[Design Pattern] Mediator Pattern

One-line pattern summary

A pattern that collects interactions between many objects into a mediator so direct dependencies between objects are reduced.

Typical Unity use cases

• When centrally controlling inventory, equipment, and shop UI interactions.
• When mutual references start becoming too complex.

Parts (roles)

• Mediator
• Concrete Mediator
• Colleague

Unity example (C#)

The code below is a simplified Unity example based on the scenario described above.

public interface IUiMediator
{
    void Notify(object sender, string eventId);
}

public sealed class LobbyUiMediator : IUiMediator
{
    public InventoryPanel InventoryPanel { get; set; }
    public EquipmentPanel EquipmentPanel { get; set; }

    public void Notify(object sender, string eventId)
    {
        if (sender == InventoryPanel && eventId == "ItemSelected")
        {
            EquipmentPanel.PreviewSelectedItem();
        }
    }
}

Advantages

• Behavior is separated into smaller units, which reduces the impact of changes.
• Adding or swapping rules is relatively safe.

Things to watch out for

• As the number of objects and indirect calls increases, the flow can become harder to follow.
• Ordering bugs should be pinned down with tests.

Interaction diagram

This shows the flow where a mediator routes communication instead of components talking directly.

flowchart LR

  mediator["GameMediator"]

  subgraph components["Components"]
    ui["UI"]
    audio["Audio"]
    quest["Quest"]
  end

  ui -->|event| mediator
  audio -->|event| mediator
  quest -->|event| mediator
  mediator -->|notify| ui
  mediator -->|notify| audio
  mediator -->|notify| quest