Lapsed listener problem

The lapsed listener problem is a common source of memory leaks for object-oriented programming languages, among the most common ones for garbage collected languages.

It originates in the observer pattern, where observers (or listeners) register with a subject (or publisher) to receive events. In basic implementation, this requires both explicit registration and explicit deregistration, as in the dispose pattern, because the publisher holds strong references to the observers, keeping them alive. The leak happens when a listener fails to unsubscribe from the publisher when it no longer needs to listen. Consequently, the publisher still holds a reference to the observer which prevents it from being garbage collected — including all other objects it is referring to — for as long as the publisher is alive, which could be until the end of the application.

This causes not only a memory leak, but also a performance degradation with an "uninterested" observer receiving and acting on unwanted events. This can be prevented by the subject holding weak references to the observers, allowing them to be garbage collected as normal without needing to be unregistered.

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