import 'package:flutter/material.dart';

GlobalKey<_ChildState> globalKey = GlobalKey();

class Child extends StatefulWidget {
  Child({
    Key key,
  }) : super(key: key);
  @override
  _ChildState createState() => _ChildState();
}
class _ChildState extends State<Child> {
  
  //子组件方法
  childMethod(){}
  ....
}

import 'package:flutter/material.dart';

class Parent extends StatefulWidget {
  Parent({}) : super(key: key);
  @override
  _ParentState createState() => _ParentState();
}
class _ParentState extends State<Parent> {
  parentMethod(){
    //父组件中调用
    globalKey.currentState.childMethod(_contractOrder)
  }
  @override
    Widget build(BuildContext context) {
      return Child(key: globalKey)
    }
}