Detailed explanation of the observer pattern in Django

"Signal dispatch" is provided in Django for decoupling when the framework performs operations. In layman's terms, a signal allows a specific sender to alert some recipients when some action occurs.

Django provides a signal mechanism. In fact, it is the observer pattern, also called publish-subscribe (Publish/Subscribe). When some action occurs, a signal is emitted, and then the function that listens to the signal will be executed.

Django has some built-in signals, such as:

django.db.models.signals.pre_save 在某个Model保存之前调用
django.db.models.signals.post_save 在某个Model保存之后调用
django.db.models.signals.pre_delete 在某个Model删除之前调用
django.db.models.signals.post_delete 在某个Model删除之后调用
django.core.signals.request_started 在建立Http请求时发送
django.core.signals.request_finished 在关闭Http请求时发送

What we have to do is to register a receiver function. For example, if you want to print a line after each request is completed.

You can use the callback method to register:

# receiver
def my_callback(sender, **kwargs):
  print("Request finished!")
 
# connect
from django.core.signalsimport request_finished
 
request_finished.connect(my_callback)

You can also use the decorator method to register. The following code is completely equivalent to the above.

from django.core.signalsimport request_finished
from django.dispatchimport receiver
 
@receiver(request_finished)
def my_callback(sender, **kwargs):
  print("Request finished!")

receiverCallback functionIn addition to using sender, you can also use some other parameters, such as for the pre_save function:

sender: sender (if it is pre_save, it is model class)
instance: instance
raw
using
update_fields
post_save() is a more practical function that can support some linkagerenew. Instead of having us write it in the view every time. For example: If a user submits a refund application, we need to change the status of the order to the "Refunded" status. You can use the signaling mechanism without having to modify it everywhere.

@receiver(post_save, sender=RefundForm)
deforder_state_update(sender, instance, created, **kwargs):
  instance.order.state = REFUNDING
  instance.order.save() # 这里，order是refundform的一个外键

Of course, you can write more and more comprehensive things here, such as canceling the refund order and changing its status back.

Observer is a very practical design pattern. Django also supports user-defined signals.

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