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 the
    signal (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 func
    return _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 receiver
        if settings.configured and settings.DEBUG:
            assert callable(receiver), "Signal receivers must be callable."

            # Check for **kwargs
            if 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.ref
            receiver_object = receiver
            # Check for bound methods
            if hasattr(receiver, '__self__') and hasattr(receiver, '__func__'):
                ref = weakref.WeakMethod
                receiver_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 only
        live 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_receivers

    def 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。