Async Tasks and Downloading Images

Reading

• Read Big Nerd Ranch
• HTTP and Background Tasks (pages 473-498)

References

• Camera Basics: https://developer.android.com/training/camera/photobasics.html
• ViewHolder: https://developer.android.com/training/improving-layouts/smooth-scrolling.html
• RecyclerView: https://developer.android.com/guide/topics/ui/layout/recyclerview.html

This class is all about creating images, picking images, and displaying images. We’ll use the camera, the gallery, and get images from the internet. Our ultimate goal is to display images in a ListView so we can scroll through them smoothly and efficiently.

Topics

• Get Bitmap from camera
• Display Bitmap in ImageView
• Save picture to Gallery
• Pick image from Gallery
• Display images in list
• Get image from URL
• Cache URL requests
• Always inflate a new view
• Use the convert view to avoid always inflating
• Build a ViewHolder and setTag to avoid having to look up ImageView each time
• Cache Bitmaps inside DownloadImageTask
• Use RecyclerView to optimize images in ListViews

ImageView ScaleTypes

This app shows off all the different possible values for the scaleType attribute on an ImageView.

Notice that setting adjustViewBounds to true eliminates extra padding that appears above or below images for certain scale types. (Note: the app actually restarts when adjustViewBounds is toggled to false because it must reset the ImageView to a clean slate.)

For a quick visual reference refer to this webpage which shows off the scale types similarly: https://robots.thoughtbot.com/android-imageview-scaletype-a-visual-guide

And, here’s a screenshot of that website:

Here’s how the Android docs describe the scale types: https://developer.android.com/reference/android/widget/ImageView.ScaleType.html

Enum values
ImageView.ScaleType	CENTER Center the image in the view, but perform no scaling.
ImageView.ScaleType	CENTER_CROP Scale the image uniformly (maintain the image's aspect ratio) so that both dimensions (width and height) of the image will be equal to or larger than the corresponding dimension of the view (minus padding).
ImageView.ScaleType	CENTER_INSIDE Scale the image uniformly (maintain the image's aspect ratio) so that both dimensions (width and height) of the image will be equal to or less than the corresponding dimension of the view (minus padding).
ImageView.ScaleType	FIT_CENTER Scale the image using CENTER.
ImageView.ScaleType	FIT_END Scale the image using END.
ImageView.ScaleType	FIT_START Scale the image using START.
ImageView.ScaleType	FIT_XY Scale the image using FILL.
ImageView.ScaleType	MATRIX Scale using the image matrix when drawing.

Here’s what the app looks like

Download Image Task

public class DownloadImageTask extends AsyncTask<Void, Void, Bitmap> {
    private static Map<String, Bitmap> cache = new HashMap<>();

    private Context mContext;
    private String mUrl;
    private ImageView mView;

    public DownloadImageTask(Context context, String url, ImageView view) {
        mContext = context;
        mUrl = url;
        mView = view;
    }

    @Override
    protected Bitmap doInBackground(Void... voids) {
        try {
            if (cache.containsKey(mUrl)) {
                return cache.get(mUrl);
            }
            InputStream stream = new URL(mUrl).openConnection().getInputStream();
            Bitmap bitmap = BitmapFactory.decodeStream(stream);

            Thread.sleep(1500);
            cache.put(mUrl, bitmap);

            return bitmap;
        } catch (IOException e) {
            return null;
        } catch (InterruptedException e) {

        }

        return null;
    }

    @Override
    protected void onPostExecute(Bitmap bitmap) {
        if (bitmap != null) {
            super.onPostExecute(bitmap);
            mView.setImageBitmap(bitmap);
        }
    }
}