Observer Design Pattern Explained

The Observer pattern lets an object (the subject) notify a list of dependent objects (observers) automatically whenever its state changes, without the subject needing to know any details about them.

Why it exists

• Many systems need one part of the code to react whenever something changes elsewhere, without tightly coupling the two parts together
• Without this pattern, the changing object would need direct references to every interested party and would have to call each one explicitly

How to apply it

• Implement it with a subscribe/unsubscribe mechanism and a notify method that calls every registered observer
• This pattern underlies many familiar tools directly: DOM event listeners, and reactive state libraries all follow the same core idea

FAQ

Is this the same as pub/sub?

They're closely related — Observer typically has direct references between subject and observers, while pub/sub usually adds an intermediary (like a message broker) decoupling publishers from subscribers entirely.