Django信号机制源码分析（观察者模式）

Django信号的实现原理本质是设计模式中的观察者模式，浅谈Python设计模式 -- 观察者模式，也可以叫做发布-订阅模式，信号对象维护一个订阅者列表，当信号被触发时，它会遍历订阅者，依次通知它们。

先来回顾一下信号的定义和使用：

from django.dispatch import receiver, Signal# 信号定义
node_approved = Signal()# 信号的注册
@receiver(node_approved)
def on_node_approved(sender, instance, **kwargs):print(‘接收到信号’)# 信号的触发
node_approved.send(sender=xx, instance=yy)

源码分析：

1、先来看receiver这个装饰器：

def receiver(signal, **kwargs):"""A decorator for connecting receivers to signals. Used by passing in thesignal (or list of signals) and keyword arguments to connect::@receiver(post_save, sender=MyModel)def signal_receiver(sender, **kwargs):...@receiver([post_save, post_delete], sender=MyModel)def signals_receiver(sender, **kwargs):..."""def _decorator(func):if isinstance(signal, (list, tuple)):for s in signal:s.connect(func, **kwargs)else:signal.connect(func, **kwargs)return funcreturn _decorator

逻辑很简单的一个装饰器，核心是调用Signal信号对象的connect方法，也就是上面举例中的node_approved这个对象的connect方法。

2、接着看Signal 的connect方法：

class Signal:def __init__(self, providing_args=None, use_caching=False):"""Create a new signal."""self.receivers = []...def connect(self, receiver, sender=None, weak=True, dispatch_uid=None):from django.conf import settings# If DEBUG is on, check that we got a good receiverif settings.configured and settings.DEBUG:assert callable(receiver), "Signal receivers must be callable."# Check for **kwargsif not func_accepts_kwargs(receiver):raise ValueError("Signal receivers must accept keyword arguments (**kwargs).")if dispatch_uid:lookup_key = (dispatch_uid, _make_id(sender))else:lookup_key = (_make_id(receiver), _make_id(sender))if weak:ref = weakref.refreceiver_object = receiver# Check for bound methodsif hasattr(receiver, '__self__') and hasattr(receiver, '__func__'):ref = weakref.WeakMethodreceiver_object = receiver.__self__receiver = ref(receiver)weakref.finalize(receiver_object, self._remove_receiver)with self.lock:self._clear_dead_receivers()if not any(r_key == lookup_key for r_key, _ in self.receivers):self.receivers.append((lookup_key, receiver))self.sender_receivers_cache.clear()

只看倒数第二行：self.receivers.append((lookup_key, receiver))，如果你了解观察者模式就很好理解这行代码的意图：将被装饰的信号处理函数(on_node_approved)注册到self.recervers属性中，也就是观察者模式中讲的主题维护观察者列表行为。

一旦有了这个观察者列表，那么就很容易做到信号被触发时，通知每个观察者的目的。下面看看源码是怎么实现的：

3、信号触发send源码：

class Signal:def __init__(self, providing_args=None, use_caching=False):"""Create a new signal."""self.receivers = []...def _live_receivers(self, sender):"""Filter sequence of receivers to get resolved, live receivers.This checks for weak references and resolves them, then returning onlylive receivers."""...receivers = None...for (receiverkey, r_senderkey), receiver in self.receivers:if r_senderkey == NONE_ID or r_senderkey == senderkey:receivers.append(receiver)...non_weak_receivers = []for receiver in receivers:if isinstance(receiver, weakref.ReferenceType):# Dereference the weak reference.receiver = receiver()if receiver is not None:non_weak_receivers.append(receiver)else:non_weak_receivers.append(receiver)return non_weak_receiversdef send(self, sender, **named):if not self.receivers or self.sender_receivers_cache.get(sender) is NO_RECEIVERS:return []return [(receiver, receiver(signal=self, sender=sender, **named))for receiver in self._live_receivers(sender)]

send理解起来也不难：遍历self._live_receivers()，依次调用各个receiver。而_live_receivers通过源码也可以看出本质还是遍历前面提到的观察者列表self.recervers属性。