@classmethod
def from_crawler(cls: Type[SchedulerTV], crawler) -> SchedulerTV:
    dupefilter_cls = load_object(crawler.settings['DUPEFILTER_CLASS'])
    return cls(
        # 创建去重过滤器实例：scrapy.dupefilters.RFPDupeFilter
        dupefilter=create_instance(dupefilter_cls, crawler.settings, crawler),
        # jobdir 用于持久化爬行状态的目录
        jobdir=job_dir(crawler.settings),
        # 持久化请求队列使用的类（文件）：scrapy.squeues.PickleLifoDiskQueue
        dqclass=load_object(crawler.settings['SCHEDULER_DISK_QUEUE']),
        # 非持久化请求队列使用的类（内存）：scrapy.squeues.LifoMemoryQueue
        mqclass=load_object(crawler.settings['SCHEDULER_MEMORY_QUEUE']),
        # 是否开启 Scheduler 的 Debug 模式
        logunser=crawler.settings.getbool('SCHEDULER_DEBUG'),
        # 爬虫统计收集器
        stats=crawler.stats,
        # 请求优先级队列使用的类：scrapy.pqueues.ScrapyPriorityQueue
        pqclass=load_object(crawler.settings['SCHEDULER_PRIORITY_QUEUE']),
        crawler=crawler,
    )

def enqueue_request(self, request: Request) -> bool:
    # 没有dont_filter标识，再让dupefilter过滤掉的话，返回False
    if not request.dont_filter and self.df.request_seen(request):
        self.df.log(request, self.spider)
        # 如果是False，引擎将发出require_dropped信号，并且不会在以后的时间里继续尝试调度该请求。
        return False
    # 尝试推入磁盘队列
    dqok = self._dqpush(request)
    if dqok:
        self.stats.inc_value('scheduler/enqueued/disk', spider=self.spider)
    else:
        # 没推入磁盘队列成功，则推入内存队列
        self._mqpush(request)
        self.stats.inc_value('scheduler/enqueued/memory', spider=self.spider)
    self.stats.inc_value('scheduler/enqueued', spider=self.spider)
    return True

def has_pending_requests(self) -> bool:
    # 如果 Scheduler 有排队的请求，则为真，否则为假。
    return len(self) > 0

def next_request(self) -> Optional[Request]:
    # 先从内存队列取 request，若为空，则从磁盘队列取 request。直至 has_pending_requests 为 False
    request = self.mqs.pop()
    if request is not None:
        self.stats.inc_value('scheduler/dequeued/memory', spider=self.spider)
    else:
        request = self._dqpop()
        if request is not None:
            self.stats.inc_value('scheduler/dequeued/disk', spider=self.spider)
    if request is not None:
        self.stats.inc_value('scheduler/dequeued', spider=self.spider)
    return request