screenOffsetFraction property
The current progress an element is going through the scrolling viewport. If the item is near the center of the scroll view, the value tends towards 0. As the item moves towards the ceiling of the scroll view, the value tends towards 1. It clamps to 1 when the item is fully out of the scroll view. As the item moves towards the floor of the scroll view, the value tends towards -1. It clamps to -1 when the item is fully out of the scroll view.
Implementation
final double screenOffsetFraction;