twisted 实现UDP服务

        udp没有连接的概念，udp套接字可以从网络上的任何服务器接收数据报，并将数据报发送到网络上的任何主机。数据报可以以任意顺序到达 or 根本不会到达 or 在传输过程中被复制。

由于没有连接，所以对于每个udp套接字我们只能使用一个对象，一个协议。然后使用已经定义的接口twisted.internet.interfaces.IReactorUDP

class IReactorUDP(Interface):
    """
    UDP socket methods.
    """

    def listenUDP(
        port: int, protocol: "DatagramProtocol", interface: str, maxPacketSize: int
    ) -> "IListeningPort":
        """
        Connects a given L{DatagramProtocol} to the given numeric UDP port.

        @param port: A port number on which to listen.
        @param protocol: A L{DatagramProtocol} instance which will be
            connected to the given C{port}.
        @param interface: The local IPv4 or IPv6 address to which to bind;
            defaults to '', ie all IPv4 addresses.
        @param maxPacketSize: The maximum packet size to accept.

        @return: object which provides L{IListeningPort}.
        """

twisted中已经帮忙实现了udp协议：twisted.internet.protocol.DatagramProtocol

@implementer(interfaces.ILoggingContext)
class DatagramProtocol(AbstractDatagramProtocol):
    """
    Protocol for datagram-oriented transport, e.g. UDP.

    @type transport: L{None} or
        L{IUDPTransport<twisted.internet.interfaces.IUDPTransport>} provider
    @ivar transport: The transport with which this protocol is associated,
        if it is associated with one.
    """

    def logPrefix(self):
        """
        Return a prefix matching the class name, to identify log messages
        related to this protocol instance.
        """
        return self.__class__.__name__

    def connectionRefused(self):
        """
        Called due to error from write in connected mode.

        Note this is a result of ICMP message generated by *previous*
        write.
        """

class AbstractDatagramProtocol:
    """
    Abstract protocol for datagram-oriented transports, e.g. IP, ICMP, ARP,
    UDP.
    """

    transport = None
    numPorts = 0
    noisy = True

    def __getstate__(self):
        d = self.__dict__.copy()
        d["transport"] = None
        return d

    def doStart(self):
        """
        Make sure startProtocol is called.

        This will be called by makeConnection(), users should not call it.
        """
        if not self.numPorts:
            if self.noisy:
                log.msg("Starting protocol %s" % self)
            self.startProtocol()
        self.numPorts = self.numPorts + 1

    def doStop(self):
        """
        Make sure stopProtocol is called.

        This will be called by the port, users should not call it.
        """
        assert self.numPorts > 0
        self.numPorts = self.numPorts - 1
        self.transport = None
        if not self.numPorts:
            if self.noisy:
                log.msg("Stopping protocol %s" % self)
            self.stopProtocol()

    def startProtocol(self):
        """
        Called when a transport is connected to this protocol.

        Will only be called once, even if multiple ports are connected.
        """

    def stopProtocol(self):
        """
        Called when the transport is disconnected.

        Will only be called once, after all ports are disconnected.
        """

    def makeConnection(self, transport):
        """
        Make a connection to a transport and a server.

        This sets the 'transport' attribute of this DatagramProtocol, and calls the
        doStart() callback.
        """
        assert self.transport == None
        self.transport = transport
        self.doStart()

    def datagramReceived(self, datagram: bytes, addr):
        """
        Called when a datagram is received.

        @param datagram: the bytes received from the transport.
        @param addr: tuple of source of datagram.
        """

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