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## Observer Pattern

### Context

1. An object, called the subject, is source of events
2. One or more observer objects want to be notified when such an event occurs.

### Solution

1. Define an observer interface type. All concrete observers implement it.
2. The subject maintains a collection of observers.
3. The subject supplies methods for attaching and detaching observers.
4. Whenever an event occurs, the subject notifies all observers.

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