Mediator

Context: Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly.

public interface IMediator
{
    void Notify(object sender, string ev);
}

public class ConcreteMediator : IMediator
{
    private ComponentA _a;
    private ComponentB _b;
    public void RegisterA(ComponentA a) => _a = a;
    public void RegisterB(ComponentB b) => _b = b;
    public void Notify(object sender, string ev)
    {
        if (ev == "A") _b.DoB();
        else if (ev == "B") _a.DoA();
    }
}

Real-world usage example

Chat room: A chat mediator handles message routing between users. Users don’t reference each other directly; they send to the mediator, which broadcasts.

Example: In .NET, the MediatR library implements mediator pattern for in‑process messaging. Also, IHttpClientFactory acts as a mediator for creating HttpClient instances.

Related sections

• 5.3 Behavioural patterns
• 5.3.4 Chain of Responsibility
• 5.3.6 Memento