简单例子：该例子实现了子View居中对齐效果

class CenterLayout extends SingleChildRenderObjectWidget {

  CenterLayout({
    Key key,
    Widget child
  }): super(key: key , child: child);

  @override
  RenderObject createRenderObject(BuildContext context) => RenderLayout();

}

class RenderLayout extends RenderShiftedBox {

  RenderLayout({RenderBox child}) : super(child);

  @override
  void performLayout() {
    child.layout(BoxConstraints(
        minHeight: 0.0,
        //maxHeight: null,
        minWidth: 0.0,
        //maxWidth: null,
    ), parentUsesSize: true);
    final BoxParentData childParentData = child.parentData;
    //控制子View坐标，（0，0）是屏幕左上角
    childParentData.offset = Offset((size.width - child.size.width) / 2, (size.height - child.size.height) / 2);
    size = Size(this.constraints.maxWidth, constraints.maxHeight);
  }

}

使用：

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Color(0xffffffff),
      body: Container(
        child: CenterLayout(
          child: Text(
              "我我我我我我我我我我我我我我我我我我我我我我我我我我我我我我我我我我我我我我我我我我我我我我我我"
          ),
        ),
      ),
    );
  }