#1 SOLID - Single Responsibility Principle | Swift | iOS Development

SRP state that a class should have only one reason to change.

Below is an implementation for a Debt calculator.

public class DebtCalculatorOld {

    private var totalDebt: Int = 0

    public init(totalCalories: Int = 0) {
        self.totalDebt = totalCalories
    }

    public func update(_ amount: Int) {
        self.totalDebt += amount

        if self.totalDebt > 500 {
            //Notify user about the change
            print("Limit Reached")
        }
    }
}

• Each class should focus on performing a single task.

• The below class is for managing debt of a user. If the total debt reaches a certain amount then we should notify the user about it.

• Here we have added everything in one class which not recommended as per single responsibility principle.

• In future, if we want to change the notification type to be email. Then we will have to change the update function in the DebtCalculator.

• There should not be multiple reasons for a class to change.
  

public class DebtCalculator {

    private var totalDebt: Int = 0
    private var notificationManager: NotificationManager

    public init(totalCalories: Int = 0,
                notificationManager: NotificationManager = NotificationManager()) {
        self.totalDebt = totalCalories
        self.notificationManager = notificationManager
    }

    public func update(_ amount: Int) {
        self.totalDebt += amount

        if self.totalDebt > 500 {
            notificationManager.notify(totalDebt)
        }
    }
}

public class NotificationManager {

    public init() {}

    func notify(_ current: Int) {
        print("Limit Reached - \(current)")
    }
}

• Here we have seperate class for Adding debt releated features and Notification releated features.
• In future if we want to chagne the Notification type to email we can change it without making changes in DebtCalculator