TextFormField數據處理

重點：TextFormField這個Widget是由TextField封裝而來，繼承了TextField的特性：
數據傳遞依靠：GlobalKey<FormState>()，
RegisterKey.currentState.save();

加上Form里面有key選項，這三項復合起來，可以達到數據傳遞的目的。具體請看如下示例：

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: HomePage(),
      ),
    );
  }
}

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  final RegisterKey=GlobalKey<FormState>();
  String username, password;

  void submitForm(){
    RegisterKey.currentState.save();
    debugPrint('username:$username');
    debugPrint('password:$password');
  }

  @override
  Widget build(BuildContext context) {
    return Form(
      key:RegisterKey,
        child: Column(
          children: <Widget>[
            Padding(
              padding: EdgeInsets.all(10.0),
              child: TextFormField(
                decoration: InputDecoration(
                  labelText: 'Please input username',
                  contentPadding: EdgeInsets.all(20),
                ),
                onSaved: (value){
                  username=value;
                },
              ),
            ),
              Container(
                padding: EdgeInsets.all(10.0),
                child: TextFormField(
                  obscureText: true,
                  decoration: InputDecoration(
                    contentPadding: EdgeInsets.all(20),
                    labelText: 'Please input password',
                  ),
                  onSaved: (value){password=value;},
                ),
              ),
            SizedBox(height: 10,),
            Container(
              padding: EdgeInsets.all(20.0),
              width: double.infinity,
              child: RaisedButton(
                onPressed:submitForm,
                color: Colors.lightGreen,
                child: Text('點我哦'),
              ),
            ),
          ],
        ),
    );
  }
}