method updateRenderObject

Source

@protected
@mustCallSuper
void updateRenderObject(@checked VirtualViewport oldWidget) {
  renderObject.virtualChildCount = _widgetProvider.virtualChildCount;

  if (startOffsetBase != null) {
    _updatePaintOffset();

    // If we don't already need layout, we need to request a layout if the
    // viewport has shifted to expose new children.
    if (!renderObject.needsLayout) {
      final double startOffset = widget.startOffset;
      bool shouldLayout = false;
      if (startOffsetBase != null) {
        if (startOffset < startOffsetBase)
          shouldLayout = true;
        else if (startOffset == startOffsetBase && oldWidget?.startOffset != startOffsetBase)
          shouldLayout = true;
      }

      if (startOffsetLimit != null) {
        if (startOffset > startOffsetLimit)
          shouldLayout = true;
        else if (startOffset == startOffsetLimit && oldWidget?.startOffset != startOffsetLimit)
          shouldLayout = true;
      }

      if (shouldLayout)
        renderObject.markNeedsLayout();
    }
  }
}