createScrollPosition method
ScrollPosition
createScrollPosition(
- ScrollPhysics physics,
- ScrollContext context,
- ScrollPosition? oldPosition
override
Creates a ScrollPosition for use by a Scrollable widget.
Subclasses can override this function to customize the ScrollPosition used by the scrollable widgets they control. For example, PageController overrides this function to return a page-oriented scroll position subclass that keeps the same page visible when the scrollable widget resizes.
By default, returns a ScrollPositionWithSingleContext.
The arguments are generally passed to the ScrollPosition being created:
physics: An instance of ScrollPhysics that determines how the ScrollPosition should react to user interactions, how it should simulate scrolling when released or flung, etc. The value will not be null. It typically comes from the ScrollView or other widget that creates the Scrollable, or, if none was provided, from the ambient ScrollConfiguration.context: A ScrollContext used for communicating with the object that is to own the ScrollPosition (typically, this is the Scrollable itself).oldPosition: If this is not the first time a ScrollPosition has been created for this Scrollable, this will be the previous instance. This is used when the environment has changed and the Scrollable needs to recreate the ScrollPosition object. It is null the first time the ScrollPosition is created.
Implementation
@override
ScrollPosition createScrollPosition(ScrollPhysics physics,
ScrollContext context, ScrollPosition? oldPosition) {
assert(_carouselState != null);
final List<int>? weights = _carouselState!._weights;
final double? itemExtent = _carouselState!._itemExtent;
int expandedItem = initialItem;
if (weights != null && !_carouselState!.allowFullyExpand) {
int smallerWeights = 0;
for (final int weight in weights) {
if (weight == weights.max) {
break;
}
smallerWeights += 1;
}
expandedItem -= smallerWeights;
}
return _CarouselPosition(
physics: physics,
context: context,
initialItem: expandedItem,
itemExtent: itemExtent,
layoutWeights: weights,
oldPosition: oldPosition,
);
}