Flutter/Dart 返回 Future<String?>

Question

I have a function, which I want to return a Future of String or null. For simplicity, the function will return null if delay for 1 second is success, or else it will return "failed";

However; I got the

  Future<String?> fcn(String str) {
    return Future.delayed(const Duration(seconds: 1)).then((value) {
      return null;
    }).onError((error, stackTrace) {
      return "failed"; // The return type 'String' isn't a 'FutureOr<Null>', as required by the closure's context.
    });
  }

Answer 1

Try to use onError as parameter of then method:

Future<String?> fcn(String str) {
  return Future.delayed(const Duration(seconds: 1)).then((value) {
    return null;
  }, onError: (error, stackTrace) {
    return "failed";
  });
}