The Mediator pattern is a behavioral design pattern.
One day you receive an order to develop a joke app – the user presses a button in the middle of the screen and a funny duck quacking sound is heard.
After uploading to the app store, the app becomes a hit: everyone quacks through your app, Elon Musk quacks on his Instagram at the latest launch of a super-high-speed tunnel on Mars, Hillary Clinton quacks Donald Trump at the debates and wins the elections in Ukraine, success!
A naive implementation of the application looks like this:
class DuckButton {
func didPress() {
print("quack!")
}
}
let duckButton = DuckButton()
duckButton.didPress()
Next you decide to add the sound of a dog barking, for this you need to show two buttons for selecting the sound – with a duck and a dog. Create two button classes DuckButton and DogButton.
Change the code:
class DuckButton {
func didPress() {
print("quack!")
}
}
class DogButton {
func didPress() {
print("bark!")
}
}
let duckButton = DuckButton()
duckButton.didPress()
let dogButton = DogButton()
dogButton.didPress()
After another success, we add the sound of a pig squeal, now three classes of buttons:
class DuckButton {
func didPress() {
print("quack!")
}
}
class DogButton {
func didPress() {
print("bark!")
}
}
class PigButton {
func didPress() {
print("oink!")
}
}
let duckButton = DuckButton()
duckButton.didPress()
let dogButton = DogButton()
dogButton.didPress()
let pigButton = PigButton()
pigButton.didPress()
Users complain that sounds overlap each other.
We add a check to prevent this from happening, and at the same time introduce the classes to each other:
class DuckButton {
var isMakingSound = false
var dogButton: DogButton?
var pigButton: PigButton?
func didPress() {
guard dogButton?.isMakingSound ?? false == false &&
pigButton?.isMakingSound ?? false == false else { return }
isMakingSound = true
print("quack!")
isMakingSound = false
}
}
class DogButton {
var isMakingSound = false
var duckButton: DuckButton?
var pigButton: PigButton?
func didPress() {
guard duckButton?.isMakingSound ?? false == false &&
pigButton?.isMakingSound ?? false == false else { return }
isMakingSound = true
print("bark!")
isMakingSound = false
}
}
class PigButton {
var isMakingSound = false
var duckButton: DuckButton?
var dogButton: DogButton?
func didPress() {
guard duckButton?.isMakingSound ?? false == false &&
dogButton?.isMakingSound ?? false == false else { return }
isMakingSound = true
print("oink!")
isMakingSound = false
}
}
let duckButton = DuckButton()
duckButton.didPress()
let dogButton = DogButton()
dogButton.didPress()
let pigButton = PigButton()
pigButton.didPress()
Based on the success of your app, the government decides to make a law that allows quack, bark and grunt on mobile devices only from 9:00 am to 3:00 pm on weekdays; at other times, the user of your app risks going to prison for 5 years for indecent sound production using personal electronic devices.
Change the code:
import Foundation
extension Date {
func mobileDeviceAllowedSoundTime() -> Bool {
let hour = Calendar.current.component(.hour, from: self)
let weekend = Calendar.current.isDateInWeekend(self)
let result = hour >= 9 && hour <= 14 && weekend == false
return result
}
}
class DuckButton {
var isMakingSound = false
var dogButton: DogButton?
var pigButton: PigButton?
func didPress() {
guard dogButton?.isMakingSound ?? false == false &&
pigButton?.isMakingSound ?? false == false &&
Date().mobileDeviceAllowedSoundTime() == true else { return }
isMakingSound = true
print("quack!")
isMakingSound = false
}
}
class DogButton {
var isMakingSound = false
var duckButton: DuckButton?
var pigButton: PigButton?
func didPress() {
guard duckButton?.isMakingSound ?? false == false &&
pigButton?.isMakingSound ?? false == false &&
Date().mobileDeviceAllowedSoundTime() == true else { return }
isMakingSound = true
print("bark!")
isMakingSound = false
}
}
class PigButton {
var isMakingSound = false
var duckButton: DuckButton?
var dogButton: DogButton?
func didPress() {
guard duckButton?.isMakingSound ?? false == false &&
dogButton?.isMakingSound ?? false == false &&
Date().mobileDeviceAllowedSoundTime() == true else { return }
isMakingSound = true
print("oink!")
