class Signal(object):
  """
  Base class for all signals

  Internal attributes:

    receivers
      { receiverkey (id) : weakref(receiver) }
  """
  def __init__(self, providing_args=None, use_caching=False):
    """
    創建一個新的Signal
    providing_args 參數，指定這個Siganl 在發出事件（調用send方法）時，可以提供給觀察者的信息參數
    比如 post_save（）會帶上 對應的instance對象，以及update_fields等
    """
    self.receivers = []
    if providing_args is None:
      providing_args = []
    self.providing_args = set(providing_args)
    self.lock = threading.Lock()
    self.use_caching = use_caching
    # For convenience we create empty caches even if they are not used.
    # A note about caching: if use_caching is defined, then for each
    # distinct sender we cache the receivers that sender has in
    # 'sender_receivers_cache'. The cache is cleaned when .connect() or
    # .disconnect() is called and populated on send().
    self.sender_receivers_cache = weakref.WeakKeyDictionary() if use_caching else {}
    self._dead_receivers = False

  def connect(self, receiver, sender=None, weak=True, dispatch_uid=None):

    from django.conf import settings

    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
      # 構造弱引用的的receiver
      if hasattr(receiver, '__self__') and hasattr(receiver, '__func__'):
        ref = WeakMethod
        receiver_object = receiver.__self__
      if sys.version_info >= (3, 4):
        receiver = ref(receiver)
        weakref.finalize(receiver_object, self._remove_receiver)
      else:
        receiver = ref(receiver, self._remove_receiver)

    with self.lock:
      #clear掉 由于弱引用 已被垃圾回收期回收的receivers
      self._clear_dead_receivers()
      for r_key, _ in self.receivers:
        if r_key == lookup_key:
          break
      else:
        self.receivers.append((lookup_key, receiver))
      self.sender_receivers_cache.clear()

  def disconnect(self, receiver=None, sender=None, weak=True, dispatch_uid=None):

    if dispatch_uid:
      lookup_key = (dispatch_uid, _make_id(sender))
    else:
      lookup_key = (_make_id(receiver), _make_id(sender))

    disconnected = False
    with self.lock:
      self._clear_dead_receivers()
      for index in range(len(self.receivers)):
        (r_key, _) = self.receivers[index]
        if r_key == lookup_key:
          disconnected = True
          del self.receivers[index]
          break
      self.sender_receivers_cache.clear()
    return disconnected

  def has_listeners(self, sender=None):
    return bool(self._live_receivers(sender))

  def send(self, sender, **named):

    responses = []
    if not self.receivers or self.sender_receivers_cache.get(sender) is NO_RECEIVERS:
      return responses

    for receiver in self._live_receivers(sender):
      response = receiver(signal=self, sender=sender, **named)
      responses.append((receiver, response))
    return responses

  def send_robust(self, sender, **named):

    responses = []
    if not self.receivers or self.sender_receivers_cache.get(sender) is NO_RECEIVERS:
      return responses

    # Call each receiver with whatever arguments it can accept.
    # Return a list of tuple pairs [(receiver, response), ... ].
    for receiver in self._live_receivers(sender):
      try:
        response = receiver(signal=self, sender=sender, **named)
      except Exception as err:
        if not hasattr(err, '__traceback__'):
          err.__traceback__ = sys.exc_info()[2]
        responses.append((receiver, err))
      else:
        responses.append((receiver, response))
    return responses

  def _clear_dead_receivers(self):
    # Note: caller is assumed to hold self.lock.
    if self._dead_receivers:
      self._dead_receivers = False
      new_receivers = []
      for r in self.receivers:
        if isinstance(r[1], weakref.ReferenceType) and r[1]() is None:
          continue
        new_receivers.append(r)
      self.receivers = new_receivers

  def _live_receivers(self, sender):
    """
    過濾掉 已經被 垃圾回收的receiver
    """
    receivers = None
    # 如果使用了cache , 并且沒有調用過_remove_receiver 函數 則去 sender_receivers_cache中查找
    if self.use_caching and not self._dead_receivers:
      receivers = self.sender_receivers_cache.get(sender)
      # We could end up here with NO_RECEIVERS even if we do check this case in
      # .send() prior to calling _live_receivers() due to concurrent .send() call.
      if receivers is NO_RECEIVERS:
        return []
    if receivers is None:
      with self.lock:
        self._clear_dead_receivers()
        senderkey = _make_id(sender)
        receivers = []
        for (receiverkey, r_senderkey), receiver in self.receivers:
          if r_senderkey == NONE_ID or r_senderkey == senderkey:
            receivers.append(receiver)
        if self.use_caching:
          if not receivers:
            self.sender_receivers_cache[sender] = NO_RECEIVERS
          else:
            # Note, we must cache the weakref versions.
            self.sender_receivers_cache[sender] = receivers
    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 _remove_receiver(self, receiver=None):

    self._dead_receivers = True

# myapp/signals/handlers.py

from django.dispatch import receiver
from django.core.signals import request_finished

## decorators 方式綁定
@receiver(request_finished, dispatch_uid="request_finished")
def my_signal_handler(sender, **kwargs):
  print("Request finished!================================")

# 普通綁定方式
def my_signal_handler(sender, **kwargs):
  print("Request finished!================================")

request_finished.connect(my_signal_handler)

#####################################################
# 針對model 的signal 
from django.dispatch import receiver
from django.db.models.signals import post_save

from polls.models import MyModel


@receiver(post_save, sender=MyModel, dispatch_uid="mymodel_post_save")
def my_model_handler(sender, **kwargs):
 print('Saved: {}'.format(kwargs['instance'].__dict__))

# myapp/__init__py

default_app_config = 'myapp.apps.MySendingAppConfig'

# myapp/apps.py

from django.apps import AppConfig


class MyAppConfig(AppConfig):
  name = 'myapp'

  def ready(self):
    # signals are imported, so that they are defined and can be used
    import myapp.signals.handlers

myapp.signals.signals.py

importdjango.dispatch

my_signal = django.dispatch.Signal(providing_args=["my_signal_arg1", "my_signal_arg_2"])

# myapp/__init__py

default_app_config = 'myapp.apps.MySendingAppConfig'
myapp/apps.py

from django.apps import AppConfig


class MyAppConfig(AppConfig):
  name = 'myapp'

  def ready(self):
    # signals are imported, so that they are defined and can be used
    import myapp.signals.handlers

# myapp/views.py

from .signals.signals import my_signal

my_signal.send(sender="some function or class",
        my_signal_arg1="something", my_signal_arg_2="something else"])

# myapp/signals/handlers.py

from django.dispatch import receiver
from myapp.signals.signals import my_signal


@receiver(my_signal, dispatch_uid="my_signal_receiver")
def my_signal_handler(sender, **kwargs):
  print('my_signal received')