铺垫
在大量的实践中，似乎我们总是通过类似的方式来使用异步编程：- 监听事件
- 事件发生执行对应的回调函数
- 回调完成（可能产生新的事件添加进监听队列）
- 回到1，监听事件
因此我们将这样的异步模式称为Reactor模式，例如在iOS开发中的Run Loop概念，实际上非常类似于Reactor loop，主线程的Run Loop监听屏幕UI事件，一旦发生UI事件则执行对应的事件处理代码，还可以通过GCD等方式产生事件至主线程执行。
上图是boost对Reactor模式的描绘，Twisted的设计就是基于这样的Reactor模式，Twisted程序就是在等待事件、处理事件的过程中不断循环。
reactor是Twisted程序中的单例对象。
reactor
reactor是事件管理器，用于注册、注销事件，运行事件循环，当事件发生时调用回调函数处理。关于reactor有下面几个结论:- Twisted的reactor只有通过调用reactor.run()来启动。
- reactor循环是在其开始的进程中运行，也就是运行在主进程中。
- 一旦启动，就会一直运行下去。reactor就会在程序的控制下（或者具体在一个启动它的线程的控制下）。
- reactor循环并不会消耗任何CPU的资源。
- 并不需要显式的创建reactor，只需要引入就OK了。
最后一条需要解释清楚。在Twisted中，reactor是Singleton（也就是单例模式），即在一个程序中只能有一个reactor，并且只要你引入它就相应地创建一个。上面引入的方式这是twisted默认使用的方法，当然了，twisted还有其它可以引入reactor的方法。例如，可以使用twisted.internet.pollreactor中的系统调用来poll来代替select方法。
若使用其它的reactor，需要在引入twisted.internet.reactor前安装它。下面是安装pollreactor的方法：
如果你没有安装其它特殊的reactor而引入了twisted.internet.reactor，那么Twisted会根据操作系统安装默认的reactor。正因为如此，习惯性做法不要在最顶层的模块内引入reactor以避免安装默认reactor，而是在你要使用reactor的区域内安装。
下面是使用 pollreactor重写上上面的程序:
那么reactor是如何实现单例的？来看一下from twisted.internet import reactor做了哪些事情就并明白了。
下面是twisted/internet/reactor.py的部分代码:
# twisted/internet/reactor.py
import sys
del sys.modules['twisted.internet.reactor']
from twisted.internet import default
default.install()注：Python中所有加载到内存的模块都放在sys.modules，它是一个全局字典。当import一个模块时首先会在这个列表中查找是否已经加载了此模块，如果加载了则只是将模块的名字加入到正在调用import的模块的命名空间中。如果没有加载则从sys.path目录中按照模块名称查找模块文件，找到后将模块载入内存，并加入到sys.modules中，并将名称导入到当前的命名空间中。
假如我们是第一次运行from twisted.internet import reactor，因为sys.modules中还没有twisted.internet.reactor，所以会运行reactory.py中的代码，安装默认的reactor。之后，如果导入的话，因为sys.modules中已存在该模块，所以会直接将sys.modules中的twisted.internet.reactor导入到当前命名空间。
default中的install：
# twisted/internet/selectreactor.py：
def install():
"""Configure the twisted mainloop to be run using the select() reactor.
"""
# 单例
reactor = SelectReactor()
from twisted.internet.main import installReactor
installReactor(reactor)
# twisted/internet/main.py：
def installReactor(reactor):
"""
Install reactor C{reactor}.
@param reactor: An object that provides one or more IReactor* interfaces.
"""
# this stuff should be common to all reactors.
import twisted.internet
import sys
if 'twisted.internet.reactor' in sys.modules:
raise error.ReactorAlreadyInstalledError("reactor already installed")
twisted.internet.reactor = reactor
sys.modules['twisted.internet.reactor'] = reactor?很明显，default中会根据平台获取相应的install。Linux下会首先使用epollreactor，如果内核还不支持，就只能使用pollreactor。Mac平台使用pollreactor，windows使用selectreactor。每种install的实现差不多，这里我们抽取selectreactor中的install来看看。
# twisted/internet/selectreactor.py：
def install():
"""Configure the twisted mainloop to be run using the select() reactor.
"""
# 单例
reactor = SelectReactor()
from twisted.internet.main import installReactor
installReactor(reactor)
# twisted/internet/main.py：
def installReactor(reactor):
"""
Install reactor C{reactor}.
@param reactor: An object that provides one or more IReactor* interfaces.
