Observer
On This Lesson 4 topics
The content for this lesson is written and waiting for review. The outline below shows what it covers. It will be published once it is approved.
In this lesson
notify interested parts when something happens.
Planned topics
- Problem: when an order is placed, email, stock, and analytics all need to know.
- The subject keeps a list of listeners and calls each one.
- The subject does not know what listeners do.
- Do not rely on the order of listeners; that is a hidden trap.
- Grows into event-driven design later.
Draft outline details
Demonstration
interface OrderListener { void onPlaced(Order o); }
listeners.forEach(l -> l.onPlaced(order));Check your understanding
Should the subject know what listeners do?
Show answer
No; that is the point of the pattern.
Practice
Apply this lesson with the module's lab, exercises and quiz.
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