1. booltest(
   1. Telement
   ), {
2. TorElse()?,

Finds the single element in this stream matchingtest.

Returns a future that is completed with the single element of this streamfor whichtest returnstrue.

If no such element is found before this stream is done, and anorElse function is provided, the result of callingorElsebecomes the value of the future. IforElse throws, the returnedfuture is completed with that error.

Only one element may match. If more than one matching element is found anerror is thrown, regardless of whetherorElse was passed.

If this stream emits an error at any point, the returned future iscompleted with that error, and the subscription is canceled.

A non-error result cannot be provided before this stream is done.

Similar tolastWhere, except that it is an error if more than onematching element occurs in this stream.

Example:

var result = await Stream.fromIterable([1, 2, 3, 6, 9, 12])    .singleWhere((element) => element % 4 == 0, orElse: () => -1);print(result); // 12result = await Stream.fromIterable([2, 6, 8, 12, 24, 32])    .singleWhere((element) => element % 9 == 0, orElse: () => -1);print(result); // -1result = await Stream.fromIterable([2, 6, 8, 12, 24, 32])    .singleWhere((element) => element % 6 == 0, orElse: () => -1);// Throws.

Implementation

Future<T> singleWhere(bool test(T element), {T orElse()?}) {  _Future<T> future = _Future<T>();  late T result;  bool foundResult = false;  StreamSubscription<T> subscription = this.listen(    null,    onError: future._completeError,    onDone: () {      if (foundResult) {        future._complete(result);        return;      }      if (orElse != null) {        _runUserCode(orElse, future._complete, future._completeError);        return;      }      var stack = StackTrace.empty;      var error = IterableElementError.noElement();      _trySetStackTrace(error, stack);      _completeWithErrorCallback(future, error, stack);    },    cancelOnError: true,  );  subscription.onData((T value) {    _runUserCode(() => test(value), (bool isMatch) {      if (isMatch) {        if (foundResult) {          var stack = StackTrace.empty;          var error = IterableElementError.tooMany();          _trySetStackTrace(error, stack);          _cancelAndErrorWithReplacement(subscription, future, error, stack);          return;        }        foundResult = true;        result = value;      }    }, _cancelAndErrorClosure(subscription, future));  });  return future;}