text_input_dialog.dart

  final _textFieldController = TextEditingController();

  Future<String?> _showTextInputDialog(BuildContext context) async {
    return showDialog(
        context: context,
        builder: (context) {
          return AlertDialog(
            title: const Text('タイトル'),
            content: TextField(
              controller: _textFieldController,
              decoration: const InputDecoration(hintText: "文字列を入力してください。"),
            ),
            actions: <Widget>[
              ElevatedButton(
                child: const Text("キャンセル"),
                onPressed: () => Navigator.pop(context),
              ),
              ElevatedButton(
                child: const Text('OK'),
                onPressed: () => Navigator.pop(context, _textFieldController.text),
              ),
            ],
          );
        },
    );
  }