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 "";
}
}
