APIs that employ observer-pattern design

Hey Folks,
Writing to inquire whether you guys use any APIs that employ an observer-pattern design; as explained here:

• Defines a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. (https://www.oodesign.com/observer-pattern.html)

Any feedback is much appreciated

 

This is built-in to C# using delegates, check those out.