python - Python中Subject已经 "deleted"时使用观察者模式删除Observer

Question

我正在尝试实现一个非常简单的 observer pattern在 python 。

这是我的 Observer 类(它实际上只是一个接口(interface)，我想我实际上并不需要它):

class Observer():
    def update(self,subject,message): pass

还有我的 Subject 类(又名 Observable，但我更喜欢 Subject):

class Subject():
    def __init__(self):
        self.observers = []
    def registerObserver(self, observer):
        if observer not in self.observers:
            self.observers.append(observer)
    def removeObserver(self, observer):
        self.observers.remove(observer)
    def notifyObservers(self, message = None):
        for observer in self.observers:
            observer.update(self,message)

类 A 包含一个嵌套的 DelNotifier 类，它是 Subject 的子类。当一个类 A 对象被删除时(实际上是垃圾回收，因为它在 __del__ 方法中)， A.DelNotifier 会通知删除的所有观察者。

class A():
    def __init__(self, name):
        self.name = name
        self.delNotifier = A.DelNotifier(self)
    class DelNotifier(Subject):
        def __init__(self, outer):
            super(A.DelNotifier,self).__init__()
            self.outer = outer
        def notifyObservers(self):
            Subject.notifyObservers(self,"This is Class A object " + self.outer.name + ": I'm dying!")
    def registerB(self,observer):
        if not isinstance(observer,B): raise ValueError("Can only register Class B objects with Class A.")
        self.delNotifier.registerObserver(observer.Aobserver)
    def deleteme(self):
        print("Must notify observers of my impending doom first...")
        self.delNotifier.notifyObservers()
    def __str__(self):
        return "Class A object " + self.name
    def __del__(self):
        self.deleteme()
        print("Done notifying everyone, time to go gentle into that good night.")

类 B 包含一个嵌套的 AObserver 类，它是 Observer 的子类，将从类 A 接收消息.DelNotifier subject 当 A 被删除时(同样，实际上这发生在 A 对象被垃圾回收时):

class B():
    def __init__(self, name, a):
        self.name = name
        self.Aobserver = B.AObserver(self)
        a.registerB(self)
    class AObserver(Observer):
        def __init__(self,outer):
            super(B.AObserver,self).__init__()
            self.outer = outer
        def update(self,subject,message):
            print(str(self.outer) + " received message: '" + str(message) + "'")
            print("Time for", self.outer, "to die, too.")
            self.outer.__del__()
    def __str__(self):
        return "Class B object " + self.name
    def __del__(self):
        print("This is " + str(self) + ": now I'm dying, too!")

这种设计在我直接调用 __del__() 时有效，但是，当 session 退出时，某些对象似乎被第二次 gc:

>>> a = A('a')
>>> b1 = B('b1', a)
>>> b2 = B('b2', a)
>>> a.__del__()
Must notify observers of my impending doom first...
Class B object b1 received message: 'This is Class A object a: I'm dying!'
Time for Class B object b1 to die, too.
This is Class B object b1: now I'm dying, too!
Class B object b2 received message: 'This is Class A object a: I'm dying!'
Time for Class B object b2 to die, too.
This is Class B object b2: now I'm dying, too!
Done notifying everyone, time to go gentle into that good night.
>>> exit()
Must notify observers of my impending doom first...
Class B object b1 received message: 'This is Class A object a: I'm dying!'
Time for Class B object b1 to die, too.
This is Class B object b1: now I'm dying, too!
Class B object b2 received message: 'This is Class A object a: I'm dying!'
Time for Class B object b2 to die, too.
This is Class B object b2: now I'm dying, too!
Done notifying everyone, time to go gentle into that good night.
This is Class B object b1: now I'm dying, too!
This is Class B object b2: now I'm dying, too!

另一个问题，我认为这更重要，是当我从列表中del 类A 项目时，该项目不会立即被垃圾回收，我无法确定是否已删除任何已注册的 B 项目:

>>> b1 = B('b1',a[0])
>>> b2 = B('b2',a[0])
>>> del a[0]
## Note that items are not deleted until session exits
>>> exit()
Must notify observers of my impending doom first...
Class B object b1 received message: 'This is Class A object a: I'm dying!'
Time for Class B object b1 to die, too.
This is Class B object b1: now I'm dying, too!
Class B object b2 received message: 'This is Class A object a: I'm dying!'
Time for Class B object b2 to die, too.
This is Class B object b2: now I'm dying, too!
Done notifying everyone, time to go gentle into that good night.
##Note that the class B objects get gc'd a second time....???
This is Class B object b1: now I'm dying, too!
This is Class B object b2: now I'm dying, too!

除了这些问题之外，我还意识到依赖 __del__ 方法来做除了在 gc 之后清理对象之外的任何事情所固有的许多问题，并且出于我尝试使用的目的，可能应该避免使用它。但我不知道另一种方法。

执行此操作的更好方法是什么？我考虑过尝试使用上下文管理器 (with) 在我使用完它们后删除它们，但我没有这样做的经验。如果那是一个不错的选择，我将如何去做？它会是什么样子？

编辑:澄清所需行为

我将尝试澄清一些(可以理解的)混淆。

我在上面简化了代码，但是 B 是一个依赖于对象 A 的对象。如果 B 的 A 消失了，那个 B 也应该消失。我将有一些 As 和 Bs 的容器(在这里使用 list):

As = [A('a'+str(i)) for i in range(10)]
Bs = [B('b'+str(i),As[i]) for i in range(10)] #Bs made of As
del As[0] #whoops, don't need As[0] anymore
assert Bs[0] is None #ERROR!
#or using pop:
As.pop(0)
assert Bs[0] is None #ERROR!

