Mediator pattern

The Mediator Pattern belongs to the behavioral design patterns.

Once you receive an order to develop a joke application – the user presses a button in the middle of the screen and a funny sound of duck quacking is heard.
After uploading to appstore, the app becomes a hit: everyone quacks through your app, Ilon Musk quacks in his instagram at the next launch of a super-high-speed tunnel on Mars, Hillary Clinton beat Donald Trump in debates and wins elections in Ukraine, success!
The 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 the dog’s bark, for this you need to show two buttons for selecting the sound – with a duck and a dog. Create two classes of buttons DuckButton and DogButton.
Change 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, already 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.
Add a check so that it does not happen, along the way we 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()

In the wake of the success of your application, the government decides to make a law on which to quack, bark and grunt on mobile devices only from 9:00 am and until 3:00 pm on weekdays.
Change 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, the flashlight application starts to force out ours from the market, let's not let it beat us, and add a flashlight by clicking on the “Oink” button, and the rest of the 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 bundle - there is no loose coupling, such a miracle is very difficult to maintain and change further because of the high chances of making a mistake.

Use Mediator

Add an intermediate class mediator - ApplicationController. This class will provide a weak connectivity of objects, provides for the division of class responsibility, and will eliminate duplicate code.
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 on user interface architecture with a user interface describe the MVC pattern and derivatives. The model is used to work with business logic data, a view or view shows information to the user in the interface / provides user interaction, the controller is a mediator providing interaction between the system components.

References

https://refactoring.guru/design-patterns/mediator

Source Code

https://gitlab.com/demensdeum/patterns/

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