Monday, March 23, 2015

memory leak in GUI event handlers - swing^c#

Here's a common java swing memory leak as described on stack overflow -- Essentially, because you register an object as a listener and forget to unregister, the notifier ends up holding onto the reference of the object, and leaks a bit of memory.

Same problem with c# events. [[effective c#]] refers to it as "runtime coupling despite compile-time loose coupling". Recall an event
is a pseudo field in a class, and the fields' type is a delegate, which is a list of 2-pointer constructs. First pointer usually points to a listener object. Imagine a bunch of large listener objects are added to the delegate's inv list (registered for event), and one of them is no longer needed. You want its memory freed, but the event source object still holds a reference via the delegate instance. See [[effective c#]] item 23. One tip is to unregister in the listener's Dispose()

I feel the dotnet event/delegate language construct has no exact java counterpart at the language level, but at runtime, swing event
paradigm == c# event paradigm. Fundamentally the same paradigm. Part of the paradigm is that the event source object holds a
collection of subscriber Objects. If the GUI stays up for 22 hours then the subscriber Objects are held "hostage" for that long, and
cannot be garbage collected.

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