The Observer pattern
Observer is a behavioral design pattern that establishes a subscription model of communication. In simple words, you use a publisher object that posts messages to a list of active subscribers either in regard to some events that occur or when a specific rule is activated. Those rules could be some points of interest. For example, say you want to send notifications after a user has registered or some part of your application needs to get an update from a different part of the application.
This way, you establish a one-to-many communication model between the publisher and the subscriber list. The publisher would not know what the subscriber list looks like. It only allows a reference to an interface that can push messages into it.
On the other side, the subscriber will receive events from the publisher that it subscribes to and has the choice of acting on or disregarding them. If the publisher somehow gets destroyed, subsequently it will remove any references...