The bounds of the individual list item

The size of the image after it’s scaled down
to fit in the list item

To look up the bounds of the Scrollable,
you pass a ScrollableState into your FlowDelegate.

To look up the bounds of your individual list item,
you pass your list item’s BuildContext into your FlowDelegate.

To look up the final size of your background image,
you assign a GlobalKey to your Image widget,
and then you pass that GlobalKey into your 
FlowDelegate.

Make this information available to ParallaxFlowDelegate.

Having all the information needed to implement
parallax scrolling, implement the shouldRepaint() method.

Now, implement the layout calculations for the parallax effect.

First, calculate the pixel position of a list
item within its ancestor Scrollable.