我正在研究是否可以在 python 中实现简单的回调功能。我以为我可以为此使用 weakref.WeakSet,但显然我遗漏了一些东西或被误解了。正如您在代码中看到的那样,我首先尝试使用“ClassA”对象中的回调方法列表,但意识到这会使已添加到回调列表中的对象保持事件状态。相反,我尝试使用 weakref.WeakSet 但这也不起作用(至少不是这样)。最后四行代码中的注释解释了我想要发生的事情。
谁能帮我解决这个问题?
from weakref import WeakSet
class ClassA:
def __init__(self):
#self.destroyCallback=[]
self.destroyCallback=WeakSet()
def __del__(self):
print('ClassA object %d is being destroyed' %id(self))
for f in self.destroyCallback:
f(self)
class ClassB:
def destroyedObjectListener(self,obj):
print('ClassB object %d is called because obj %d is being destroyed'%(id(self),id(obj)))
a1=ClassA()
a2=ClassA()
b=ClassB()
a1.destroyCallback.add(b.destroyedObjectListener)
#a1.destroyCallback.append(b.destroyedObjectListener)
print('destroyCallback len() of obj: %d is: %d'%(id(a1),len(a1.destroyCallback))) # should be 1
a2.destroyCallback.add(b.destroyedObjectListener)
#a2.destroyCallback.append(b.destroyedObjectListener)
print('destroyCallback len() of obj: %d is: %d'%(id(a2),len(a2.destroyCallback))) # should be 1
del a1 # Should call b.destroyedObjectListener(self) in its __del__ method
del b # should result in no strong refs to b so a2's WeakSet should automatically remove added item
print('destroyCallback len() of obj: %d is: %d'%(id(a2),len(a2.destroyCallback))) # should be 0
del a2 # Should call __del__ method
更新:可以在 github 上找到基于已接受答案的解决方案:git@github.com:thgis/PythonEvent.git
最佳答案
您不能创建对方法对象的弱引用。方法对象是短暂的;它们是在您访问实例上的名称时即时创建的。查看descriptor howto这是如何运作的。
当您访问方法名称时,会为您创建一个新方法对象,然后当您将该方法添加到WeakSet
时,不存在对它的其他引用再也没有了,所以垃圾收集很乐意再次清理它。
您必须存储一些不那么短暂的东西。存储实例对象本身会起作用,然后在注册的回调上调用预定义的方法:
def __del__(self):
for f in self.destroyCallback:
f.destroyedObjectListener(self)
并注册:
a1.destroyCallback.add(b)
你也可以通过给它一个__call__
方法使b
本身成为可调用的:
class ClassB:
def __call__(self,obj):
print('ClassB object %d is called because obj %d '
'is being destroyed' % (id(self), id(obj)))
另一种方法是存储对底层函数对象的引用以及对实例的引用:
import weakref
class ClassA:
def __init__(self):
self._callbacks = []
def registerCallback(self, callback):
try:
# methods
callback_ref = weakref.ref(callback.__func__), weakref.ref(callback.__self__)
except AttributeError:
callback_ref = weakref.ref(callback), None
self._callbacks.append(callback_ref)
def __del__(self):
for callback_ref in self._callbacks:
callback, arg = callback_ref[0](), callback_ref[1]
if arg is not None:
# method
arg = arg()
if arg is None:
# instance is gone
continue
callback(arg, self)
continue
else:
if callback is None:
# callback has been deleted already
continue
callback(self)
演示:
>>> class ClassB:
... def listener(self, deleted):
... print('ClassA {} was deleted, notified ClassB {}'.format(id(deleted), id(self)))
...
>>> def listener1(deleted):
... print('ClassA {} was deleted, notified listener1'.format(id(deleted)))
...
>>> def listener2(deleted):
... print('ClassA {} was deleted, notified listener2'.format(id(deleted)))
...
>>> # setup, one ClassA and 4 listeners (2 methods, 2 functions)
...
>>> a = ClassA()
>>> b1 = ClassB()
>>> b2 = ClassB()
>>> a.registerCallback(b1.listener)
>>> a.registerCallback(b2.listener)
>>> a.registerCallback(listener1)
>>> a.registerCallback(listener2)
>>>
>>> # deletion, we delete one instance of ClassB, and one function
...
>>> del b1
>>> del listener1
>>>
>>> # Deleting the ClassA instance will only notify the listeners still remaining
...
>>> del a
ClassA 4435440336 was deleted, notified ClassB 4435541648
ClassA 4435440336 was deleted, notified listener2
关于python - 使用 python WeakSet 启用回调功能,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/21826700/