def add_item(self, click_position):
        """
        Event handler to add new obstacles, humans and zombies
        """
        row, col = self._simulation.get_index(click_position, CELL_SIZE)
        if self._item_type == OBSTACLE:
            if not self.is_occupied(row, col):
                self._simulation.set_full(row, col)
        elif self._item_type == ZOMBIE:
            if self._simulation.is_empty(row, col):
                self._simulation.add_zombie(row, col)
        elif self._item_type == HUMAN:
            if self._simulation.is_empty(row, col):
                self._simulation.add_human(row, col)