The Observer design pattern provides a solution:
Define Subject and Observer objects
so that when a subject changes state,
all registered observers are notified and updated automatically.
Describing the Observer design in more detail is the theme of the following sections.
See Applicability section for all problems Observer can solve.

• The key idea in this pattern is to establish a flexible notification-registration interaction by notifying (calling update on) all registered observers automatically when an event of interest occurs.
• Define Subject and Observer objects:
  • Subject defines an interface for registering and unregistering observers (attach(o), detach(o)) and for notifying observers (notify()), i.e., calling update() on all registered observers.
  • Observer defines an interface for updating state (update()), i.e., synchronizing observer's state with subject's state.
    
• When a subject changes state, all registered observers are notified and updated automatically (for each o in observers: o.update()).
• This enables loose coupling between subject and observers.
  Subject and observers have no explicit knowledge of each other. An open-ended number of observers can observe a subject. New observers can be added to and existing ones can be removed from the subject independently and dynamically.
  
• "This kind of interaction is also known as publish-subscribe.
  The subject is the publisher of notifications. It sends out these notifications without having to know who its observers are. Any number of observers can subscribe to receive notifications." [GoF, p294]