An object that can be used to control the position to which this scroll view is scrolled.
Must be null if primary is true.
A ScrollController serves several purposes. It can be used to control the initial scroll position (see ScrollController.initialScrollOffset). It can be used to control whether the scroll view should automatically save and restore its scroll position in the PageStorage (see ScrollController.keepScrollOffset). It can be used to read the current scroll position (see ScrollController.offset), or change it (see ScrollController.animateTo).
final ScrollController controller