#4 SOLID - Interface Segregation Principle | Swift | iOS Development

In the following example we have defined a PrinterProtocol which we can use to create multiple type of printers.

We have added every methods in one protocol. So, even if we want to create a simple printer we will have to implement every function in the protocol. We can provide default implimention for the protocols but still it's violates the ISP because the SimplePrinter could call the default implementation as well. That is not expected behaviour. A SimplePrinter doesn't have to call receiveFax() method.

protocol PrinterProtocol {
    func printDocument()
    func scanDocument()
    func sendFax()
    func receiveFax()
}

extension PrinterProtocol {
    func receiveFax() {
        print("Default: Receiving fax...")
    }
}

class SimplePrinter: PrinterProtocol {
    func printDocument() {
        print("Printing document...")
    }

    func scanDocument() {
        print("Scanning document...")
    }

    func sendFax() {
        print("Sending fax...")
    }

    func receiveFax() {
        print("Receiving fax...")
    }
}

Let's check how we can implement as per ISP 🚀

• Clients should not be forced to depend on interfaces they don't use.
• Larger, general-purpose interfaces should be segregated into smaller, more specific interfaces that are tailored to the needs of individual clients.
• The goal is to avoid the situation where a client is required to implement methods that are irrelevant to its use case.
• This promotes modularity, maintainability, flexibility, and testability in software systems.

protocol Printer {
    func printDocument()
}

protocol Scanner {
    func scanDocument()
}

protocol FaxMachine {
    func sendFax()
    func receiveFax()
}

class AllInOnePrinter: Printer, Scanner, FaxMachine {
    func printDocument() {
        print("Printing document...")
    }

    func scanDocument() {
        print("Scanning document...")
    }

    func sendFax() {
        print("Sending fax...")
    }

    func receiveFax() {
        print("Receiving fax...")
    }
}

class SimplePrinter: Printer {
    func printDocument() {
        print("Printing document...")
    }
}