class IOLoop(Configurable):
    ......

    @classmethod
    def configurable_default(cls):
        if hasattr(select, "epoll"):     # 因为我们假设我们的系统为Linux，且支持epoll，所以这里为True
            from tornado.platform.epoll import EPollIOLoop
            return EPollIOLoop 
        if hasattr(select, "kqueue"):
            # Python 2.6+ on BSD or Mac
            from tornado.platform.kqueue import KQueueIOLoop
            return KQueueIOLoop
        from tornado.platform.select import SelectIOLoop
        return SelectIOLoop

class EPollIOLoop(PollIOLoop):　　# 该类只有这么短短的几句，可见主要的方法是在其父类PollIOLoop中实现。
    def initialize(self, **kwargs):
        super(EPollIOLoop, self).initialize(impl=select.epoll(), **kwargs) # 执行了父类PollIOLoop的initialize方法，并将select.epoll()传入

class PollIOLoop(IOLoop):  # 从属配置子类

    def initialize(self, impl, time_func=None, **kwargs):
        super(PollIOLoop, self).initialize(**kwargs)                # 调用IOLoop的initialize方法
        self._impl = impl                               # self._impl = select.epoll()
        if hasattr(self._impl, ‘fileno‘):               # 文件描述符的close_on_exec属性
            set_close_exec(self._impl.fileno())
        self.time_func = time_func or time.time
        self._handlers = {}                             # 文件描述符对应的fileno()作为key，(文件描述符对象,处理函数)作为value
        self._events = {}                               # 用来存储epoll_obj.poll()返回的事件，也就是哪个fd发生了什么事件{(fd1, event1), (fd2, event2)……}
        self._callbacks = []
        self._callback_lock = threading.Lock()          # 添加线程锁
        self._timeouts = []                             # 存储定时任务
        self._cancellations = 0
        self._running = False
        self._stopped = False
        self._closing = False
        self._thread_ident = None                       # 获得当前线程标识符
        self._blocking_signal_threshold = None
        self._timeout_counter = itertools.count()

        # Create a pipe that we send bogus data to when we want to wake
        # the I/O loop when it is idle
        self._waker = Waker()
        self.add_handler(self._waker.fileno(),
                         lambda fd, events: self._waker.consume(),
                         self.READ)

    def initialize(self, make_current=None):
        if make_current is None:
            if IOLoop.current(instance=False) is None:
                self.make_current()
        elif make_current:
            if IOLoop.current(instance=False) is not None:
                raise RuntimeError("current IOLoop already exists")
            self.make_current()

    @staticmethod
    def current(instance=True):
        current = getattr(IOLoop._current, "instance", None)
        if current is None and instance:
            return IOLoop.instance()
        return current

    def make_current(self):
        IOLoop._current.instance = self

    def add_handler(self, fd, handler, events):
        # 向epoll中注册事件 ， 并在self._handlers[fd]中为该文件描述符添加相应处理函数
        fd, obj = self.split_fd(fd)   # fd.fileno(),fd
        self._handlers[fd] = (obj, stack_context.wrap(handler))
        self._impl.register(fd, events | self.ERROR)

    def update_handler(self, fd, events):
        fd, obj = self.split_fd(fd)
        self._impl.modify(fd, events | self.ERROR)

    def remove_handler(self, fd):
        fd, obj = self.split_fd(fd)
        self._handlers.pop(fd, None)
        self._events.pop(fd, None)
        try:
            self._impl.unregister(fd)
        except Exception:
            gen_log.debug("Error deleting fd from IOLoop", exc_info=True)

    def add_callback(self, callback, *args, **kwargs):
        # 因为Python的GIL的限制，导致Python线程并不算高效。加上tornado实现了多进程 + 协程的模式，所以我们略过源码中的部分线程相关的一些操作
        if self._closing:
            return
        self._callbacks.append(functools.partial(stack_context.wrap(callback), *args, **kwargs))
    def add_future(self, future, callback):
        # 为future对象添加经过包装后的回调函数，该回调函数会在future对象被set_done后添加至_callbacks中
        assert is_future(future)
        callback = stack_context.wrap(callback)
        future.add_done_callback(
            lambda future: self.add_callback(callback, future))

def add_timeout(self, deadline, callback, *args, **kwargs):
        """
            ``deadline``可能是一个数字，表示相对于当前时间的时间（与“IOLoop.time”通常为“time.time”相同的大小），或者是datetime.timedelta对象。 
            自从Tornado 4.0以来，`call_later`是一个比较方便的替代方案，因为它不需要timedelta对象。

        """
        if isinstance(deadline, numbers.Real):
            return self.call_at(deadline, callback, *args, **kwargs)
        elif isinstance(deadline, datetime.timedelta):
            return self.call_at(self.time() + timedelta_to_seconds(deadline),
                                callback, *args, **kwargs)
        else:
            raise TypeError("Unsupported deadline %r" % deadline)

def start(self):
        try:
            while True:    
                callbacks = self._callbacks
                self._callbacks = []
                due_timeouts = []
                # 将时间已到的定时任务放置到due_timeouts中，过程省略
                for callback in callbacks:          # 执行callback
                    self._run_callback(callback)
                for timeout in due_timeouts:        # 执行定时任务
                    if timeout.callback is not None:
                        self._run_callback(timeout.callback)       
                callbacks = callback = due_timeouts = timeout = None    # 释放内存
                # 根据情况设置poll_timeout的值，过程省略
                if not self._running:    # 终止ioloop运行时，在执行完了callback后结束循环
                    breaktry:
                    event_pairs = self._impl.poll(poll_timeout)
                except Exception as e:
                    if errno_from_exception(e) == errno.EINTR:  # 系统调用被信号处理函数中断，进行下一次循环
                        continue
                    else:
                        raise 
                self._events.update(event_pairs)
                while self._events: 
                    fd, events = self._events.popitem()             # 获取一个fd以及对应事件
                    try:
                        fd_obj, handler_func = self._handlers[fd]   # 获取该fd对应的事件处理函数
                        handler_func(fd_obj, events)                # 运行该事件处理函数
                    except (OSError, IOError) as e:         
                        if errno_from_exception(e) == errno.EPIPE:     # 当客户端关闭连接时会产生EPIPE错误                         
                            pass
                        # 其他异常处理已经省略
                fd_obj = handler_func = None       # 释放内存空间          

def close(self, all_fds=False):
        with self._callback_lock:
            self._closing = True
        self.remove_handler(self._waker.fileno())
        if all_fds:    # 该参数若为True，则表示会关闭所有文件描述符
            for fd, handler in self._handlers.values():
                self.close_fd(fd)
        self._waker.close()
        self._impl.close() 
        self._callbacks = None
        self._timeouts = None