还有 see my previous question from the other day这帮助我想到了首先使用观察者模式的整个想法。

Answer 1

这是一个很大的代码差异，可以满足您自动维护引用列表并在删除引用时进行清理的要求。我添加了一个 Manager 类来完成此操作，并添加了第二个传递 deleted() 事件，该事件是 Manager 清理其维护的列表的 Hook 。我将在此处重新发布完整的修改后的代码，因为更新我之前的答案并非易事。

我相信这完全可以满足您提出的问题。也许不是您一开始就需要这个的原因为什么，但我认为您在另一个问题中问过这个问题。

我们需要弱引用来完成这项工作:

import weakref

Observer 接口(interface)获取一个新方法，在 deleted()

之后调用

class Observer(object):
    def update(self,subject,message): pass

    def deleting(self,subject):
        ''' the subject is being deleted '''
        pass

    def deleted(self,subject):
        pass

管理类维护主题和观察者列表

class Manager(Observer):

    def __init__(self, ):
        self._subjects = []
        self._observers = []

    def monitorSubject(self, subject):
        self._subjects.append( weakref.ref(subject) )

        # observe the subject for deletion to
        # trigger list maintenance on "deleted"
        subject.registerObserver(self)

    def monitorObserver(self, observer):
        self._observers.append( weakref.ref(observer) )

    def deleted(self, subject):
        ''' a subject was deleted, remove it from the list.
        deleting() is called first, and the observers delete themselves.
        deleted() is called next, and is a hook for the manager to
        cleanup any dead subjects and observers '''

        # calling a weakref returns the original object, and `None` when the
        # reference is dead
        def isNotDead(r):
            return not r()==None

        # remove any dead subjects
        print 'Removing dead subjects...'
        self._subjects = filter(isNotDead, self._subjects)

        # remove any dead observers            
        print 'Removing dead observers...'
        self._observers = filter(isNotDead, self._observers, )

    def __str__(self,):
        return "[Manager] Subjects:{0}, Observers:{1}".format(
            ','.join([str(r()) for r in self._subjects]),
            ','.join([str(r()) for r in self._observers])
        )

主题的差异在评论中注明，但主要是有第二次调用 deleted。 deleting 的第一遍通知观察者，而 deleted 的第二遍通知管理器。此外，__del__ 例程使用弱引用进行迭代，因为其中一些引用会在此过程中被删除。

class Subject(object):
    def __init__(self):
        self.observers = []

    def __del__(self, ):
        ''' on deletion, notify the observers '''
        print "{0}.__del__".format(self)

        # copy the observer list, then remove all references to the observers
        # NEW - use weakrefs here, or they will not be properly deleted later
        obs = [weakref.ref(o) for o in self.observers]

        # clear all strong references to the observers
        self.observers = []

        # notify all observers that we were deleted
        # ** only if they are not already deleted **
        for o in obs:
            if not o() == None:
                o().deleting(self)

        # NEW - second pass to allow the Manager to cleanup
        # ** only if they are not already deleted **
        for o in obs:
            if not o() == None:
                o().deleted(self)

    def registerObserver(self, observer):
        if observer not in self.observers:
            self.observers.append(observer)

    def removeObserver(self, observer):
        self.observers.remove(observer)

    def notifyObservers(self, message = None):
        for observer in self.observers:
            observer.update(self,message)

和以前一样，简化了字符串格式

class A(Subject):
    ''' A is just a subject '''
    def __init__(self, name):
        super(A,self).__init__()
        self.name = name

    def __str__(self):
        return "A[ {0} ]".format(self.name)

B和之前一样

class B(Observer):
    ''' class B is an observer of A '''

    def __init__(self, name, a):
        self.name = name

        # don't keep a reference to 'a', or 'a' will not be deleted!
        a.registerObserver(self)

    def __str__(self):
        return "B[ {0} ]".format(self.name)

    def __del__(self):
        print("{0}.__del__".format(self))

    def deleting(self, subject):
        ''' notification from the subject (A) that it is being deleted. I'm
        assuming here that the subject is actually the one we registered in
        __init__, since I couldn't store a reference or else __del__ would
        not have been called! '''
        print "B[{0}].deleting, subject={1}".format(self.name, subject)

        del self

执行文件的一些代码:

if __name__ == '__main__':

    mgr = Manager()

    # keep strong references to the subjects, because
    # we will delete them explicitly
    a1 = A('a1')
    a2 = A('a2')
    mgr.monitorSubject(a1)
    mgr.monitorSubject(a2)

    # monitor observers directly, and do NOT keep
    # strong references or they will not be deleted
    mgr.monitorObserver( B('b1', a1) )
    mgr.monitorObserver( B('b2', a1) )
    mgr.monitorObserver( B('b3', a2) )
    mgr.monitorObserver( B('b4', a2) )

    # show the starting state
    print mgr

    print "Deleting a1..."
    del a1
    print mgr

    print "Deleting a2..."
    del a2
    print mgr

输出:

    #  OUTPUT (some newlines added)
    #
    #  [Manager] Subjects:A[ a1 ],A[ a2 ], Observers:B[ b1 ],B[ b2 ],B[ b3 ],B[ b4 ]
    #
    #  Deleting a1...
    #  A[ a1 ].__del__
    #  B[ b1 ].__del__
    #  B[ b2 ].__del__
    #  Removing dead subjects...
    #  Removing dead observers...
    #  [Manager] Subjects:A[ a2 ], Observers:B[ b3 ],B[ b4 ]
    #
    #  Deleting a2...
    #  A[ a2 ].__del__
    #  B[ b3 ].__del__
    #  B[ b4 ].__del__
    #  Removing dead subjects...
    #  Removing dead observers...
    #
    #  [Manager] Subjects:, Observers: