java 如何指定占位符图像？

How can I specify a placeholder image?

提问by Sheehan Alam

I am using Google's ImageDownloader as seen in their multithreading for performance blog article: http://android-developers.blogspot.com/2010/07/multithreading-for-performance.html

我正在使用 Google 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

I want to be able to specify a default placeholder image while images are downloaded off the internet. How can I do this?

我希望能够在从 Internet 下载图像时指定默认占位符图像。我怎样才能做到这一点？

Here is the code:

这是代码：

/**
 * 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);
    }
}

采纳答案by CommonsWare

Set the ImageViewto the placeholder in download()before calling forceDownload().

在调用 之前将 设置ImageView为占位符。download()forceDownload()

回答by Sylphe

Decode first the Bitmap you want to place. Pass it to your DownloadedDrawable which extends BitmapDrawable. When I create the class, I fill mContext with a valid context.

首先解码要放置的位图。将它传递给扩展 BitmapDrawable 的 DownloadedDrawable。当我创建这个类时，我用一个有效的上下文填充 mContext。

in force download :

有效下载：

Bitmap bitmap = BitmapFactory.decodeResource(mContext.getResources(), R.resource.placeholder);

imageView.setImageDrawable(new DownloadedDrawable(mContext, handler, bitmap));

The downloadedDrawable class :

下载的Drawable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();
    }
}

Hope it helps :>

希望它有帮助：>

回答by kamil zych

I'm not sure that I fully understand your question, but here is some code, that maybe will help you.

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);
        }
    }
}