Observer is a behavioral pattern that lets you define a subscription mechanism so that multiple objects are automatically notified when the state of another object changes. The notified objects are called observers (or listeners); the object they watch is the subject (or publisher). Think of a newspaper subscription: the publisher prints an edition and delivers it to all subscribers — the publisher doesn't call each subscriber individually, and subscribers can join or leave at any time.
ℹ️Core idea: define a one-to-many dependency between objects so that when one object (Subject) changes state, all its dependents (Observers) are notified and updated automatically — without the Subject knowing anything about its observers.
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