onError<E extends Object> method

Future<T> onError<E extends Object>(
  1. FutureOr<T> handleError(
    1. E error,
    2. StackTrace stackTrace
  2. {bool test(
    1. E error

Handles errors on this future.

Catches errors of type E that this future complete with. If test is supplied, only catches errors of type E where test returns true. If E is Object, then all errors are potentially caught, depending only on a supplied test.toString()

If the error is caught, the returned future completes with the result of calling handleError with the error and stack trace. This result must be a value of the same type that this future could otherwise complete with. For example, if this future cannot complete with null, then handleError also cannot return null. Example:

Future<T> retryOperation<T>(Future<T> operation(), T onFailure()) =>
    operation().onError<RetryException>((e, s) {
      if (e.canRetry) {
        return retryOperation(operation, onFailure);
      return onFailure();

If handleError throws, the returned future completes with the thrown error and stack trace, except that if it throws the same error object again, then it is considered a "rethrow" and the original stack trace is retained. This can be used as an alternative to skipping the error in test. Example:

// Unwraps an exceptions cause, if it has one.
someFuture.onError<SomeException>((e, _) {
  throw e.cause ?? e;
// vs.
someFuture.onError<SomeException>((e, _) {
  throw e.cause!;
}, test: (e) => e.cause != null);

If the error is not caught, the returned future completes with the same result, value or error, as this future.

This method is effectively a more precisely typed version of Future.catchError. It makes it easy to catch specific error types, and requires a correctly typed error handler function, rather than just Function. Because of this, the error handlers must accept the stack trace argument.


Future<T> onError<E extends Object>(
    FutureOr<T> handleError(E error, StackTrace stackTrace),
    {bool test(E error)?}) {
  FutureOr<T> onError(Object error, StackTrace stackTrace) {
    if (error is! E || test != null && !test(error)) {
      // Counts as rethrow, preserves stack trace.
      throw error;
    return handleError(error, stackTrace);
  if (this is _Future<Object?>) {
    // Internal method working like `catchError`,
    // but allows specifying a different result future type.
    return unsafeCast<_Future<T>>(this)._safeOnError<T>(onError);

  return this.then<T>((T value) => value, onError: onError);