Question

Hello fellow developers 🙂

I’ve made a very basic image fetcher for Android to download and display bitmaps from the web on my application the code for it is:

public class BitmapFetcher {

    private static HashMap<String, SoftReference<Bitmap>> bitmapCache = new HashMap<String, SoftReference<Bitmap>>();

    public static Bitmap fetchBitmap(String urlString) {
        SoftReference<Bitmap> cachedBitmap = bitmapCache.get(urlString);

        if (cachedBitmap != null && cachedBitmap.get() != null) {
            return cachedBitmap.get();
        }

        try {
            InputStream is = fetch(urlString);

            Bitmap bitmap = BitmapFactory.decodeStream(is);
            SoftReference<Bitmap> softReferencedBitmap = new SoftReference<Bitmap>(bitmap);
            bitmapCache.put(urlString, softReferencedBitmap);

            return bitmap;
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

        return null;
    }

    public static void fetchBitmapAsync(final String urlString, final ImageView view) {
        final Handler handler = new Handler() {
            public void handleMessage(Message message) {
                AsyncImageContainer imageContainer = (AsyncImageContainer) message.obj;

                imageContainer.applyImageToView();
            }
        };

        BitmapTaskRunnable asyncImageFetcherTask = new BitmapTaskRunnable(view, urlString, handler);

        new Thread(asyncImageFetcherTask).start();
    }

    public static InputStream fetch(String urlString) throws MalformedURLException, IOException {
        Log.d("BitmapFetcher", "fetch: " + urlString);
        DefaultHttpClient httpClient = new DefaultHttpClient();
        HttpGet request = new HttpGet(urlString);
        HttpResponse response = httpClient.execute(request);

        return response.getEntity().getContent();
    }

}

BitmapTaskRunnable.java:

public class BitmapTaskRunnable implements Runnable {

    private ImageView imageView;
    private String imageUrl;
    private Handler handler;

    public BitmapTaskRunnable() {
    }

    public BitmapTaskRunnable(ImageView imageView, String imageUrl, Handler handler) {
        setImageView(imageView);
        setImageUrl(imageUrl);
        setHandler(handler);
    }

    public void run() {
        Bitmap bitmap = BitmapFetcher.fetchBitmap(getImageUrl());

        AsyncImageContainer imageContainer = new AsyncImageContainer(getImageView(), bitmap);

        handler.sendMessage(handler.obtainMessage(0, imageContainer));
    }

    public ImageView getImageView() {
        return imageView;
    }

    public void setImageView(ImageView imageView) {
        this.imageView = imageView;
    }

    public String getImageUrl() {
        return imageUrl;
    }

    public void setImageUrl(String imageUrl) {
        this.imageUrl = imageUrl;
    }

    public Handler getHandler() {
        return handler;
    }

    public void setHandler(Handler handler) {
        this.handler = handler;
    }

}

AsyncImageContainer.java:

public class AsyncImageContainer {

    private ImageView imageView;
    private Bitmap bitmap;

    public AsyncImageContainer() {
    }

    public AsyncImageContainer(ImageView imageView, Bitmap bitmap) {
        setImageView(imageView);
        setBitmap(bitmap);
    }

    public void applyImageToView() {
        getImageView().setImageBitmap(getBitmap());
    }

    public ImageView getImageView() {
        return imageView;
    }

    public void setImageView(ImageView imageView) {
        this.imageView = imageView;
    }

    public Bitmap getBitmap() {
        return bitmap;
    }

    public void setBitmap(Bitmap bitmap) {
        this.bitmap = bitmap;
    }

}

As I said, it’s very basic, with a very basic caching and no throttling on threads being created (this is already planned to be done soon).

Well, I’m currently experiencing a weird behavior from this when I need to load images in a ListView, what I did is: I have some classes that extend ArrayAdapter and overrides getView to display my layout, whenever I need an image on it, I will do the following:

BitmapFetcher.fetchBitmapAsync(news.getChannelAvatar(), holder.channelAvatarView);

And this should start make BitmapFetcher start a new thread which will download the bitmap and send a message to the handler to it apply the image to the view (as only the thread which created a view hierarchy can modify it).

Everything is fine for the 2nd to n-th ImageViews in the list, but the 1st result ALWAYS goes crazy, changing to images which have been downloaded until all the images are loaded and it settles to it. Then if I drag the list a little until the 1st result disappears and goes back to the top, it displays the correct image.

This is really bugging me, as I had done a much simpler version of the code (one which the handler handled directly placing the Bitmap in the ImageView, the Runnable did not exist, it was a simple anonymous Thread object with run() overriden, etc) and tried this version thinking that somehow fetchBitmapAsync was losing reference to the correct ImageView or something like that.

Does this happen for some thing that Android does to recycle Views inside a ListView? If not, what could be the reason? Am I being silly somewhere and after a couple days working on this code I’ve gone blind? 🙁

Thanks for all the help 😀

Answer 1

For all of my lazy image loading I use Prime. It even has an example of how to use it within a ListView.