pocketmine\tile\MobSpawner::canUpdate PHP Method

    public function canUpdate() : bool
    {
        if ($this->getEntityId() === 0) {
            return false;
        }
        $hasPlayer = false;
        $count = 0;
        foreach ($this->getLevel()->getEntities() as $e) {
            if ($e instanceof Player) {
                if ($e->distance($this->getBlock()) <= 15) {
                    $hasPlayer = true;
                }
            }
            if ($e::NETWORK_ID == $this->getEntityId()) {
                $count++;
            }
        }
        if ($hasPlayer and $count < 15) {
            // Spawn limit = 15
            return true;
        }
        return false;
    }