isMakingSound = false
}
}
let duckButton = DuckButton()
let dogButton = DogButton()
let pigButton = PigButton()
duckButton.dogButton = dogButton
duckButton.pigButton = pigButton
dogButton.duckButton = duckButton
dogButton.pigButton = pigButton
pigButton.duckButton = duckButton
pigButton.dogButton = dogButton
duckButton.didPress()
dogButton.didPress()
pigButton.didPress()
Suddenly a flashlight app starts to push ours out of the market, let's not let it defeat us and add a flashlight by pressing the "oink-oink" button, and so-no to the other buttons:
import Foundation
extension Date {
func mobileDeviceAllowedSoundTime() -> Bool {
let hour = Calendar.current.component(.hour, from: self)
let weekend = Calendar.current.isDateInWeekend(self)
let result = hour >= 9 && hour <= 14 && weekend == false
return result
}
}
class Flashlight {
var isOn = false
func turn(on: Bool) {
isOn = on
}
}
class DuckButton {
var isMakingSound = false
var dogButton: DogButton?
var pigButton: PigButton?
var flashlight: Flashlight?
func didPress() {
flashlight?.turn(on: true)
guard dogButton?.isMakingSound ?? false == false &&
pigButton?.isMakingSound ?? false == false &&
Date().mobileDeviceAllowedSoundTime() == true else { return }
isMakingSound = true
print("quack!")
isMakingSound = false
}
}
class DogButton {
var isMakingSound = false
var duckButton: DuckButton?
var pigButton: PigButton?
var flashlight: Flashlight?
func didPress() {
flashlight?.turn(on: true)
guard duckButton?.isMakingSound ?? false == false &&
pigButton?.isMakingSound ?? false == false &&
Date().mobileDeviceAllowedSoundTime() == true else { return }
isMakingSound = true
print("bark!")
isMakingSound = false
}
}
class PigButton {
var isMakingSound = false
var duckButton: DuckButton?
var dogButton: DogButton?
var flashlight: Flashlight?
func didPress() {
flashlight?.turn(on: true)
guard duckButton?.isMakingSound ?? false == false &&
dogButton?.isMakingSound ?? false == false &&
Date().mobileDeviceAllowedSoundTime() == true else { return }
isMakingSound = true
print("oink!")
isMakingSound = false
}
}
let flashlight = Flashlight()
let duckButton = DuckButton()
let dogButton = DogButton()
let pigButton = PigButton()
duckButton.dogButton = dogButton
duckButton.pigButton = pigButton
duckButton.flashlight = flashlight
dogButton.duckButton = duckButton
dogButton.pigButton = pigButton
dogButton.flashlight = flashlight
pigButton.duckButton = duckButton
pigButton.dogButton = dogButton
pigButton.flashlight = flashlight
duckButton.didPress()
dogButton.didPress()
pigButton.didPress()
As a result, we have a huge application that contains a lot of copy-paste code, the classes inside are connected to each other by a dead link - there is no weak coupling, such a miracle is very difficult to maintain and change in the future due to the high chances of making a mistake.
Using Mediator
Let's add an intermediate class mediator - ApplicationController. This class will provide weak coupling of objects, ensure separation of class responsibilities, and eliminate duplicate code.
Let's rewrite:
import Foundation
class ApplicationController {
private var isMakingSound = false
private let flashlight = Flashlight()
private var soundButtons: [SoundButton] = []
func add(soundButton: SoundButton) {
soundButtons.append(soundButton)
}
func didPress(soundButton: SoundButton) {
flashlight.turn(on: true)
guard Date().mobileDeviceAllowedSoundTime() &&
isMakingSound == false else { return }
isMakingSound = true
soundButton.didPress()
isMakingSound = false
}
}
class SoundButton {
let soundText: String
init(soundText: String) {
self.soundText = soundText
}
func didPress() {
print(soundText)
}
}
class Flashlight {
var isOn = false
func turn(on: Bool) {
isOn = on
}
}
extension Date {
func mobileDeviceAllowedSoundTime() -> Bool {
let hour = Calendar.current.component(.hour, from: self)
let weekend = Calendar.current.isDateInWeekend(self)
let result = hour >= 9 && hour <= 14 && weekend == false
return result
}
}
let applicationController = ApplicationController()
let pigButton = SoundButton(soundText: "oink!")
let dogButton = SoundButton(soundText: "bark!")
let duckButton = SoundButton(soundText: "quack!")
applicationController.add(soundButton: pigButton)
applicationController.add(soundButton: dogButton)
applicationController.add(soundButton: duckButton)
pigButton.didPress()
dogButton.didPress()
duckButton.didPress()
Many articles devoted to the architecture of applications with a user interface describe the MVC pattern and its derivatives. The model is used to work with business logic data, the view or presentation shows information to the user in the interface/provides interaction with the user, the controller is a mediator providing interaction between system components.
Sources
https://refactoring.guru/ru/design-patterns/ mediator
Source code
https://gitlab.com/demensdeum/patterns/< /p>
