For example, if growthDirection is GrowthDirection.forward and axisDirection is AxisDirection.down, then a ScrollDirection.reverse means that the user is scrolling down, in the positive scrollOffset direction.
This is used by some slivers to determine how to react to a change in scroll offset. For example, RenderSliverFloatingPersistentHeader will only expand a floating app bar when the userScrollDirection is in the positive scroll offset direction.
To create a local project with this code sample, run:
flutter create --sample=rendering.ScrollDirection.1 mysample
final ScrollDirection userScrollDirection;