Observer

في هذا الدرس

notify interested parts when something happens.

الموضوعات المخطط لها

• 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.