@implementer(interfaces.IStreamClientEndpoint)
class TCP4ClientEndpoint(object):
    """
    TCP client endpoint with an IPv4 configuration.
    """

def __init__(self, reactor, host, port, timeout=30, bindAddress=None):
      
        self._reactor = reactor
        self._host = host
        self._port = port
        self._timeout = timeout
        self._bindAddress = bindAddress

def connect(self, protocolFactory):
        """
        Implement L{IStreamClientEndpoint.connect} to connect via TCP.
        """
        try:
            wf = _WrappingFactory(protocolFactory)
            self._reactor.connectTCP(
                self._host, self._port, wf,
                timeout=self._timeout, bindAddress=self._bindAddress)
            return wf._onConnection
        except:
            return defer.fail()

class _WrappingFactory(ClientFactory):
  
    protocol = _WrappingProtocol

def __init__(self, wrappedFactory):
       
        self._wrappedFactory = wrappedFactory
        self._onConnection = defer.Deferred(canceller=self._canceller)#返回了延迟对象

def startedConnecting(self, connector):
        """
        A connection attempt was started.  Remember the connector which started
        said attempt, for use later.
        """
        self._connector = connector

def _canceller(self, deferred):
       
推论：调用reactor.connectTCP()时会生成一个connetor，同时connector会加入到reactor的selectable中，当相应的fd有数据时，会通过connector调用doread，doread函数就会调用到protocol中的datarecieved函数，当协议的connectionLost时就是deferred对象回调时。connetor的回调对象在哪里呢？如果是通过TCP4ClientEndpoint作为tranport的话，deferred对象是保存在factory中的。又因为connetor中有factory属性，所以通过factory属性可以找到deferred对象，并调用callback。从而完成延迟对象的回调。