DependencyInjection.swift

// Without Dependency Injection
class Company {
    var name: String
    // `ceo` is dependency here, but Company initializes the ceo object,
    // `Company` knows how to initilize the ceo
    var ceo: Employee = Employee(name: "Steve", salary: 12344)

    init(name: String) {
        self.name = name
    }
}

class Employee {
    var name: String
    var salary: Int

    init(name: String, salary: Int) {
        self.name = name
        self.salary = salary
    }
}

// here we do not need to pass `ceo` object as inside Company class, it is already been taken care by the Company class itself
let microsoft = Company(name: "Microsoft")

//******************************************************************************************************************


// Dependency Injection
class Company {
    var name: String
    // `ceo` is dependency here, but does not know how to initialize ceo object, Company expecting that whoever will initilize Company knows how to initialize ceo also.
    var ceo: Employee
    
    init(name: String, ceo: Employee) {
        self.name = name
        self.ceo = ceo
    }
}

class Employee {
    var name: String
    var salary: Int
    
    init(name: String, salary: Int) {
        self.name = name
        self.salary = salary
    }
}

// here we have to pass the ceo object by using the initializer of Employee class. So company does not know how to initialize or it does not have any responsibility to initialize ceo, it only knows how to use ceo. this is dependency Injection
let microsoft = Company(name: "Microsoft", ceo: Employee(name: "Steve", salary: 12344))