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List:       openjdk-openjfx-dev
Subject:    Re: WeakXXListener - when not to use?
From:       Jeanette Winzenburg <fastegal () swingempire ! de>
Date:       2020-03-27 15:03:39
Message-ID: 20200327160339.Horde.p-BCB8UvZNS-vKImc_XXBQ1 () webmail ! df ! eu
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Zitat von Nir Lisker <nlisker@gmail.com>:

> BTW, Tomas Mikula wrote about this on
> http://tomasmikula.github.io/blog/2015/02/10/the-trouble-with-weak-listeners.html
> .
> There is a comment at the end that is worth a read too.

also interesting is the old (closed as wontfix) bug  
https://bugs.openjdk.java.net/browse/JDK-8095375 - it's about a very  
long lived observable (f.i. one holding the current Locale, which  
hardly ever changes but might) to which all nodes are listening with  
WeakListeners. Those listeners hang around until the value actually  
changes - at which time it might be too late to really do any cleanup  
(either directly throwing a overflow or make the ui unresponsive).

Always thought that the weakListeners are removed from two ends  
(provided their weakRef is reclaimed):

1. from the listener itself when being notified: it removes itself  
from the observable if ref is null
2. from the observable itself via its ExpressionHelper which has a  
trim method that's called on addListener: it seems to throw away all  
garbage collected weak listeners

1 is ineffective without change notification, and 2 seems to rarely be  
effective: it's only ever called when the current size of the  
listeners is exactly its capacity .. which in the example of the bug  
is after it's too late.

Just musing (didn't test anything) - maybe this is part of the very  
bad performance of table scrolling (see  
https://github.com/openjdk/jfx/pull/108) and maybe it is worth  
improving, after all :)

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