Mediator Design Pattern in Scala

1. Definition

The Mediator Design Pattern aims to reduce connections between multiple classes or objects by centralizing external communications. It allows you to decouple or reduce the direct references between the classes that would otherwise be tightly coupled.

2. Problem Statement

In a complex system, if each component communicates with each other directly, the system can become unmanageable. This tight coupling between classes or objects can introduce a plethora of issues, including high interdependency, code maintenance challenges, and difficulty in scaling.

3. Solution

The Mediator pattern suggests introducing a new component, called the mediator, that encapsulates how a set of objects interact. Instead of talking with each other directly, objects communicate only through the mediator.

4. Real-World Use Cases

1. Air traffic control center mediating communication between planes.

2. Chat rooms where individual participants don't communicate directly but only through the server.

3. MVC frameworks where the controller acts as a mediator between the view and the model.

5. Implementation Steps

1. Create a mediator interface that encapsulates interactions between different components.

2. Implement concrete mediator classes.

3. Components should only communicate using the mediator and not directly with each other.

6. Implementation in Scala Programming

// Step 1: Define the Mediator trait
trait Mediator {
  def notify(sender: Component, event: String): Unit
}
// Step 2: Create concrete Components
class Component(val mediator: Mediator) {
  def triggerEvent(event: String): Unit = {
    mediator.notify(this, event)
  }
}
// Step 3: Implement a concrete Mediator
class ConcreteMediator extends Mediator {
  var component1: Component = _
  var component2: Component = _
  override def notify(sender: Component, event: String): Unit = {
    if (event == "A") {
      println("Mediator reacts on A and triggers related operations.")
      component2.triggerEvent("B")
    } else {
      println("Mediator reacts on B.")
    }
  }
}
// Client and Tester
object MediatorClient extends App {
  val mediator = new ConcreteMediator()
  val comp1 = new Component(mediator)
  val comp2 = new Component(mediator)
  mediator.component1 = comp1
  mediator.component2 = comp2
  println("Component 1 triggers event A.")
  comp1.triggerEvent("A")
}

Output:

Component 1 triggers event A.
Mediator reacts on A and triggers related operations.
Mediator reacts on B.

Explanation:

1. We define a Mediator trait that is the interface for our mediators.

2. The Component class represents components that wish to communicate with each other. They don't communicate directly but send their communications through the mediator.

3. ConcreteMediator defines how it will react to notifications from components.

4. In the client code, two components trigger events, but their communication is mediated by the mediator, decoupling the direct interaction between them.

7. When to use?

The Mediator pattern is particularly useful when:

1. A set of objects communicate with each other in well-defined but complex ways, making them hard to understand or maintain.

2. Reusing an object is difficult because it communicates with many other objects.

3. An abstraction should be able to encapsulate interactions between a set of objects.