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。
from twisted.internet import reactor reactor.run()
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：
from twisted.internet import pollreactor pollreactor.install()
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:
from twited.internet import pollreactor pollreactor.install() from twisted.internet import reactor reactor.run()
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/default.py def _getInstallFunction(platform): """ Return a function to install the reactor most suited for the given platform. @param platform: The platform for which to select a reactor. @type platform: L{twisted.python.runtime.Platform} @return: A zero-argument callable which will install the selected reactor. """ try: if platform.isLinux(): try: from twisted.internet.epollreactor import install except ImportError: from twisted.internet.pollreactor import install elif platform.getType() == 'posix' and not platform.isMacOSX(): from twisted.internet.pollreactor import install else: from twisted.internet.selectreactor import install except ImportError: from twisted.internet.selectreactor import install return install install = _getInstallFunction(platform)
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.ReactorAlreadyInstalledError("reactor already installed") twisted.internet.reactor = reactor sys.modules['twisted.internet.reactor'] = reactor
在installReactor中，向sys.modules添加twisted.internet.reactor键，值就是再install中创建的单例reactor。以后要使用reactor，就会导入这个单例了。
SelectReactor # twisted/internet/selectreactor.py @implementer(IReactorFDSet) class SelectReactor(posixbase.PosixReactorBase, _extraBase) implementer表示SelectReactor实现了IReactorFDSet接口的方法，这里用到了zope.interface，它是python中的接口实现，有兴趣的同学可以去看下。
IReactorFDSet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。
至此，一个简单的流程，从创建监听事件，到接收客户端数据就此结束了。
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