Composite.md

Composite

Introduction

The Composite Design Pattern is a structural design pattern that allows us to use recursive composition so that client code can treat individual objects and compositions uniformly. It's particularly useful when dealing with a hierarchy of objects.

In simpler terms, it lets you compose objects into tree structures and then work with these structures as if they were individual objects.

Let's take a closer look at how you can implement the Composite Design Pattern in Swift.

Components of Composite Design Pattern

The Composite Pattern consists of three primary components:

1. Component: This is the base interface or class which defines the common methods that will be implemented by all child parts.
2. Leaf: These are the building blocks of the composite pattern. They implement all the methods defined by the Component.
3. Composite: This is an object which consists of Components. It implements the Component methods and also defines methods for accessing its child Components.

Implementing Composite Design Pattern in Swift

Let's consider an example of an organization structure to demonstrate the composite pattern. The organization has employees, and each employee can either be a manager or a developer.

We'll start by defining the Component:

protocol Employee {
    func work()
}

Next, let's define the Leaf classes:

class Manager: Employee {
    func work() {
        print("Manager is managing the team")
    }
}

class Developer: Employee {
    func work() {
        print("Developer is writing code")
    }
}

Then, let's create the Composite:

class Organization {
    private var employees: [Employee] = []
    
    func add(employee: Employee) {
        employees.append(employee)
    }
    
    func work() {
        employees.forEach { $0.work() }
    }
}

In this example, Employee is the Component, Manager and Developer are Leaf classes, and Organization is the Composite.

Now we can use it like this:

let john = Manager()
let alice = Developer()
let bob = Developer()

let organization = Organization()
organization.add(employee: john)
organization.add(employee: alice)
organization.add(employee: bob)

organization.work()

When we call work() on the organization, it delegates the call to all its employees, whether they're managers or developers. This is the power of the Composite Design Pattern.

Conclusion

The Composite Design Pattern provides a way to work with tree structures in a way that abstracts away the difference between individual objects and groups of objects. It's a powerful tool for dealing with complex hierarchies, and Swift's strong protocol-oriented programming features make it an excellent fit for implementing this pattern.