获取 _getVin.text 的空值

Question

在下面的代码中，我实现了一个文本字段，它的值将由 _getVin 存储。但是在用户能够输入值之前，我收到了空值。那么什么是正确的解决方案？有哪些改变？

 @override
  Widget build(BuildContext context) {
    TextEditingController _getVin = TextEditingController();
     var vin =  VIN(number: _getVin.text, extended: true);


return new Scaffold (

             body:  Container(
                padding: new EdgeInsets.only(top: 16.0),
              child: TextField(
                controller: _getVin,
                decoration:InputDecoration(
                  border :OutlineInputBorder(
                      borderRadius: BorderRadius.all(Radius.circular(5.0)),
                      borderSide: BorderSide(color:Colors.black)
                  ),
                  labelText: "Enter VIN",
                  labelStyle: TextStyle(
                      fontSize: 15,
                      color:Colors.black
                  ),
                ),

              ),
            ),
               GestureDetector(
                   onTap:() {
                     print(_getVin);

                     print(vin.modelYear());

                         },


                child:Card(
                 child: Text( "Press Here"),
                                    )
                 ),

Answer 1

糟糕，您不应该在构建方法中创建控制器，每次颤振重建该小部件时，您将有效地创建一个新控制器，并且您不会丢弃它。正确的做法是使用 Stateful 小部件：

class VinWidget extends StatefulWidget {
  const VinWidget({Key? key}) : super(key: key);

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

class _VinWidgetState extends State<VinWidget> {
  late TextEditingController _getVin = TextEditingController();

  @override
  Widget build(BuildContext context) {
    
 return  Scaffold (

             body:  Container(
                padding:  EdgeInsets.only(top: 16.0),
              child: TextField(
                controller: _getVin,
                decoration:InputDecoration(
                  border :OutlineInputBorder(
                      borderRadius: BorderRadius.all(Radius.circular(5.0)),
                      borderSide: BorderSide(color:Colors.black)
                  ),
                  labelText: "Enter VIN",
                  labelStyle: TextStyle(
                      fontSize: 15,
                      color:Colors.black
                  ),
                ),

              ),
            ),
               GestureDetector(
                   onTap:() {
var vin = VIN(number: _getVin.text, extended: true);
                     print(_getVin);

                     print(vin.modelYear());

                         },


                child:Card(
                 child: Text( "Press Here"),
                                    )
                 ),
  }

  @override
  void dispose() {
    super.dispose();
    _getVin.dispose();
  }
}