问我正在使用Flutter文本字段(不是文本表单字段)制作温度转换器应用程序，我需要限制多个点和多个符号(+ - )？

EN

class MyHomePage extends StatefulWidget {
  @override
  MyHomePageState createState() {
    return new MyHomePageState();
  }
}

class MyHomePageState extends State<MyHomePage> {
  final _text = TextEditingController();
  bool _validate = false;

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('TextField Demo'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            TextField(
              controller: _text,
              decoration: InputDecoration(
                labelText: 'Enter the Value',
                errorText: !_validate ? 'Your Message' : null,
              ),
            ),
            RaisedButton(
              onPressed: () {
                setState(() {
                  // **The Condition**
                  if ('.'.allMatches(_text.text).length > 1 &&
                      ('+'.allMatches(_text.text).length > 1 ||
                          '-'.allMatches(_text.text).length > 1)) {
                    _validate = true;
                  } else {
                    _validate = false;
                  }
                });
              },
              child: Text('Submit'),
              textColor: Colors.white,
              color: Colors.blueAccent,
            )
          ],
        ),
      ),
    );
  }
}