An example implementation of Null Object pattern for reference types using protocol

NullObjectReferenceTypeProtocol.swift

protocol PhoneCall {
    var identifier: Int { get }
    var isNil: Bool { get }
    func hangUp()
}

class RealPhoneCall: PhoneCall {

    let identifier: Int
    let isNil = false

    init(identifier: Int) {
        self.identifier = identifier
    }

    func hangUp() {
        print("Hanging up call")
    }
}

class NullPhoneCall: PhoneCall {

    let identifier = 0
    let isNil = true

    func hangUp() {
        print("Not hanging up call")
    }
}

protocol PhoneCallRegistry {
    func callWithIdentifier(identifier: Int) -> PhoneCall
}

class PhoneCallRegistryImpl {

    private var phoneCalls = [Int: PhoneCall]()

    func addCall(call: PhoneCall) {
        phoneCalls[call.identifier] = call
    }
}

extension PhoneCallRegistryImpl: PhoneCallRegistry {

    func callWithIdentifier(identifier: Int) -> PhoneCall {
        if let phoneCall = phoneCalls[identifier] {
            return phoneCall
        } else {
            return NullPhoneCall()
        }
    }
}

// Usage

let registry = PhoneCallRegistryImpl()
let phoneCall = RealPhoneCall(identifier: 1)
registry.addCall(phoneCall)

func hangUpCallWithIdentifier(identifier: Int) {
    registry.callWithIdentifier(identifier).hangUp()
}

hangUpCallWithIdentifier(1)  // Prints 'Hanging up call'
hangUpCallWithIdentifier(2)  // Prints 'Not hanging up call'

let realCall = registry.callWithIdentifier(1)
print(realCall.isNil)  // Prints 'false'

let nullCall = registry.callWithIdentifier(2)
print(nullCall.isNil)  // Prints 'true'