flutter 弹出框键盘遮挡 获取键盘高度

键盘高度：MediaQuery.of(context).viewInsets.bottom （此高度只能在键盘唤醒时在build中获取，弹出键盘会执行build）

看代码：
iimport ‘package:flutter/cupertino.dart’;
import ‘package:flutter/material.dart’;

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
title: ‘Flutter Demo’,
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: MyHomePage(title: ‘Flutter Demo Home Page’),
);
}
}

class MyHomePage extends StatefulWidget {
MyHomePage({Key key, this.title}) : super(key: key);

final String title;

@override
_MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State {
String hint = ‘请输入…’;

@override
Widget build(BuildContext context) {