问ScrollablePositionedList初始滚动偏移量

EN

int initialIndex = 1;

@override
Widget build(BuildContext context) {
   return LayoutBuilder(builder: (context, constraints) {
       return ScrollablePositionedList.builder(
           initialScrollIndex: initialIndex,
           initialAlignment: getAlignment( 
              index: initialIndex,
              itemHeight: list[initialIndex].height,
              viewPortHeight: constraints.maxHeight,
              alignmentOnItem: 0.5, //middle of item
           ),

double getAlignment({
    required int index,
    required double viewPortHeight,

    ///The item height at the [index]. If all your items have the same height this
    ///can be a static value
    required double itemHeight,

    ///A double value between |0, 1| is expected, where percentage of the item
    ///should be positioned based on the center of the screen.
    ///(0 is top of the item, 1 is bottom if the item)
    double alignmentOnItem = 0.5,
  }) {
    assert(alignmentOnItem >= 0 && alignmentOnItem <= 1,
        "Alignment on item is expected to be within 0 and 1");
    final relativePageHeight = 1 / viewPortHeight * itemHeight;
    return 0.5 - relativePageHeight * alignmentOnItem;
  }