[Haskell-cafe] threepenny-gui: garbage collection

Heinrich Apfelmus apfelmus at quantentunnel.de
Wed Jan 7 14:52:01 UTC 2015

Tom Ellis wrote:
> On Mon, Jan 05, 2015 at 06:31:57PM +0100, Heinrich Apfelmus wrote:
>> Tom Ellis wrote:
>>> Heinrich Apfelmus wrote:
>>>> Tom Ellis wrote:
>>>>> Suppose that each time I make a particular threepenny-gui widget, call it a
>>>>> `Foo`, I register an event handler using `on` so that the `Foo` can change
>>>>> its appearance based on some `Event`.
>>>>> Suppose I create, show, and hide a billion `Foo`s, each one never to be
>>>>> shown again.  Can they be garbage collected?  Presumably there is something
>>>>> storing the callback that was registered for the `Event` and the callback
>>>>> refers to the `Foo`.  How then can the `Foo`s be freed?
>>>>> [..]
>>>> This is a classic example of a circular reference that occurs in
>>>> every GUI program: A widget keeps a reference to an event handler,
>>>> which in turn keeps a reference to the widget.
>>> Hi Heinrich, thanks for your reply.
>>> Actually I don't think I am thinking of a circular reference problem.  The
>>> problem I am envisaging is that the `Event` keeps a reference to the event
>>> handler which in turn keeps a reference to the widget.  Thus it is hard to
>>> see how the widget can be freed before the `Event`.  A quick Google shows
>>> this indeed a common problem in GUI programming, e.g.
>>>    http://stackoverflow.com/questions/4526829/why-and-how-to-avoid-event-handler-memory-leaks
>> Consider this: Who keeps a reference to the `Event`?
>> (If no one keeps a reference to the `Event`, then it can be garbage
>> collected, and so can the event handler and the widget.)
> I am suggesting that the `Event` originates elsewhere, perhaps from some
> other widget that we wish to have a long life, but we want to attach
> short-lived widgets to it via event handlers.  Can we do that without having
> to explicitly unregister the handlers when we want the short-lived widgets
> to be garbage collected?

Well, of course then you have to unregister the event handlers, 
otherwise they will reference the widget. If you don't unregister them, 
then they will, in fact, be long-lived by definition, because the widget 
could still react to the event.

Best regards,
Heinrich Apfelmus


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