src/Observable.php
public function groupBy(callable $keySelector, callable $elementSelector = null, callable $keySerializer = null): Observable
    {
        return $this->groupByUntil($keySelector, $elementSelector, function () {
            return static::never();
        }, $keySerializer);
    }
public function groupBy(callable $keySelector, callable $elementSelector = null, callable $keySerializer = null): Observable
    {
        return $this->groupByUntil($keySelector, $elementSelector, function () {
            return static::never();
        }, $keySerializer);
    }
public function groupBy ( callable $ keySelector , callable $ elementSelector = null , callable $ keySerializer = null ) : Observable { return $ this -> groupByUntil ( $ keySelector , $ elementSelector , function ( ) { return static :: never ( ) ; } , $ keySerializer ) ; }
Groups the elements of an observable sequence according to a specified key selector function and comparer and selects the resulting elements by using a specified function.

@param callable $keySelector
@param callable|null $elementSelector
@param callable|null $keySerializer
@return Observable

@demo groupBy/groupBy.php
@operator
@reactivex groupBy
Groups the elements of an observable sequence according to a specified key selector function and comparer and selects the resulting elements by using a specified function .