"""
# this stuff should be common to all reactors.
import twisted.internet
import sys
if 'twisted.internet.reactor' in sys.modules:
raise error.ReactoIReactorFDSet接口主要对描述符的获取、添加、删除等操作的方法。这些方法看名字就能知道意思，所以我就没有加注释。
# twisted/internet/interfaces.py
class IReactorFDSet(Interface):
def addReader(reader):
def addWriter(writer):
def removeReader(reader):
def removeWriter(writer):
def removeAll():
def getReaders():
def getWriters():
reactor.listenTCP()示例中的reactor.listenTCP()注册了一个监听事件，它是父类PosixReactorBase中方法。
# twisted/internet/posixbase.py
@implementer(IReactorTCP, IReactorUDP, IReactorMulticast)
class PosixReactorBase(_SignalReactorMixin, _DisconnectSelectableMixin,
ReactorBase):
def listenTCP(self, port, factory, backlog=50, interface=''):
p = tcp.Port(port, factory, backlog, interface, self)
p.startListening()
return p
# twisted/internet/tcp.py
@implementer(interfaces.IListeningPort)
class Port(base.BasePort, _SocketCloser):
def __init__(self, port, factory, backlog=50, interface='', reactor=None):
"""Initialize with a numeric port to listen on.
"""
base.BasePort.__init__(self, reactor=reactor)
self.port = port
self.factory = factory
self.backlog = backlog
if abstract.isIPv6Address(interface):
self.addressFamily = socket.AF_INET6
self._addressType = address.IPv6Address
self.interface = interface
...
def startListening(self):
"""Create and bind my socket, and begin listening on it.
创建并绑定套接字，开始监听。
This is called on unserialization, and must be called after creating a
server to begin listening on the specified port.
"""
if self._preexistingSocket is None:
# Create a new socket and make it listen
try:
# 创建套接字
skt = self.createInternetSocket()
if self.addressFamily == socket.AF_INET6:
addr = _resolveIPv6(self.interface, self.port)
else:
addr = (self.interface, self.port)
# 绑定
skt.bind(addr)
except socket.error as le:
raise CannotListenError(self.interface, self.port, le)
# 监听
skt.listen(self.backlog)
else:
# Re-use the externally specified socket
skt = self._preexistingSocket
self._preexistingSocket = None
# Avoid shutting it down at the end.
self._shouldShutdown = False
# Make sure that if we listened on port 0, we update that to
# reflect what the OS actually assigned us.
self._realPortNumber = skt.getsockname()[1]
log.msg("%s starting on %s" % (
self._getLogPrefix(self.factory), self._realPortNumber))
# The order of the next 5 lines is kind of bizarre. If no one
# can explain it, perhaps we should re-arrange them.
self.factory.doStart()
self.connected = True
self.socket = skt
self.fileno = self.socket.fileno
self.numberAccepts = 100
# startReading调用reactor的addReader方法将Port加入读集合
self.startReading()整个逻辑很简单，和正常的server端一样，创建套接字、绑定、监听。不同的是将套接字的描述符添加到了reactor的读集合。那么假如有了client连接过来的话，reactor会监控到，然后触发事件处理程序。
reacotr.run()事件主循环
# twisted/internet/posixbase.py
@implementer(IReactorTCP, IReactorUDP, IReactorMulticast)
class PosixReactorBase(_SignalReactorMixin, _DisconnectSelectableMixin,
ReactorBase)
# twisted/internet/base.py
class _SignalReactorMixin(object):
def startRunning(self, installSignalHandlers=True):
"""
PosixReactorBase的父类_SignalReactorMixin和ReactorBase都有该函数，但是
_SignalReactorMixin在前，安装mro顺序的话，会先调用_SignalReactorMixin中的。
"""
self._installSignalHandlers = installSignalHandlers
ReactorBase.startRunning(self)
def run(self, installSignalHandlers=True):
self.startRunning(installSignalHandlers=installSignalHandlers)
self.mainLoop()
def mainLoop(self):
while self._started:
try:
while self._started:
# Advance simulation time in delayed event
# processors.
self.runUntilCurrent()
t2 = self.timeout()
t = self.running and t2
# doIteration是关键，select,poll,epool实现各有不同
self.doIteration(t)
except:
log.msg("Unexpected error in main loop.")
log.err()
else:
log.msg('Main loop terminated.')mianLoop就是最终的主循环了，在循环中，调用doIteration方法监控读写描述符的集合，一旦发现有描述符准备好读写，就会调用相应的事件处理程序。
# twisted/internet/selectreactor.py
@implementer(IReactorFDSet)
class SelectReactor(posixbase.PosixReactorBase, _extraBase):
def __init__(self):
"""
Initialize file descriptor tracking dictionaries and the base class.
"""
self._reads = set()
self._writes = set()
posixbase.PosixReactorBase.__init__(self)
def doSelect(self, timeout):
"""
Run one iteration of the I/O monitor loop.
This will run all selectables who had input or output readiness
waiting for them.
