What is a Dependency Inversion Principle?

The Dependency Inversion Principle means that high-level modules should not depend on low-level modules.

Source

Source

What problems does it solve?

The Dependency Inversion Principle (DIP) helps solve:

  • Rigidity
  • Fragility
  • Immobility problems

Real-world code example

Violation of DIP

// High-level module directly depending on low-level modules
class MessageService {
    func sendMessageViaEmail(message: String) {
        let emailSender = EmailSender()
        emailSender.sendMessage(message: message)
    }
    
    func sendMessageViaSMS(message: String) {
        let smsSender = SMSSender()
        smsSender.sendMessage(message: message)
    }
    
    func sendMessageViaPushNotification(message: String) {
        let pushNotificationSender = PushNotificationSender()
        pushNotificationSender.sendMessage(message: message)
    }
}

Adhering to DIP

// Protocol defining the interface for sending messages
protocol MessageSender {
    func sendMessage(message: String)
}

// High-level module depending on abstraction (MessageSender protocol)
class MessageService {
    private let messageSender: MessageSender
    
    init(messageSender: MessageSender) {
        self.messageSender = messageSender
    }
    
    func sendMessage(message: String) {
        messageSender.sendMessage(message: message)
    }
}

// Concrete implementations of MessageSender protocol for different channels
class EmailSender: MessageSender {
    func sendMessage(message: String) {
        print("Sending email: \(message)")
    }
}

class SMSSender: MessageSender {
    func sendMessage(message: String) {
        print("Sending SMS: \(message)")
    }
}

class PushNotificationSender: MessageSender {
    func sendMessage(message: String) {
        print("Sending push notification: \(message)")
    }
}

// Example usage
let emailSender = EmailSender()
let smsSender = SMSSender()
let pushNotificationSender = PushNotificationSender()

let emailService = MessageService(messageSender: emailSender)
let smsService = MessageService(messageSender: smsSender)
let pushNotificationService = MessageService(messageSender: pushNotificationSender)

emailService.sendMessage(message: "Hello via email")
smsService.sendMessage(message: "Hello via SMS")
pushNotificationService.sendMessage(message: "Hello via push notification")

Thank you for reading! 😊