Mediator

M5 · Communication Between ObjectsSession 10-11BehavioralStill needed as-isOutline

Intent. Replace direct references between objects with a hub that coordinates them.

In Kotlin. No language feature replaces it. Watch the hub for God Object growth.

The other way to cut coupling. Where Observer’s publisher knows nothing about its subscribers, a mediator knows all of them and holds the coordination logic. That is its value and its failure mode: every rule you cannot place anywhere else ends up in the hub, and the hub becomes a God Object.

Structure

classDiagram
    class Mediator {
        <<interface>>
        +notify(sender, event)
    }
    class DialogMediator {
        +notify(sender, event)
    }
    class Component {
        #mediator: Mediator
    }
    class Button
    class TextField
    class Checkbox
    Mediator <|.. DialogMediator
    Component o--> Mediator : knows only the hub
    Component <|-- Button
    Component <|-- TextField
    Component <|-- Checkbox

Telling It Apart

Patterns whose structure looks the same but whose intent does not. See the Confusing Pairs reference.

• Observer — Notify a set of subscribers automatically when state changes; the publisher does not know who is listening.