Rx\Observable::groupBy PHP Method

    public function groupBy(callable $keySelector, callable $elementSelector = null, callable $keySerializer = null)
    {
        return $this->groupByUntil($keySelector, $elementSelector, function () {
            // observable that never calls
            return new AnonymousObservable(function () {
                // todo?
                return new EmptyDisposable();
            });
        }, $keySerializer);
    }

 /**
  * @param Observable $observable
  * @return Observable\AnonymousObservable
  */
 public function summarize(Observable $observable)
 {
     return $observable->groupBy(function (FileWithDate $file) {
         return $file->getDatePath();
     }, function (FileWithDate $file) {
         return $file->getDatePath();
     }, function ($key) {
         return $key;
     })->flatMap(function (GroupedObservable $g) {
         return $g->zip([$g->distinct(), $g->count()], function ($_, $b, $c) {
             return "Created {$b} with {$c} files";
         });
     });
 }