MyPanListener.java

private class MyPanListener extends GestureDetector.SimpleOnGestureListener {
  @Override
  public boolean onScroll(MotionEvent e1, MotionEvent e2, float distanceX, float distanceY) {
    mDrawingMatrix.postTranslate(distanceX, distanceY);
    invalidate();
    return true;
  }
}

MyScaleListener.java

private class MyScaleListener extends ScaleGestureDetector.SimpleOnScaleGestureListener {
  @Override
  public boolean onScale(final ScaleGestureDetector detector) {
    final float scale = detector.getScaleFactor();
    mDrawingMatrix.postScale(scale, scale, detector.getFocusX(), detector.getFocusY());
    mDrawingMatrix.mapRect(outRect, bitmapSizeRect);
    if (bitmapSizeRect.width() / outRect.width() > 2) {
      // 2x zoomed in, start loading the next level.
      // outRect shows the currently-visible region of the bitmap, can just pass to BitmapRegionDecoder
    }
    invalidate();
    return true;
  }
}

MyView.java

public class MyView extends View {
  // or whatever
  private GestureDetector scaleDetector = new GestureDetectorCompat(new MyScaleListener());
  private GestureDetector panDetector = new GestureDetectorCompat(new MyPanListener());

  @Override
  public boolean onTouchEvent(final MotionEvent event) {
    boolean result = super.onTouchEvent(event); // handles listeners, etc
    result |= scaleDetector.onTouchEvent(event);
    result |= panDetector.onTouchEvent(event);
    return result;
  }

  @Override
  protected void onDraw(@NotNull final Canvas canvas) {
    canvas.drawBitmap(bitmap, mDrawingMatrix, mFilterBitmapPaint);
  }
}