鋪墊
在大量的實踐中，似乎我們總是通過類似的方式來使用異步編程：
- 監聽事件
- 事件發生執行對應的回調函數
- 回調完成（可能產生新的事件添加進監聽隊列）
- 回到1，監聽事件
因此我們將這樣的異步模式稱為Reactor模式，例如在iOS開發中的Run Loop概念，實際上非常類似于Reactor loop，主線程的Run Loop監聽屏幕UI事件，一旦發生UI事件則執行對應的事件處理代碼，還可以通過GCD等方式產生事件至主線程執行。
上圖是boost對Reactor模式的描繪，Twisted的設計就是基于這樣的Reactor模式，Twisted程序就是在等待事件、處理事件的過程中不斷循環。
from twisted.internet import reactor reactor.run()
reactor是Twisted程序中的單例對象。
reactor
reactor是事件管理器，用于注冊、注銷事件，運行事件循環，當事件發生時調用回調函數處理。關于reactor有下面幾個結論:
- Twisted的reactor只有通過調用reactor.run()來啟動。
- reactor循環是在其開始的進程中運行，也就是運行在主進程中。
- 一旦啟動，就會一直運行下去。reactor就會在程序的控制下（或者具體在一個啟動它的線程的控制下）。
- reactor循環并不會消耗任何CPU的資源。
- 并不需要顯式的創建reactor，只需要引入就OK了。
最后一條需要解釋清楚。在Twisted中，reactor是Singleton（也就是單例模式），即在一個程序中只能有一個reactor，并且只要你引入它就相應地創建一個。上面引入的方式這是twisted默認使用的方法，當然了，twisted還有其它可以引入reactor的方法。例如，可以使用twisted.internet.pollreactor中的系統調用來poll來代替select方法。
若使用其它的reactor，需要在引入twisted.internet.reactor前安裝它。下面是安裝pollreactor的方法：
from twisted.internet import pollreactor pollreactor.install()
如果你沒有安裝其它特殊的reactor而引入了twisted.internet.reactor，那么Twisted會根據操作系統安裝默認的reactor。正因為如此，習慣性做法不要在最頂層的模塊內引入reactor以避免安裝默認reactor，而是在你要使用reactor的區域內安裝。
下面是使用 pollreactor重寫上上面的程序:
from twited.internet import pollreactor pollreactor.install() from twisted.internet import reactor reactor.run()
那么reactor是如何實現單例的？來看一下from twisted.internet import reactor做了哪些事情就并明白了。
下面是twisted/internet/reactor.py的部分代碼:
# twisted/internet/reactor.py import sys del sys.modules['twisted.internet.reactor'] from twisted.internet import default default.install()
注：Python中所有加載到內存的模塊都放在sys.modules，它是一個全局字典。當import一個模塊時首先會在這個列表中查找是否已經加載了此模塊，如果加載了則只是將模塊的名字加入到正在調用import的模塊的命名空間中。如果沒有加載則從sys.path目錄中按照模塊名稱查找模塊文件，找到后將模塊載入內存，并加入到sys.modules中，并將名稱導入到當前的命名空間中。
假如我們是第一次運行from twisted.internet import reactor，因為sys.modules中還沒有twisted.internet.reactor，所以會運行reactory.py中的代碼，安裝默認的reactor。之后，如果導入的話，因為sys.modules中已存在該模塊，所以會直接將sys.modules中的twisted.internet.reactor導入到當前命名空間。
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)
很明顯，default中會根據平臺獲取相應的install。Linux下會首先使用epollreactor，如果內核還不支持，就只能使用pollreactor。Mac平臺使用pollreactor，windows使用selectreactor。每種install的實現差不多，這里我們抽取selectreactor中的install來看看。
# twisted/internet/selectreactor.py：
def install():
"""Configure the twisted mainloop to be run using the select() reactor.
"""
# 單例
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鍵，值就是再install中創建的單例reactor。以后要使用reactor，就會導入這個單例了。
SelectReactor # twisted/internet/selectreactor.py @implementer(IReactorFDSet) class SelectReactor(posixbase.PosixReactorBase, _extraBase)
implementer表示SelectReactor實現了IReactorFDSet接口的方法，這里用到了zope.interface，它是python中的接口實現，有興趣的同學可以去看下。
IReactorFDSet接口主要對描述符的獲取、添加、刪除等操作的方法。這些方法看名字就能知道意思，所以我就沒有加注釋。
# 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()注冊了一個監聽事件，它是父類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.
創建并綁定套接字，開始監聽。
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:
# 創建套接字
skt = self.createInternetSocket()
if self.addressFamily == socket.AF_INET6:
addr = _resolveIPv6(self.interface, self.port)
else:
addr = (self.interface, self.port)
# 綁定
skt.bind(addr)
except socket.error as le:
raise CannotListenError(self.interface, self.port, le)
# 監聽
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調用reactor的addReader方法將Port加入讀集合
self.startReading()
整個邏輯很簡單，和正常的server端一樣，創建套接字、綁定、監聽。不同的是將套接字的描述符添加到了reactor的讀集合。那么假如有了client連接過來的話，reactor會監控到，然后觸發事件處理程序。
reacotr.run()事件主循環
# 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的父類_SignalReactorMixin和ReactorBase都有該函數，但是
_SignalReactorMixin在前，安裝mro順序的話，會先調用_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是關鍵，select,poll,epool實現各有不同
self.doIteration(t)
except:
log.msg("Unexpected error in main loop.")
log.err()
else:
log.msg('Main loop terminated.')
mianLoop就是最終的主循環了，在循環中，調用doIteration方法監控讀寫描述符的集合，一旦發現有描述符準備好讀寫，就會調用相應的事件處理程序。
# 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:
# 調用select方法監控讀寫集合，返回準備好讀寫的描述符
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.
# 調用_doReadOrWrite方法
_logrun(selectable, _drdw, selectable, method)
doIteration = doSelect
def _doReadOrWrite(self, selectable, method):
try:
# 調用method，doRead或者是doWrite，
# 這里的selectable可能是我們監聽的tcp.Port
why = getattr(selectable, method)()
except:
why = sys.exc_info()[1]
log.err()
if why:
self._disconnectSelectable(selectable, why, method=="doRead")
那么假如客戶端有連接請求了，就會調用讀集合中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.
當套接字準備好讀的時候調用
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:
# 調用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初始化的過程中，會將自身假如到reactor的讀集合中，那么當它準備
# 好讀的時候，就可以調用它的doRead方法讀取客戶端發過來的數據了
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，調用accept產生了用于接收客戶端數據的套接字，將套接字與transport綁定，然后把transport加入到reactor的讀集合。當客戶端有數據到來時，就會調用transport的doRead方法進行數據讀取了。
Connection是Server（transport實例的類）的父類，它實現了doRead方法。
# twisted/internet/tcp.py
@implementer(interfaces.ITCPTransport, interfaces.ISystemHandle)
class Connection(_TLSConnectionMixin, abstract.FileDescriptor, _SocketCloser,
_AbortingMixin):
def doRead(self):
try:
# 接收數據
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
# 調用我們自定義protocol的dataReceived方法處理數據
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中調用了示例中我們自定義的EchoProtocol的dataReceived方法處理數據。
至此，一個簡單的流程，從創建監聽事件，到接收客戶端數據就此結束了。
