handleError method Null safety

Stream<T> handleError(
  1. Function onError,
  2. {bool test(
    1. dynamic error

Creates a wrapper Stream that intercepts some errors from this stream.

If this stream sends an error that matches test, then it is intercepted by the onError function.

The onError callback must be of type void Function(Object error) or void Function(Object error, StackTrace). The function type determines whether onError is invoked with a stack trace argument. The stack trace argument may be StackTrace.empty if this stream received an error without a stack trace.

An asynchronous error error is matched by a test function if test(error) returns true. If test is omitted, every error is considered matching.

If the error is intercepted, the onError function can decide what to do with it. It can throw if it wants to raise a new (or the same) error, or simply return to make this stream forget the error. If the received error value is thrown again by the onError function, it acts like a rethrow and it is emitted along with its original stack trace, not the stack trace of the throw inside onError.

If you need to transform an error into a data event, use the more generic Stream.transform to handle the event by writing a data event to the output sink.

The returned stream is a broadcast stream if this stream is. If a broadcast stream is listened to more than once, each subscription will individually perform the test and handle the error.


Stream.periodic(const Duration(seconds: 1), (count) {
  if (count == 2) {
    throw Exception('Exceptional event');
  return count;

// Outputs:
// 0
// 1
// Exception: Exceptional event
// 3
// 4


Stream<T> handleError(Function onError, {bool test(error)?}) {
  final void Function(Object, StackTrace) callback;
  if (onError is void Function(Object, StackTrace)) {
    callback = onError;
  } else if (onError is void Function(Object)) {
    callback = (Object error, StackTrace _) {
  } else {
    throw ArgumentError.value(
        "Error handler must accept one Object or one Object and a StackTrace"
            " as arguments.");
  return new _HandleErrorStream<T>(this, callback, test);