Revision Notes/Design Patterns
Field Entry - Design Patterns
Observer Pattern
Defines a one-to-many dependency so when one object (subject) changes state, all its dependents (observers) are notified automatically.
Read the full Design Patterns guide → · Or jump to the cheat sheet →
Field Note - Real-world Analogy
“Like subscribing to a YouTube channel. You (observer) click Subscribe on a channel (subject). When the channel uploads a video (state change), YouTube notifies all subscribers automatically. You can Unsubscribe any time. The creator doesn't personally know or contact each subscriber — it all goes through the platform's notification system.”
Field Observations - Key Topics Covered
The problem: one object changes and several other objects need to react. Without Observer, the subject would need to know about every dependent and call them directly — tight coupling that gets messy fast. Adding a new dependent means modifying the subject.
4 more observations inside
Sign in to read every key point in full
Ready to go deeper?
Open the complete revision note.
Sign in to read every field observation in full, including a runnable code example and side-by-side comparisons with related patterns.
See Also in Design Patterns