Android的ListView异步下载图片官方提供的源码

源文：

http://android-developers.blogspot.com/2010/07/multithreading-for-performance.html

一个图片的下载体

static Bitmap downloadBitmap(String url) {    final AndroidHttpClient client = 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();                 final Bitmap bitmap = BitmapFactory.decodeStream(inputStream);                return bitmap;            } finally {                if (inputStream != null) {                    inputStream.close();                  }                entity.consumeContent();            }        }    } catch (Exception e) {        // Could provide a more explicit error message for IOException or IllegalStateException        getRequest.abort();        Log.w("ImageDownloader", "Error while retrieving bitmap from " + url, e.toString());    } finally {        if (client != null) {            client.close();        }    }    return null;}

作者说BitmapFactory.decodeStream 是有bug的，怎样fixed呢？？

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 byte = read();                  if (byte < 0) {                      break;  // we reached EOF                  } else {                      bytesSkipped = 1; // we read one byte                  }           }            totalBytesSkipped += bytesSkipped;        }        return totalBytesSkipped;    }}

异步下载

public class ImageDownloader {    public void download(String url, ImageView imageView) {            BitmapDownloaderTask task = new BitmapDownloaderTask(imageView);            task.execute(url);        }    }    /* class BitmapDownloaderTask, see below */}

class BitmapDownloaderTask extends AsyncTask<String, Void, Bitmap> {    private String url;    private final WeakReference<ImageView> imageViewReference;    public BitmapDownloaderTask(ImageView imageView) {        imageViewReference = new WeakReference<ImageView>(imageView);    }    @Override    // Actual download method, run in the task thread    protected Bitmap doInBackground(String... params) {         // params comes from the execute() call: params[0] is the url.         return downloadBitmap(params[0]);    }    @Override    // Once the image is downloaded, associates it to the imageView    protected void onPostExecute(Bitmap bitmap) {        if (isCancelled()) {            bitmap = null;        }        if (imageViewReference != null) {            ImageView imageView = imageViewReference.get();            if (imageView != null) {                imageView.setImageBitmap(bitmap);            }        }    }}

处理并发(Handling concurrency)

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 download(String url, ImageView imageView) {     if (cancelPotentialDownload(url, imageView)) {         BitmapDownloaderTask task = new BitmapDownloaderTask(imageView);         DownloadedDrawable downloadedDrawable = new DownloadedDrawable(task);         imageView.setImageDrawable(downloadedDrawable);         task.execute(url, cookie);     }}

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;}

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;}

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;}

if (imageViewReference != null) {    ImageView imageView = imageViewReference.get();    BitmapDownloaderTask bitmapDownloaderTask = getBitmapDownloaderTask(imageView);    // Change bitmap only if this process is still associated with it    if (this == bitmapDownloaderTask) {        imageView.setImageBitmap(bitmap);    }}