Observer Pattern
The observer pattern is a software design pattern in which an object, named the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.
Angular uses the Observer pattern which simply means — Observable objects are registered, and other objects observe (in Angular using the subscribe method) them and take action when the observable object is acted on in some way.
Visit the following resources to learn more: