firstWhere method

Future<T> firstWhere(
  1. bool test(
    1. T element
    ), {
  2. T orElse()?,
})

Finds the first element of this stream matching test.

Returns a future that is completed with the first element of this stream for which test returns true.

If no such element is found before this stream is done, and an orElse function is provided, the result of calling orElse becomes the value of the future. If orElse throws, the returned future is completed with that error.

If this stream emits an error before the first matching element, the returned future is completed with that error, and processing stops.

Stops listening to this stream after the first matching element or error has been received.

Internally the method cancels its subscription after the first element that matches the predicate. This means that single-subscription (non-broadcast) streams are closed and cannot be reused after a call to this method.

If an error occurs, or if this stream ends without finding a match and with no orElse function provided, the returned future is completed with an error.

Example:

var result = await Stream.fromIterable([1, 3, 4, 9, 12])
    .firstWhere((element) => element % 6 == 0, orElse: () => -1);
print(result); // 12

result = await Stream.fromIterable([1, 2, 3, 4, 5])
    .firstWhere((element) => element % 6 == 0, orElse: () => -1);
print(result); // -1

Implementation

Future<T> firstWhere(bool test(T element), {T orElse()?}) {
  _Future<T> future = new _Future();
  StreamSubscription<T> subscription =
      this.listen(null, onError: future._completeError, onDone: () {
    if (orElse != null) {
      _runUserCode(orElse, future._complete, future._completeError);
      return;
    }
    try {
      // Sets stackTrace on error.
      throw IterableElementError.noElement();
    } catch (e, s) {
      _completeWithErrorCallback(future, e, s);
    }
  }, cancelOnError: true);

  subscription.onData((T value) {
    _runUserCode(() => test(value), (bool isMatch) {
      if (isMatch) {
        _cancelAndValue(subscription, future, value);
      }
    }, _cancelAndErrorClosure(subscription, future));
  });
  return future;
}