iOS Development Made Simple!

Through reading this short article you’ll be taking yet another big step towards familiarising yourself with Object Relations in Object Oriented Programming.

In this article you’ll be served with explanation about Composition.

To get a better grasp on today’s topic, before we go on with this article, I highly encourage you to read my article on Aggregation first. Then come back and read this article further.

As Aggregation is a specialised way of Association. Composition is considered to be a special kind of Aggregation. In a such relation a single object i.e container is always composed of one of multiple instances of other object(s).

Composition On A UML Diagram Is Shown With A Simple Arrow In front 
And A Filled Diamond/ Squared Tail.
————-
Object School knows about object Department, con­sists of Department, and man­ages Departments life cycle, for instance a school can closedown an entire department. 
Class University depends on Department.

Here is an example, think of a school which serves as a container for multiple departments. The main difference between this type of rela­tion and oth­er relation types, is that here objects can only exist as a parts of the main con­tain­er. Below you will find some Swift code which explain Composition further.

import Foundation

class Student {
    let name: String
    let lastName: String
    let age: Int
    
    init(name: String, lastName: String, age: Int) {
        self.name = name
        self.lastName = lastName
        self.age = age
    }
}

class Teacher {
    let name: String
    let lastName: String
    let age: Int
    
    init(name: String, lastName: String, age: Int) {
        self.name = name
        self.lastName = lastName
        self.age = age
    }
}

class SoftwareDepartment {
    
    private var teachers: [Teacher] = []
    private var students: [Student] = []
    
    func addTeacher( _ teacher: Teacher) {
        self.teachers.append(teacher)
    }
    
    func addStudent( _ student: Student) {
        self.students.append(student)
    }
    
    func getStudent() -> [Student] {
        return self.students
    }
    
    func getTeacher() -> [Teacher] {
        return self.teachers
    }
}

class DesignDepartment {
    private var teachers: [Teacher] = []
    private var students: [Student] = []
    
    func addTeacher( _ teacher: Teacher) {
        self.teachers.append(teacher)
    }
    
    func addStudent( _ student: Student) {
        self.students.append(student)
    }
    
    func getStudent() -> [Student] {
        return self.students
    }
    
    func getTeacher() -> [Teacher] {
        return self.teachers
    }
    
}

class School {
    
    private var designDepartment: DesignDepartment
    private var softwareDepartment: SoftwareDepartment
    
    init( _ designDepartment: DesignDepartment,
          _ softwareDepartment: SoftwareDepartment){ //Dependency
        self.designDepartment = designDepartment
        self.softwareDepartment = softwareDepartment
    }
    
    func getDepartments() -> (design: DesignDepartment, 
            software: SoftwareDepartment){ //Dependency
        return (design: self.designDepartment, 
                software: self.softwareDepartment)
    }
}

let student = Student(name: "Joseph", lastName: "Doe", age: 25)
let teacher = Teacher(name: "Yasser", lastName: "Farahi", age: 34)
let softwareDepartment = SoftwareDepartment()
let designDepartment = DesignDepartment()
let school = School(designDepartment, softwareDepartment)
softwareDepartment.addTeacher(teacher)
designDepartment.addTeacher(teacher)
softwareDepartment.addStudent(student)
designDepartment.addStudent(student)

print(school.getDepartments().design.getTeacher().first!.name)
print(school.getDepartments().software.getStudent().first!.name)
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