In some ( non-polling) implementations of the publish-subscribe pattern, this is solved by creating a dedicated message queue server (and sometimes an extra message handler object) as an extra stage between the observer and the object being observed, thus decoupling the components. This type of implementation is considered tightly coupled, forcing both the observers and the subject to be aware of each other and have access to their internal parts, creating possible issues of scalability, speed, message recovery and maintenance (also called event or notification loss), the lack of flexibility in conditional dispersion and possible hindrance to desired security measures. ![]() Typically, the observer pattern is implemented so that the subject being observed is part of the object for which state changes are being observed (and communicated to the observers). This can be prevented if the subject holds weak references to the observers.Ĭoupling and typical publish-subscribe implementations The observer pattern can cause memory leaks, known as the lapsed listener problem, because in a basic implementation, it requires both explicit registration and explicit deregistration, as in the dispose pattern, because the subject holds strong references to the observers, keeping them alive. This notification-registration interaction is also known as publish-subscribe. Observers can be added and removed independently at run time. ![]() Subject and observers have no explicit knowledge of each other. This makes subject and observers loosely coupled. The responsibility of observers is to register and unregister themselves with a subject (in order to be notified of state changes) and to update their state (to synchronize their state with the subject's state) when they are notified. The sole responsibility of a subject is to maintain a list of observers and to notify them of state changes by calling their update() operation.
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