Mediator

Intent, Motivation, Applicability, Structure, Participants, Collaborations, Consequences, Implementation, Sample Code, Known Uses, Related Patterns

Known Uses

Both ET++ and the THINK C class library use director-like objects in dialogs as mediators between widgets.

The application architecture of Smalltalk/V for Windows is based on a mediator structure. In that environment, an application consists of a Window containing a set of panes. The library contains several predefined Pane objects; examples include TextPane, ListBox, Button, and so on. These panes can be used without subclassing. An application developer only subclasses from ViewManager, a class that's responsible for doing inter-pane coordination. ViewManager is the Mediator, and each pane only knows its view manager, which is considered the ``owner'' of the pane. Panes don't refer to each other directly.

The following object diagram shows a snapshot of an application at run-time:

Smalltalk/V uses an event mechanism for Pane-ViewManager communication. A pane generates an event when it wants to get information from the mediator or when it wants to inform the mediator that something significant happened. An event defines a symbol (e.g., #select) that identifies the event. To handle the event, the view manager registers a method selector with the pane. This selector is the event's handler; it will be invoked whenever the event occurs.

The following code excerpt shows how a ListPane object gets created inside a ViewManager subclass and how ViewManager registers an event handler for the #select event:

self addSubpane: (ListPane new paneName: 'myListPane'; owner: self; when: #select perform: #listSelect:).

Another application of the Mediator pattern is in coordinating complex updates. An example is the ChangeManager class mentioned in Observer. ChangeManager mediates between subjects and observers to avoid redundant updates. When an object changes, it notifies the ChangeManager, which in turn coordinates the update by notifying the object's dependents.

A similar application appears in the Unidraw drawing framework and uses a class called CSolver to enforce connectivity constraints between ``connectors.'' Objects in graphical editors can appear to stick to one another in different ways. Connectors are useful in applications that maintain connectivity automatically, like diagram editors and circuit design systems. CSolver is a mediator between connectors. It solves the connectivity constraints and updates the connectors' positions to reflect them.

Known Uses of Abstract Factory, Adapter, Bridge, Builder, Chain of Responsibility, Command, Composite, Decorator, Facade, Factory Method, Flyweight, Interpreter, Iterator, Mediator, Memento, Observer, Prototype, Proxy, Singleton, State, Strategy, Template Method, Visitor