapDownloaderTask.execute("http://url/to/your/image.jpg")
}
class BitmapDownloaderTask extends AsyncTask<String, Void, Bitmap> {
@Override
protected onPreExecute() {
someImageView.setImageBitmap(getResources().getDrawable(R.drawable.placeholder));
}
@Override
protected Bitmap doInBackground(String... params) {
return downloadBitmap(params[0]);
}
@Override
protected void onPostExecute(Bitmap bitmap) {
someImageView.setImageBitmap(bitmap);
}
}
}