关于作业题，找到了源代码注释，哈哈哈。

/// If the pre-built subtree is passed as the [child] parameter, the
/// [AnimatedBuilder] will pass it back to the [builder] function so that it
/// can be incorporated into the build.
///
/// Using this pre-built child is entirely optional, but can improve
/// performance significantly in some cases and is therefore a good practice.
/// can be incorporated into the build.
///
/// Using this pre-built child is entirely optional, but can improve
/// performance significantly in some cases and is therefore a good practice.

AnimatedBulder 其实就是封装了下 AnimatedWdiget，在 AnimatedBulder 内部调用了 builder 函数，构建 widget 样式

这样的好处就是给真实要渲染的widget上加了一层 Container，所有的动画都是在那个 Container 上完成。

回答问题：
1. 为了渲染和动画分离
2. 可以的就是渲染和动画又揉一起了

老师有没有遇到这个问题，Widget myFormWidget(BuildContext context){在这里吗创建一个form组件}
，class BodyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return Column(
      mainAxisAlignment: MainAxisAlignment.start,
      children: <Widget>[
        // textFieldWidget(context),
        myFormWidget(context),
      ],
    );
  }
然后通过这种方式调用，键盘刚获取焦点立马就是去焦点了，无法输入，这是什么原因