The first naïve idea would be to change the level of blurriness depending on the position of
scrollView, but this will be really inefficient because the blur operation is CPU-intensive, and it won't be smooth on older devices.
So, the idea is to trick the user. Instead of blurring the image at every change of position of
scrollView, we blur the image, only before setting it to
UIImageView. Then, we set the alpha channel to 0 (which means transparent). Next, we change the alpha depending on the position, reaching opaque when the
scrollView offset reaches half.
First of all, we need to import the framework to blur the image:
Then, we need to create the overlay view and set it as
private let overlayView ...