Observer Pattern Implementor

When to Use

A data source is directly calling update methods on its consumers, making it impossible to add or remove consumers without modifying the source. Apply this skill when:

• An abstraction has two separable aspects — a subject (data) and observers (views/reactions) — and tightly coupling them limits reuse of each independently
• A change to one object requires notifying an unknown or variable number of other objects, and you don't want to hard-code who those are
• Objects should be able to notify others without making assumptions about who those objects are — you want zero coupling from subject to observer

Do not apply if:

• Only one consumer will ever exist and that is certain by design (direct call is simpler)
• Observers trigger further changes that trigger more observers — cascade control is complex; use ChangeManager (Step 9)
• The language/framework already provides this mechanism natively (e.g., React state, RxJS, Qt signals) — use those instead

Before starting, verify you can answer: What is the subject (the thing that changes)? What are the observers (the things that react)? What state do observers need from the subject after a change?