在下面的代码中，分别声明了三个函数，即创建文件目录函数、写文件函数与读文件函数。

//创建文件目录

Future<File> get _localFile async {

        final directory = await getApplicationDocumentsDirectory();

        final path = directory.path;

        return File('$path/content.txt');

}

//将字符串写入文件

Future<File> writeContent(String content) async {

        final file = await _localFile;

        return file.writeAsString(content);

}

//从文件读出字符串

Future<String> readContent() async {

        try {

                final file = await _localFile;

                String contents = await file.readAsString();

                return contents;

        } catch (e) {

                return "";

        }

}