singleWhere method

Future<T> singleWhere (bool test(T element), { T orElse() })

Finds the single element in this stream matching test.

Like lastWhere, except that it is an error if more than one matching element occurs in the stream.

Implementation 

Future<T> singleWhere(bool test(T element), {T orElse()}) {
  _Future<T> future = new _Future<T>();
  T result = null;
  bool foundResult = false;
  StreamSubscription subscription;
  subscription = this.listen(
      (T value) {
        _runUserCode(() => true == test(value), (bool isMatch) {
          if (isMatch) {
            if (foundResult) {
              try {
                throw IterableElementError.tooMany();
              } catch (e, s) {
                _cancelAndErrorWithReplacement(subscription, future, e, s);
              }
              return;
            }
            foundResult = true;
            result = value;
          }
        }, _cancelAndErrorClosure(subscription, future));
      },
      onError: future._completeError,
      onDone: () {
        if (foundResult) {
          future._complete(result);
          return;
        }
        try {
          if (orElse != null) {
            _runUserCode(orElse, future._complete, future._completeError);
            return;
          }
          throw IterableElementError.noElement();
        } catch (e, s) {
          _completeWithErrorCallback(future, e, s);
        }
      },
      cancelOnError: true);
  return future;
}