PDFRenderer for subsampling-scale-image-view

PDFDecoder.java

/**
 * Decodes and renders a {@link PdfRenderer.Page} into a {@link Bitmap}
 * Created by Lindner Stefan on 03.05.16.
 */
@SuppressLint("NewApi")
public class PDFDecoder implements ImageDecoder {

  private float scale;

  /**
   * the current pdf site
   */
  private int position;

  /**
   * the pdf file to render
   */
  private File file;

  /**
   * basic constructor for PDFDecoder.
   * @param scale the scale to get from {@link Point} to Pixel
   */
  public PDFDecoder(int position, File file,  float scale) {
    this.file = file;
    this.scale = scale;
    this.position = position;
  }

  /**
   * Creates a {@link Bitmap}in the correct size and renders the {@link PdfRenderer.Page} into it.
   * @param context not used
   * @param uri not used
   * @return a bitmap, containing the pdf page
   * @throws Exception, if rendering fails
   */
  @Override public Bitmap decode(Context context, Uri uri) throws Exception {
    ParcelFileDescriptor descriptor = ParcelFileDescriptor.open(file, ParcelFileDescriptor.MODE_READ_ONLY);;
    PdfRenderer renderer = new PdfRenderer(descriptor);
    final PdfRenderer.Page page = renderer.openPage(position);

    Bitmap bitmap = Bitmap.createBitmap((int)(page.getWidth()*scale+0.5), (int)(page.getHeight()*scale+0.5f),
        Bitmap.Config.ARGB_8888);

    page.render(bitmap,null,null,PdfRenderer.Page.RENDER_MODE_FOR_DISPLAY);

    page.close();
    renderer.close();
    descriptor.close();

    return bitmap;
  }
}

PDFPagerAdapter.java

/**
 * Created by Lindner Stefan on 05.05.2016
 */
@SuppressLint("NewApi") public class PDFPagerAdapter extends PagerAdapter {

  /**
   * context for the view
   */
  private Context context;

  /**
   * pdf file to show
   */
  private File file;

  /**
   * file descriptor of the PDF
   */
  private ParcelFileDescriptor mFileDescriptor;

  /**
   * this scale sets the size of the {@link PdfRenderer.Page} in the {@link
   * PDFRegionDecoder}.
   * Since it rescales the picture, it also sets the possible zoom level.
   */
  private float scale;

  /**
   * this renderer is only used to count the pages
   */
  private PdfRenderer renderer;

  /**
   * @param file the pdf file
   */
  public PDFPagerAdapter(Context context, File file) {
    super();
    this.context = context;
    this.file = file;
    this.scale = 8;
    try {
      mFileDescriptor = ParcelFileDescriptor.open(file, ParcelFileDescriptor.MODE_READ_ONLY);
      renderer = new PdfRenderer(mFileDescriptor);
    } catch (IOException e) {
      throw new RuntimeException(e);
    }
  }

  /**
   * Instantiates an item. Therefor a {@link SubsamplingScaleImageView} with special decoders is
   * initialized and rendered.
   *
   * @param container isn't used here
   * @param position the current pdf page position
   */
  public Object instantiateItem(ViewGroup container, int position) {

    SubsamplingScaleImageView imageView = new SubsamplingScaleImageView(context);

    // the smaller this number, the smaller the chance to get a "outOfMemoryException"
    // still, values lower than 100 really do affect the quality of the pdf picture
    int minimumTileDpi = 100;
    imageView.setMinimumTileDpi(minimumTileDpi);

    imageView.setBitmapDecoderFactory(() -> new PDFDecoder(position, file, scale));

    imageView.setRegionDecoderFactory(() -> new PDFRegionDecoder(position, file, scale));

    ImageSource source = ImageSource.uri(file.getAbsolutePath());

    imageView.setImage(source);

    container.addView(imageView);
    return imageView;
  }

  /**
   * gets the pdf site count
   *
   * @return pdf site count
   */
  public int getCount() {
    return renderer.getPageCount();
  }

  @Override public void destroyItem(ViewGroup container, int position, Object view) {
    container.removeView((View) view);
  }

  @Override public boolean isViewFromObject(View view, Object object) {
    return view == object;
  }
}

PDFRegionDecoder.java

/**
 * Decodes and renders a given rect out of a {@link PdfRenderer.Page} into a {@link Bitmap}
 * Created by Lindner Stefan on 03.05.16.
 */
@SuppressLint("NewApi") public class PDFRegionDecoder implements ImageRegionDecoder {

  /**
   * the page that will be rendered to a bitmap.
   */
  private PdfRenderer renderer;

  /**
   * scale: defined in {@link DocumentActivity}
   */
  private float scale;

  /**
   * the current page position in the pdf
   */
  private int position;

  /**
   * the pdf page
   */
  private PdfRenderer.Page page;

  /**
   * the file descriptor
   */
  private ParcelFileDescriptor descriptor;

  /**
   * the pdf file
   */
  private File file;

  /**
   * basic constructor for PDFDecoder.
   *
   * @param scale the scale to get from {@link Point} to Pixel
   */
  public PDFRegionDecoder(int position,File file,  float scale) {
    this.file = file;
    this.scale = scale;
    this.position = position;
  }

  /**
   * Initializes the region decoder. This method initializes
   * @param context not used here
   * @param uri not used here (file is already loaded)
   * @return the rescaled point
   * @throws Exception
   */
  @Override public Point init(Context context, Uri uri) throws Exception {

    this.descriptor = ParcelFileDescriptor.open(file, ParcelFileDescriptor.MODE_READ_ONLY);
    this.renderer = new PdfRenderer(descriptor);
    page = renderer.openPage(position);

    return new Point((int) ((page.getWidth() * scale + 0.5f)),
        (int) ((page.getHeight() * scale + 0.5f)));
  }

  /**
   * Creates a {@link Bitmap} in the correct size and renders the region defined by rect of the
   * {@link PdfRenderer.Page} into it.
   *
   * @param rect the rect of the {@link PdfRenderer.Page} to be rendered to the bitmap
   * @param sampleSize the sample size
   * @return a bitmap containing the rendered rect of the page
   */
  @Override public Bitmap decodeRegion(Rect rect, int sampleSize) {

    int bitmapWidth = rect.width() / sampleSize;
    int bitmapHeight = rect.height() / sampleSize;

    Bitmap bitmap = Bitmap.createBitmap(bitmapWidth , bitmapHeight,
        Bitmap.Config.ARGB_8888);

    Matrix matrix = new Matrix();
    matrix.setScale(scale/sampleSize,scale/sampleSize);
    matrix.postTranslate(-rect.left/sampleSize, -rect.top/sampleSize);
    page.render(bitmap, null, matrix, PdfRenderer.Page.RENDER_MODE_FOR_DISPLAY);

    return bitmap;
  }

  @Override public boolean isReady() {
    return true;
  }

  /**
   * close everything
   */
  @Override public void recycle() {
    page.close();
    renderer.close();
    try {
      descriptor.close();
    } catch (IOException e) {
      e.printStackTrace();
    }
  }
}

To be more generic and close to the original API, I prefer to work with Uri instead of File in the renderers.

So I replaced this line:
this.descriptor = ParcelFileDescriptor.open(file, ParcelFileDescriptor.MODE_READ_ONLY)

by this line:
this.descriptor = context.contentResolver.openFileDescriptor(uri, "r")