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?
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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-handle...
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? Tom