跟踪 twisted 里deferred 的Callback

                 twisted 提供了 deferred 机制，而关键点就是回调。通过查看deferred 源码 （version 8.2.0）我们可以 看到

deferred的addCallback是怎么工作的，以及相关的操作流程。 一个deferred 有一对回调链，一条处理正确的结果，一条处理错误的结果，addCallbacks 与 addCallback ,addBoth 函数则是向这对回调链里添加函数，addCallback 后，deferred 是怎么做的呢，我们看源码：

        

def addCallbacks(self, callback, errback=None,
                     callbackArgs=None, callbackKeywords=None,
                     errbackArgs=None, errbackKeywords=None):
        """Add a pair of callbacks (success and error) to this Deferred.

        These will be executed when the 'master' callback is run.
        """
        assert callable(callback)
        assert errback == None or callable(errback)
        cbs = ((callback, callbackArgs, callbackKeywords),
               (errback or (passthru), errbackArgs, errbackKeywords))
        self.callbacks.append(cbs)

        if self.called:
            self._runCallbacks()
        return self

def addCallback(self, callback, *args, **kw):
        """Convenience method for adding just a callback.

        See L{addCallbacks}.
        """
        
        return self.addCallbacks(callback, callbackArgs=args,
                                 callbackKeywords=kw)

             

addCallback第二个参数是要添加的函数，第三个参数是要传递给回调函数的参数，第四个是要传递给回调函数
的关键字，然后，addCallback里调用了addCallbacks 。 我们可以看到，在deferred初始化 时，这对回调链self.callbacks 是空的 

def __init__(self):
        self.callbacks = [] 
        if self.debug:
            self._debugInfo = DebugInfo()
            self._debugInfo.creator = traceback.format_stack()[:-1]
        cbs = ((callback, callbackArgs, callbackKeywords),
               (errback or (passthru), errbackArgs, errbackKeywords))
        self.callbacks.append(cbs)

 看addCallbacks源码，你会发现， 如果没有指定要添加的errback 时，errback 则是pass-through 的。这时，这对回调函数就会被添加到回调链的尾， 效果如图所示：

                                                            

                         

            当调用一个回调函数时，deferred 又会怎么做呢？假设当前调用的是 callback：

             

def callback(self, result):
        """Run all success callbacks that have been added to this Deferred.

        Each callback will have its result passed as the first
        argument to the next; this way, the callbacks act as a
        'processing chain'. Also, if the success-callback returns a Failure
        or raises an Exception, processing will continue on the *error*-
        callback chain.
        """
        assert not isinstance(result, Deferred)
        self._startRunCallbacks(result)

              看作者的注释，deferred 把上一个回调函数的结果result传递给 _startRunCallbacks：

def _startRunCallbacks(self, result):
        if self.called:
            if self.debug:
                if self._debugInfo is None:
                    self._debugInfo = DebugInfo()
                extra = "\n" + self._debugInfo._getDebugTracebacks()
                raise AlreadyCalledError(extra)
            raise AlreadyCalledError
        if self.debug:
            if self._debugInfo is None:
                self._debugInfo = DebugInfo()
            self._debugInfo.invoker = traceback.format_stack()[:-2]
        self.called = True
        self.result = result 
        if self.timeoutCall:
            try:
                self.timeoutCall.cancel()
            except:
                pass

            del self.timeoutCall
        self._runCallbacks()

                   但这时还没有真正调用了我们刚才添加的回调函数，在这个_startRunCallbacks 里是找不到回调链
“self.callbacks ”的字眼的。这个函数是“开始执行回调函数”，对执行之前处理了一些事务，但它最后调用了
_runCallbacks：

      

def _runCallbacks(self):
        if self._runningCallbacks:
            # Don't recursively run callbacks
            return
        if not self.paused:
            while self.callbacks:
                item = self.callbacks.pop(0) 
                callback, args, kw = item[   
                    isinstance(self.result, failure.Failure)] 
                args = args or ()            
                kw = kw or {}               

                try:
                    self._runningCallbacks = True
                    try:
                        self.result = callback(self.result, *args, **kw) 
                    finally:                                             
                        self._runningCallbacks = False                
                    if isinstance(self.result, Deferred):                
                        # note: this will cause _runCallbacks to be called
                        # recursively if self.result already has a result.
                        # This shouldn't cause any problems, since there is no
                        # relevant state in this stack frame at this point.
                        # The recursive call will continue to process
                        # self.callbacks until it is empty, then return here,
                        # where there is no more work to be done, so this call
                        # will return as well.
                        self.pause()
                        self.result.addBoth(self._continue)
                        break
                except:
                    self.result = failure.Failure()

        if isinstance(self.result, failure.Failure):
            self.result.cleanFailure()
            if self._debugInfo is None:
                self._debugInfo = DebugInfo()
            self._debugInfo.failResult = self.result
        else:
            if self._debugInfo is not None:
                self._debugInfo.failResult = None

       self.callbacks.pop(0)                                                       #弹出回调链里的第一对回调函数,
    callback, args, kw = item[   
                    isinstance(self.result, failure.Failure)]            #根据self.result是否为 failure.Failure类型，来选择回调函数
                                                                                     
     self.result = callback(self.result, *args, **kw)                #这才真正执行了回调函数并把执行结果 作为参数传递给下                                                                                            #一个回调函数

    通过作者的注释，我们可以看到，当执行完回调函数之后，
（1）若self.result 为deferred，即如果回调函数返回一个新的deferred，则当前的deferred（外部deferred）会

         暂停下来，去执行新的deferred（内部deferred）里的回调函数，直到内部deferred把控制权转交给外部  
         deferred，如图：

                            
        

              看代码：

                        self.pause()
                        self.result.addBoth(self._continue)

                     而addBoth会调用addCallbacks,  addCallbacks 则调用_runCallbacks，

def addBoth(self, callback, *args, **kw):
        """Convenience method for adding a single callable as both a callback
        and an errback.

        See L{addCallbacks}.
        """
        return self.addCallbacks(callback, callback,
                                 callbackArgs=args, errbackArgs=args,
                                 callbackKeywords=kw, errbackKeywords=kw)
  
def addCallbacks(self, callback, errback=None,
                     callbackArgs=None, callbackKeywords=None,
                     errbackArgs=None, errbackKeywords=None):
        """Add a pair of callbacks (success and error) to this Deferred.

        These will be executed when the 'master' callback is run.
        """
        assert callable(callback)
        assert errback == None or callable(errback)
        cbs = ((callback, callbackArgs, callbackKeywords),
               (errback or (passthru), errbackArgs, errbackKeywords))
        self.callbacks.append(cbs)

        if self.called:
            self._runCallbacks()
        return self

          调用顺序为：
          addBoth --> addCallbacks --> _runCallbacks, 如此递归执行。
（2）若self.result 不为deferred,则执行下一个回调函数，并把self.result 传递给下一个回调函数，直到self.result为              空。整个跟踪过程就到这里把！


     参考：         
        http://twistedmatrix.com/trac/browser/tags/releases/twisted-.2.0/twisted/internet/defer.py
        http://krondo.com/?page_id=1327
        http://blog.sina.com.cn/s/blog_704b6af70100py9n.html 
   

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