"""
try:
# 调用select方法监控读写集合，返回准备好读写的描述符
r, w, ignored = _select(self._reads,
self._writes,
[], timeout)
except ValueError:
# Possibly a file descriptor has gone negative?
self._preenDescriptors()
return
except TypeError:
# Something *totally* invalid (object w/o fileno, non-integral
# result) was passed
log.err()
self._preenDescriptors()
return
except (select.error, socket.error, IOError) as se:
# select(2) encountered an error, perhaps while calling the fileno()
# method of a socket. (Python 2.6 socket.error is an IOError
# subclass, but on Python 2.5 and earlier it is not.)
if se.args[0] in (0, 2):
# windows does this if it got an empty list
if (not self._reads) and (not self._writes):
return
else:
raise
elif se.args[0] == EINTR:
return
elif se.args[0] == EBADF:
self._preenDescriptors()
return
else:
# OK, I really don't know what's going on. Blow up.
raise
_drdw = self._doReadOrWrite
_logrun = log.callWithLogger
for selectables, method, fdset in ((r, "doRead", self._reads),
(w,"doWrite", self._writes)):
for selectable in selectables:
# if this was disconnected in another thread, kill it.
# ^^^^ --- what the !@#*? serious! -exarkun
if selectable not in fdset:
continue
# This for pausing input when we're not ready for more.
# 调用_doReadOrWrite方法
_logrun(selectable, _drdw, selectable, method)
doIteration = doSelect
def _doReadOrWrite(self, selectable, method):
try:
# 调用method，doRead或者是doWrite，
# 这里的selectable可能是我们监听的tcp.Port
why = getattr(selectable, method)()
except:
why = sys.exc_info()[1]
log.err()
if why:
self._disconnectSelectable(selectable, why, method=="doRead")那么假如客户端有连接请求了，就会调用读集合中tcp.Port的doRead方法。
# twisted/internet/tcp.py
@implementer(interfaces.IListeningPort)
class Port(base.BasePort, _SocketCloser):
def doRead(self):
"""Called when my socket is ready for reading.
当套接字准备好读的时候调用
This accepts a connection and calls self.protocol() to handle the
wire-level protocol.
"""
try:
if platformType == "posix":
numAccepts = self.numberAccepts
else:
numAccepts = 1
for i in range(numAccepts):
if self.disconnecting:
return
try:
# 调用accept
skt, addr = self.socket.accept()
except socket.error as e:
if e.args[0] in (EWOULDBLOCK, EAGAIN):
self.numberAccepts = i
break
elif e.args[0] == EPERM:
continue
elif e.args[0] in (EMFILE, ENOBUFS, ENFILE, ENOMEM, ECONNABORTED):
log.msg("Could not accept new connection (%s)" % (
errorcode[e.args[0]],))
break
raise
fdesc._setCloseOnExec(skt.fileno())
protocol = self.factory.buildProtocol(self._buildAddr(addr))
if protocol is None:
skt.close()
continue
s = self.sessionno
self.sessionno = s+1
# transport初始化的过程中，会将自身假如到reactor的读集合中，那么当它准备
# 好读的时候，就可以调用它的doRead方法读取客户端发过来的数据了
transport = self.transport(skt, protocol, addr, self, s, self.reactor)
protocol.makeConnection(transport)
else:
self.numberAccepts = self.numberAccepts+20
except:
log.deferr()doRead方法中，调用accept产生了用于接收客户端数据的套接字，将套接字与transport绑定，然后把transport加入到reactor的读集合。当客户端有数据到来时，就会调用transport的doRead方法进行数据读取了。
Connection是Server（transport实例的类）的父类，它实现了doRead方法。
# twisted/internet/tcp.py
@implementer(interfaces.ITCPTransport, interfaces.ISystemHandle)
class Connection(_TLSConnectionMixin, abstract.FileDescriptor, _SocketCloser,
_AbortingMixin):
def doRead(self):
try:
# 接收数据
data = self.socket.recv(self.bufferSize)
except socket.error as se:
if se.args[0] == EWOULDBLOCK:
return
else:
return main.CONNECTION_LOST
return self._dataReceived(data)
def _dataReceived(self, data):
if not data:
return main.CONNECTION_DONE
# 调用我们自定义protocol的dataReceived方法处理数据
rval = self.protocol.dataReceived(data)
if rval is not None:
offender = self.protocol.dataReceived
warningFormat = (
'Returning a value other than None from %(fqpn)s is '
'deprecated since %(version)s.')
warningString = deprecate.getDeprecationWarningString(
offender, versions.Version('Twisted', 11, 0, 0),
format=warningFormat)
deprecate.warnAboutFunction(offender, warningString)
return rval_dataReceived中调用了示例中我们自定义的EchoProtocol的dataReceived方法处理数据。
本文大致分析Twisted reactor流程，从创建监听事件到接收客户端数据就此结束。