Swift Example

/// Mediator Design Pattern
///
/// Intent: Lets you reduce chaotic dependencies between objects. The pattern
/// restricts direct communications between the objects and forces them to
/// collaborate only via a mediator object.

import XCTest

/// The Mediator interface declares a method used by components to notify the
/// mediator about various events. The Mediator may react to these events and
/// pass the execution to other components.
protocol Mediator: AnyObject {

    func notify(sender: BaseComponent, event: String)
}

/// Concrete Mediators implement cooperative behavior by coordinating several
/// components.
class ConcreteMediator: Mediator {

    private var component1: Component1
    private var component2: Component2

    init(_ component1: Component1, _ component2: Component2) {
        self.component1 = component1
        self.component2 = component2

        component1.update(mediator: self)
        component2.update(mediator: self)
    }

    func notify(sender: BaseComponent, event: String) {
        if event == "A" {
            print("Mediator reacts on A and triggers following operations:")
            self.component2.doC()
        }
        else if (event == "D") {
            print("Mediator reacts on D and triggers following operations:")
            self.component1.doB()
            self.component2.doC()
        }
    }
}

/// The Base Component provides the basic functionality of storing a mediator's
/// instance inside component objects.
class BaseComponent {

    fileprivate weak var mediator: Mediator?

    init(mediator: Mediator? = nil) {
        self.mediator = mediator
    }

    func update(mediator: Mediator) {
        self.mediator = mediator
    }
}

/// Concrete Components implement various functionality. They don't depend on
/// other components. They also don't depend on any concrete mediator classes.
class Component1: BaseComponent {

    func doA() {
        print("Component 1 does A.")
        mediator?.notify(sender: self, event: "A")
    }

    func doB() {
        print("Component 1 does B.\n")
        mediator?.notify(sender: self, event: "B")
    }
}

class Component2: BaseComponent {

    func doC() {
        print("Component 2 does C.")
        mediator?.notify(sender: self, event: "C")
    }

    func doD() {
        print("Component 2 does D.")
        mediator?.notify(sender: self, event: "D")
    }
}

/// Let's see how it all works together.
class MediatorConceptual: XCTestCase {

    func testMediatorConceptual() {

        let component1 = Component1()
        let component2 = Component2()

        let mediator = ConcreteMediator(component1, component2)
        print("Client triggers operation A.")
        component1.doA()

        print("\nClient triggers operation D.")
        component2.doD()

        print(mediator)
    }
}

Client triggers operation A.
Component 1 does A.
Mediator reacts on A and triggers following operations:
Component 2 does C.

Client triggers operation D.
Component 2 does D.
Mediator reacts on D and triggers following operations:
Component 1 does B.

Component 2 does C.