Reference: https://refactoring.guru/design-patterns

Here some quick notes

• Things that notify others about their changes in state should implement the publisher interface
• Things that want to track changes to the publisher's state should implement the subscriber interface

The publisher interface includes:

• an array storing references to subscriber objects
• method to add/remove subscriber from above list

The subscriber interface includes:

• the notification method

Others

• all subscribers should implement the same interface
• and the publisher communicate with them via that only interface
• if you have many subscriber interface and want subcribers to be compatible with them, you can go further and make those subsriber implement one interface