public Scheduler getScheduler() {
        return scheduler.getScheduler();
    }
Whether add urls extracted to download.<br>
Add urls to download when it is true, and just download seed urls when it is false. <br>
DO NOT set it unless you know what it means!

@param spawnUrl spawnUrl
@return this
@since 0.4.0
webmagic-core/src/main/java/us/codecraft/webmagic/Spider.java