如何在 python 中连接到 GObject 信号，而不保留对连接器的引用？

Question

问题基本上是这样的，在 python 的 gobject 和 gtk 绑定中。假设我们有一个在构造时绑定到信号的类：

class ClipboardMonitor (object):
  def __init__(self):
    clip = gtk.clipboard_get(gtk.gdk.SELECTION_CLIPBOARD)
    clip.connect("owner-change", self._clipboard_changed)

现在的问题是，没有 ClipboardMonitor 实例会消亡。 gtk 剪贴板是一个应用程序范围的对象，连接到它会保留对该对象的引用，因为我们使用回调 self._clipboard_changed。

我正在讨论如何使用弱引用（weakref 模块）来解决这个问题，但我还没有拿出一个计划。任何人都知道如何将回调传递给信号注册，并使其表现得像弱引用（如果在 ClipboardMonitor 实例超出范围时调用信号回调，则它应该是无操作）。

添加： 独立于 GObject 或 GTK+ 的表述：

如何使用弱引用语义为不透明对象提供回调方法？如果连接对象超出范围，则应将其删除，并且回调应充当无操作；连接者不应持有对连接器的引用。

澄清一下：我明确希望避免调用“析构函数/终结器”方法

Answer 1

标准的方法是断开信号。然而，这需要在您的类中具有类似析构函数的方法，由维护对象的代码显式调用。这是必要的，因为否则你会得到循环依赖。

class ClipboardMonitor(object):
    [...]

    def __init__(self):
        self.clip = gtk.clipboard_get(gtk.gdk.SELECTION_CLIPBOARD)
        self.signal_id = self.clip.connect("owner-change", self._clipboard_changed)

    def close(self):
        self.clip.disconnect(self.signal_id)

正如您所指出的，如果您想避免显式破坏，则需要弱引用。我会编写一个弱回调工厂，例如：（

import weakref

class CallbackWrapper(object):
    def __init__(self, sender, callback):
        self.weak_obj = weakref.ref(callback.im_self)
        self.weak_fun = weakref.ref(callback.im_func)
        self.sender = sender
        self.handle = None

    def __call__(self, *things):
        obj = self.weak_obj()
        fun = self.weak_fun()
        if obj is not None and fun is not None:
            return fun(obj, *things)
        elif self.handle is not None:
            self.sender.disconnect(self.handle)
            self.handle = None
            self.sender = None

def weak_connect(sender, signal, callback):
    wrapper = CallbackWrapper(sender, callback)
    wrapper.handle = sender.connect(signal, wrapper)
    return wrapper

这是概念验证代码，对我有用 - 您可能应该根据您的需要调整这一部分）。几点说明：

• 我分别存储回调对象和函数。您不能简单地创建绑定方法的weakref，因为绑定方法是非常临时的对象。实际上，weakref.ref(obj.method)会在创建weakref后立即销毁绑定的方法对象。我也没有检查是否需要存储函数的弱引用...我想如果你的代码是静态的，你可能可以避免这种情况。
• 当对象包装器注意到弱引用不再存在时，它会将自身从信号发送者中删除。这对于破坏 CallbackWrapper 和信号发送者对象之间的循环依赖也是必要的。

Answer 2

（这个答案跟踪我的进度）

第二个版本也将断开连接；我有一个用于 gobjects 的便利函数，但实际上我需要此类来处理更一般的情况 - 无论是 D-Bus 信号回调还是 GObject 回调。

无论如何，我们可以将 WeakCallback 实现风格称为什么？它是对弱回调的非常干净的封装，但不知不觉地附加了 gobject/dbus 专业化。胜过为这两种情况编写两个子类。

import weakref

class WeakCallback (object):
    """A Weak Callback object that will keep a reference to
    the connecting object with weakref semantics.

    This allows to connect to gobject signals without it keeping
    the connecting object alive forever.

    Will use @gobject_token or @dbus_token if set as follows:
        sender.disconnect(gobject_token)
        dbus_token.remove()
    """
    def __init__(self, obj, attr):
        """Create a new Weak Callback calling the method @obj.@attr"""
        self.wref = weakref.ref(obj)
        self.callback_attr = attr
        self.gobject_token = None
        self.dbus_token = None

    def __call__(self, *args, **kwargs):
        obj = self.wref()
        if obj:
            attr = getattr(obj, self.callback_attr)
            attr(*args, **kwargs)
        elif self.gobject_token:
            sender = args[0]
            sender.disconnect(self.gobject_token)
            self.gobject_token = None
        elif self.dbus_token:
            self.dbus_token.remove()
            self.dbus_token = None

def gobject_connect_weakly(sender, signal, connector, attr, *user_args):
    """Connect weakly to GObject @sender's @signal,
    with a callback in @connector named @attr.
    """
    wc = WeakCallback(connector, attr)
    wc.gobject_token = sender.connect(signal, wc, *user_args)

Answer 3

还没有真正尝试过，但是：

class WeakCallback(object):
    """
    Used to wrap bound methods without keeping a ref to the underlying object.
    You can also pass in user_data and user_kwargs in the same way as with
    rpartial. Note that refs will be kept to everything you pass in other than
    the callback, which will have a weakref kept to it.
    """
    def __init__(self, callback, *user_data, **user_kwargs):
        self.im_self = weakref.proxy(callback.im_self, self._invalidated)
        self.im_func = weakref.proxy(callback.im_func)
        self.user_data = user_data
        self.user_kwargs = user_kwargs

    def __call__(self, *args, **kwargs):
        kwargs.update(self.user_kwargs)
        args += self.user_data
        self.im_func(self.im_self, *args, **kwargs)

    def _invalidated(self, im_self):
        """Called by the weakref.proxy object."""
        cb = getattr(self, 'cancel_callback', None)
        if cb is not None:
            cb()

    def add_cancel_function(self, cancel_callback):
        """
        A ref will be kept to cancel_callback. It will be called back without
        any args when the underlying object dies.
        You can wrap it in WeakCallback if you want, but that's a bit too
        self-referrential for me to do by default. Also, that would stop you
        being able to use a lambda as the cancel_callback.
        """
        self.cancel_callback = cancel_callback

def weak_connect(sender, signal, callback):
    """
    API-compatible with the function described in
    http://stackoverflow.com/questions/1364923/. Mostly used as an example.
    """
    cb = WeakCallback(callback)
    handle = sender.connect(signal, cb)
    cb.add_cancel_function(WeakCallback(sender.disconnect, handle))