对于此任务，建议使用TextEditingController沿着TextFormField的onChanged回调函数。下面的示例清楚地说明了所需的方法。

class TextFormFieldDemo extends StatefulWidget {
  const TextFormFieldDemo({super.key});

  @override
  State<TextFormFieldDemo> createState() => _TextFormFieldDemoState();
}

class _TextFormFieldDemoState extends State<TextFormFieldDemo> {
  late final TextEditingController textField1Controller;
  late final TextEditingController textField2Controller;

  @override
  void initState() {
    super.initState();
    textField1Controller = TextEditingController();
    textField2Controller = TextEditingController();
  }

  @override
  void dispose() {
    textField1Controller.dispose();
    textField2Controller.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: Column(
        children: [
          TextFormField(
            controller: textField1Controller,
            onChanged: (value) {
              final double? val = double.tryParse(value);
              if (val != null) {
                textField2Controller.text = (val + 1).toString();
              }
            },
          ),
          const SizedBox(height: 12),
          TextFormField(
            controller: textField2Controller,
          )
        ],
      ),
    );
  }
}

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您可以简单地实现这一点，将相同的textEditingController分配给两个textFormField小部件，如下所示

class TextFormFieldDemo extends StatefulWidget {
          const TextFormFieldDemo({super.key});
        
          @override
          State<TextFormFieldDemo> createState() => _TextFormFieldDemoState();
}
        
class _TextFormFieldDemoState extends State<TextFormFieldDemo> {
          late final TextEditingController textFieldController=TextEditingController();
        
          @override
          Widget build(BuildContext context) {
            return Scaffold(
              appBar: AppBar(),
              body: Column(
                children: [
                  TextFormField(
                    controller: textFieldController,
                  ),
                  const SizedBox(height: 10),
                  TextFormField(
                    controller: textFieldController,
                  )
                ],
              ),
            );
          }
        }

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您可以在文本字段1的onChanged方法中更新文本字段2的值。
示例：-

TextFormField(
                controller: textController1,
                onChanged: (String? value){
                  textController2.text="new value";
                },
              ),

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