flutter 滚动小部件原理

由上图可知，滚动的实现为：

• 监听手势，并将滑动手势转换为offset
• 重新layout，并将offset传递下去，直到列表RenderObject
• 列表RenderObject利用scrollOffset重新paint child，列表的child为链表方式存储，paint firstChild之后累加offset继续paint后续child renderObject

#class RenderSliverMultiBoxAdaptor
@override
  void paint(PaintingContext context, Offset offset) {
    if (firstChild == null)
      return;
    // offset is to the top-left corner, regardless of our axis direction.
    // originOffset gives us the delta from the real origin to the origin in the axis direction.
    Offset mainAxisUnit, crossAxisUnit, originOffset;
    bool addExtent;
    switch (applyGrowthDirectionToAxisDirection(constraints.axisDirection, constraints.growthDirection)) {
      case AxisDirection.up:
        mainAxisUnit = const Offset(0.0, -1.0);
        crossAxisUnit = const Offset(1.0, 0.0);
        originOffset = offset + Offset(0.0, geometry.paintExtent);
        addExtent = true;
        break;
      case AxisDirection.right:
        mainAxisUnit = const Offset(1.0, 0.0);
        crossAxisUnit = const Offset(0.0, 1.0);
        originOffset = offset;
        addExtent = false;
        break;
      case AxisDirection.down:
        mainAxisUnit = const Offset(0.0, 1.0);
        crossAxisUnit = const Offset(1.0, 0.0);
        originOffset = offset;
        addExtent = false;
        break;
      case AxisDirection.left:
        mainAxisUnit = const Offset(-1.0, 0.0);
        crossAxisUnit = const Offset(0.0, 1.0);
        originOffset = offset + Offset(geometry.paintExtent, 0.0);
        addExtent = true;
        break;
    }
    assert(mainAxisUnit != null);
    assert(addExtent != null);
    RenderBox child = firstChild;
    while (child != null) {
      final double mainAxisDelta = childMainAxisPosition(child);
      final double crossAxisDelta = childCrossAxisPosition(child);
      Offset childOffset = Offset(
        originOffset.dx + mainAxisUnit.dx * mainAxisDelta + crossAxisUnit.dx * crossAxisDelta,
        originOffset.dy + mainAxisUnit.dy * mainAxisDelta + crossAxisUnit.dy * crossAxisDelta,
      );
      if (addExtent)
        childOffset += mainAxisUnit * paintExtentOf(child);

      // If the child's visible interval (mainAxisDelta, mainAxisDelta + paintExtentOf(child))
      // does not intersect the paint extent interval (0, constraints.remainingPaintExtent), it's hidden.
      if (mainAxisDelta < constraints.remainingPaintExtent && mainAxisDelta + paintExtentOf(child) > 0)
        context.paintChild(child, childOffset);

      child = childAfter(child);
    }
  }