另外，flutter还分别实现了几个子类，进一步封装了RenderObjectWidget，它们分别是LeafRenderObjectWidget、SingleChildRenderObjectWidget、MultiChildRenderObjectWidget。其中，LeafRenderObjectWidget是叶节点，不含子Widget；SingleChildRenderObjectWidget仅有一个child；而MultiChildRenderObjectWidget则是含有children列表。这几个子类根据child的情况分别创建了对应的Element，所以通过这几个子类，我们只需要关注RenderObject的创建和更新。
（2）用法以最简单的LeafRenderObjectWidget为例，我们需要实现createRenderObject、updateRenderObject两个方法：
class CustomRenderWidget extends LeafRenderObjectWidget { CustomRenderWidget({ this.width = 0, this.height = 0, this.color, }); final double width; final double height; final Color color; @override RenderObject createRenderObject(BuildContext context) { return CustomRenderBox(width, height, color); } @override void updateRenderObject(BuildContext context, RenderObject renderObject) { CustomRenderBox renderBox = renderObject as CustomRenderBox; renderBox ..width = width ..height = height ..color = color; } }