Builder组件
文档如下：
class Builder extends StatelessWidget {
/// Creates a widget that delegates its build to a callback.
///
/// The [builder] argument must not be null.
const Builder({
Key key,
@required this.builder,
}) : assert(builder != null),
super(key: key);
/// Called to obtain the child widget.
///
/// This function is called whenever this widget is included in its parent's
/// build and the old widget (if any) that it synchronizes with has a distinct
/// object identity. Typically the parent's build method will construct
/// a new tree of widgets and so a new Builder child will not be [identical]
/// to the corresponding old one.
final WidgetBuilder builder;
@override
Widget build(BuildContext context) => builder(context);
}
如果翻阅flutter源码的话可以看到很多源码中很多组件都是依赖于Builder实现的，那么Builder价值何在？
Builder 是一个继承自 Stateless 的组件，需要实现 build 抽象方法，通过 BuildContext 对象，来构建 Widget 对象。而 Builder#build 只是使用了构造传入的 builder 函数，并将 当前的 BuildContext 作为回调传递出去。也就是将构建组件的逻辑交由外界处理，自身只是将 context 对象回调出去而已。
所以 Builder 组件唯一的价值，就是回调当前的 context ，让用户根据这个 context 进行组件的构建。
