Form表单

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: '登录表单验证 Form',
      theme: ThemeData(

        primarySwatch: Colors.orange,
      ),
      home: MyHomePage(title: '登录表单验证 Form'),
    );
  }
}

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

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

class _MyHomePageState extends State<MyHomePage> {
  TextEditingController _unameController = new TextEditingController();
  TextEditingController _pwdController = new TextEditingController();
  GlobalKey _formKey = new GlobalKey<FormState>(); // 为了通过 Globalkey找到该state

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Padding(
        padding: const EdgeInsets.symmetric(vertical: 16.0,horizontal: 24.0),
        child: Form(
          key: _formKey,
          autovalidate: true, // 开启自动校验
          child: Column(
            children: <Widget>[
              TextFormField(
                autofocus: true,
                controller: _unameController,
                decoration: InputDecoration(
                  labelText: '用户名',
                  hintText:'用户名或邮箱',
                  icon: Icon(Icons.perm_contact_calendar),
                ),
                // 校验
                validator: (v){
                  return v
                      .trim()
                      .length > 0 ? null: '用户名不能为空';
                },
              ),
              TextFormField(
                controller: _pwdController,
                decoration: InputDecoration(
                  labelText: '密码',
                  hintText: '输入密码',
                  icon: Icon(Icons.lock),
                ),
                obscureText: true,
                validator: (v){
                  return v
                      .trim()
                      .length > 5 ? null : '密码不能少于6位';
                },
              ),
              // 登录按钮
              Padding(
                padding: const EdgeInsets.only(top: 28.0),
                child: Row(
                   children: <Widget>[
                      Expanded(
                        child: RaisedButton(
                          padding:  EdgeInsets.all(15.0),
                          child: Text('登录'),
                          color: Theme.of(context).primaryColor,
                          textColor: Colors.white,
                          onPressed: (){
                            if ((_formKey.currentState as FormState).validate()){
                              debugPrint('验证通过提交');
                            }
                          },
                        ),
                      )
                   ],
                ),
              )
            ],
          )
        ),
      ),
    );
  }
}