Amp\Observer::next PHP 메소드

    public function next() : Promise
    {
        if (isset($this->deferreds[$this->position])) {
            $future = $this->deferreds[$this->position];
            unset($this->values[$this->position], $this->deferreds[$this->position]);
            $future->resolve();
        }
        ++$this->position;
        if (\array_key_exists($this->position, $this->values)) {
            return new Success(true);
        }
        if ($this->resolved) {
            --$this->position;
            if ($this->exception) {
                return new Failure($this->exception);
            }
            return new Success(false);
        }
        $this->deferred = new Deferred();
        return $this->deferred->promise();
    }