Future<T> catchError(Function onError, { bool test(Object error) })

Handles errors emitted by this Future.

This is the asynchronous equivalent of a "catch" block.

Returns a new Future that will be completed with either the result of this future or the result of calling the onError callback.

If this future completes with a value, the returned future completes with the same value.

If this future completes with an error, then test is first called with the error value.

If test returns false, the exception is not handled by this catchError, and the returned future completes with the same error and stack trace as this future.

If test returns true, onError is called with the error and possibly stack trace, and the returned future is completed with the result of this call in exactly the same way as for then's onError.

If test is omitted, it defaults to a function that always returns true. The test function should not throw, but if it does, it is handled as if the onError function had thrown.

Example:

foo
  .catchError(..., test: (e) => e is ArgumentError)
  .catchError(..., test: (e) => e is NoSuchMethodError)
  .then((v) { ... });

This method is equivalent to:

Future catchError(onError(error),
                  {bool test(error)}) {
  this.then((v) => v,  // Forward the value.
            // But handle errors, if the [test] succeeds.
            onError: (e, stackTrace) {
              if (test == null || test(e)) {
                if (onError is ZoneBinaryCallback) {
                  return onError(e, stackTrace);
                }
                return onError(e);
              }
              throw e;
            });
}

Source 

// The `Function` below can stand for several types:
// - (dynamic) -> T
// - (dynamic, StackTrace) -> T
// - (dynamic) -> Future<T>
// - (dynamic, StackTrace) -> Future<T>
// Given that there is a `test` function that is usually used to do an
// `isCheck` we should also expect functions that take a specific argument.
// Note: making `catchError` return a `Future<T>` in non-strong mode could be
// a breaking change.
Future<T> catchError(Function onError,
                         {bool test(Object error)});