protocol ServiceAProtocol {
    func performTaskA()
}

protocol ServiceBProtocol {
    func performTaskB()
}

class ServiceA: ServiceAProtocol {
    func performTaskA() {
        print("performing task A ....")
    }
}

class ServiceB: ServiceBProtocol {
    func performTaskB() {
        print("performing task B ....")
    }
}

class Client {
    var serviceA: ServiceAProtocol?
    var serviceB: ServiceBProtocol?
    init() {}
    func performTasks() {
        // will optionally call services
        self.serviceA?.performTaskA()
        self.serviceB?.performTaskB()
    }
}

// Injector code
let client = Client()
client.serviceA = ServiceA()
client.serviceB = ServiceB()
client.performTasks()