// Grows the cropping rectange by (dx, dy) in image space.
private void moveBy(float dx, float dy)
{
Rect invalRect = new Rect(DrawRect);
cropRect.Offset(dx, dy);
// Put the cropping rectangle inside image rectangle.
cropRect.Offset(
Math.Max(0, imageRect.Left - cropRect.Left),
Math.Max(0, imageRect.Top - cropRect.Top));
cropRect.Offset(
Math.Min(0, imageRect.Right - cropRect.Right),
Math.Min(0, imageRect.Bottom - cropRect.Bottom));
DrawRect = computeLayout();
invalRect.Union(DrawRect);
invalRect.Inset(-10, -10);
context.Invalidate(invalRect);
}