futter 通过判定输入框组件是否都有值，对按钮进行禁用/启用。

onPressed 函数 为空则按钮禁用，不为空则可操作。

于是想到三元操作 ， 以下是踩坑踩完爬上来的结果

 onPressed: checkInput() ? null :  tapFunction,

坑一 onPressed:

onPressed:调用的是函数签名，而不是执行函数，也就是带括号和不带括号的区别，带上括号是执行函数，然后得到的返回值塞到 onPressed:后面去了，所以当然不行。由于dart继承了js的良好（草率）风格，导致像我这样的新手就很容易踩坑。

坑二 获取输入值

光在onchanged里面赋值是没用的，要用setState，大概是由于statefulwidget的属性是放在 state里面的。

//从TextFormField中获取值
 onChanged: (val) {
                   setState(() {
                     print(number);
                     number = val;
                   });
                 },
···
 onChanged: (val) {
       setState(() {
             password = val;
           });
         print('$password');
       },

坑三 输入框组件是否都有值

调试了一大圈，判断每次输入框输入的值才发现了

null
1
12
1
12
121
12
1

121
12

输入框最开始的时候是null，输入输入再删除删除之后，不会再回到null 的状态。哎，我这笨脑袋。
所以判定是否同时都有值的方法如下

 checkInput(){
   return number==null||number==''||password==null||password=='';
 }

方法比较笨，但是挺好使
贴一下源码吧 ,免得我忘记了

//  Description 仿钉钉登录页面
//  Author Mr.wang
//  Date 2020-05-05 17:38
import 'package:flutter/material.dart';

main() => runApp(MaterialApp(
     home: Login(),
   ));

class Login extends StatefulWidget {
 Login({Key key}) : super(key: key);
 @override
 _LoginState createState() => _LoginState();
}

class _LoginState extends State<Login> {

 TextEditingController _phoneNumberController;

 TextEditingController _passwordController;

 //账号
 String number;

 //密码
 String password;


 @override
 Widget build(BuildContext context) {
   @override
   void initState() {
     _phoneNumberController = TextEditingController();
     _passwordController = TextEditingController();


     var size = MediaQuery.of(context).size;

   }
   return Scaffold(
       appBar: buildAppBar(),
       body: Container(
         padding: EdgeInsets.only(left: 40, right: 40),
         child: Column(
             crossAxisAlignment: CrossAxisAlignment.start,
             children: <Widget>[
               buildIcon(),
               Container(
                 child: Text('欢迎使用老年宝', style: TextStyle(fontSize: 28)),
               ),
               Container(
                 padding: EdgeInsets.only(top: 40),
                 child: Text('手机号码', style: TextStyle(fontSize: 14)),
               ),
               buildNumber(),
               Container(
                 padding: EdgeInsets.only(top: 10),
                 child: Text('密码', style: TextStyle(fontSize: 14)),
               ),
               buildPassWord(),
               buildLoginButton(),
               GestureDetector(
                 child: Text('忘记密码',
                     style: TextStyle(fontSize: 14, color: Colors.grey)),
                 onTap: () {
                   print('忘记密码');
//                   Navigator.push(context, MaterialPageRoute(builder: (context) =>Agreement()));
                 },
               ),
               buildRegisterButton()
             ]),
       ));
 }

 Container buildLoginButton() {
   return Container(
       padding: EdgeInsets.only(top: 20, bottom: 20), child: LoginButton());
 }

 Container buildRegisterButton() {
   return Container(
     width: double.infinity,
     alignment: Alignment(0.0, -1.0),
     padding: EdgeInsets.only(top: 50),
     child: GestureDetector(
       child: Text('注册账号',
           textAlign: TextAlign.center,
           style: TextStyle(fontSize: 18, color: Colors.grey)),
       onTap: () {
         print('注册账号');
//                   Navigator.push(context, MaterialPageRoute(builder: (context) =>Agreement()));
       },
     ),
   );
 }

 Container buildIcon() {
   return Container(
     padding: EdgeInsets.only(bottom: 20),
     child: CircleAvatar(
         backgroundImage: NetworkImage(
             'https://t8.baidu.com/it/u=3571592872,3353494284&fm=79&app=86&size=h300&n=0&g=4n&f=jpeg?sec=1589212401&t=35519f0def1b6d8e99be80a142dea0f3')
//                    AssetImage('assets/images/app'),//debug用网络图片
         ),
   );
 }

 Container buildNumber() {
   return Container(
       decoration: BoxDecoration(
           border: Border(bottom: BorderSide(width: 0.4, color: Colors.grey))),
       child: Row(
//                      mainAxisAlignment: MainAxisAlignment.spaceAround,
         children: <Widget>[
           Expanded(
             flex: 2,
             child: Container(
                 decoration: BoxDecoration(
                     border: Border(
                         right: BorderSide(width: 0.4, color: Colors.grey))),
                 child: Container(
                   child: Text('+86',
                       style: TextStyle(
                         color: Colors.grey,
                         fontSize: 18,
                       )),
                 )),
           ),
           Expanded(
             flex: 1,
             child: Text(' '),
           ),
           Expanded(
               flex: 8,
               child: TextFormField(
                 onChanged: (val) {
                   setState(() {
                     print(number);
                     number = val;
                   });
                 },
                 keyboardType: TextInputType.number,
//                                focusNode: passwordFocusNode,
                 controller: _phoneNumberController,
                 decoration: InputDecoration(
                   hintText: "请输入手机号码",
                   border: InputBorder.none,
                 ),
               ))
         ],
       ));
 }

 Container buildPassWord() {
   return Container(
     decoration: BoxDecoration(
         border: Border(bottom: BorderSide(width: 0.4, color: Colors.grey))),
     child: TextFormField(
       onChanged: (val) {
         print('$password');
           setState(() {
             password = val;
           });
       },
       keyboardType: TextInputType.number,
//                                focusNode: passwordFocusNode,
       controller: _passwordController,
       decoration: InputDecoration(
         hintText: "请输入密码",
         border: InputBorder.none,
       ),
     ),
   );
 }

 AppBar buildAppBar() {
   return AppBar(
     brightness: Brightness.light,
     elevation: 0,
     backgroundColor: Colors.transparent,
     iconTheme: IconThemeData(color: Colors.black),
   );
 }
 checkInput(){
   return number==null||number==''||password==null||password=='';
 }
 //按钮是否禁用，返回为函数，null
 VoidCallback tapFunction() {
//    print(_phoneNumberController.text);
//    print(_passwordController.text);
//    print(context);
   //  输入账号密码检测
   showDialog(
       context: context,
       child: AlertDialog(
         title: Text('登录失败'),
         content: Text('账号或密码错误'),
       ));
 }

 Widget LoginButton() => Container(
     width: double.infinity,
     child: FlatButton(

       disabledColor: Colors.lightBlueAccent,
       disabledTextColor: Colors.white,
       onPressed: checkInput() ? null :  tapFunction,
       child: Container(
           padding: EdgeInsets.all(15),
           child:
               Text("登录", style: TextStyle(fontSize: 16, letterSpacing: 2))),
       color: Colors.blue,
       textColor: Colors.white,
       shape: RoundedRectangleBorder(
         borderRadius: BorderRadius.all(Radius.circular(10.0)),
       ),
     ));
}