getVerticalDragScrollData function

DragScrollData getVerticalDragScrollData({ required double displacement, required double globalOffset, required double localOffset, required double gestureOriginOffset, required double viewportExtent, required double screenHeight, required double frozenExtent, required double positionPixel, required ScrollingData scrollingData, })

Given the pointer local and global offsets return DragScrollData for the vertical axis.

Parameters:

• displacement see VirtualizationState.displacement.
• globalOffset The y offset from the events globalPosition.
• localOffset The y offset from the events localPosition.
• gestureOriginOffset The y offset from the coordinate where the drag gesture started.
• viewportExtent The total extent of the viewport in the vertical axis.
• screenHeight the height of the screen
• frozenExtent How much of the viewport is dedicated to frozen headers.
• positionPixel the current scrolled amount in the scroll view
• scrollingData see VirtualizationState.scrollingData

Implementation

DragScrollData getVerticalDragScrollData({
  required double displacement,
  required double globalOffset,
  required double localOffset,
  required double gestureOriginOffset,
  required double viewportExtent,
  required double screenHeight,
  required double frozenExtent,
  required double positionPixel,
  required ScrollingData scrollingData,
}) {
  // The point that represents the top edge of the scrollable part.
  final topThreshold = displacement.abs() + frozenExtent;
  if (gestureOriginOffset < topThreshold) {
    // If the gesture started in a frozen pane
    if (localOffset >= topThreshold) {
      // Going to the scrollable area should reset scroll
      return ResetScrollDragScrollData();
    }
    // Keeping the drag in the frozen area should not scroll
    return DoNotScrollDragScrollData();
  }

  final precedingScrollExtent = scrollingData.constraints.precedingScrollExtent;

  final pointerDistanceToTopThreshold = topThreshold - localOffset;
  if (pointerDistanceToTopThreshold > 0) {
    return AutoScrollDragScrollData(
      direction: AxisDirection.up,
      maxToScroll: positionPixel - precedingScrollExtent + displacement.abs(),
      pointerDistance: pointerDistanceToTopThreshold,
    );
  }

  final bottomThreshold = screenHeight - kAutoScrollTriggerThreshold;
  final pointerDistanceToBottomThreshold = globalOffset - bottomThreshold;
  if (pointerDistanceToBottomThreshold > 0) {
    // The bottom of the table is the amount scrolled on previous slivers
    // plus the total extent of the table (which includes headers and top
    // padding).
    final tableBottomPixel = precedingScrollExtent + scrollingData.totalExtent;

    // The current bottom of the table is the which is the amount of pixels
    // already scrolled plus the leadingPadding (header + top padding) and
    // the current viewport extent.
    final viewportTableBottom = positionPixel + viewportExtent + scrollingData.leadingPadding;
    return AutoScrollDragScrollData(
      direction: AxisDirection.down,
      maxToScroll: tableBottomPixel - viewportTableBottom + config.kColumnHeaderHeight,
      pointerDistance: pointerDistanceToBottomThreshold,
    );
  }

  return DoNotScrollDragScrollData();
}