Design Patterns in life and Ruby – gain an intuitive understanding of OO design patterns by linking them with real-life examples.

The Composite Pattern gives the ability to treat grouped objects and individual objects the same.

Let’s look at its definition:

A Real Life Example: Group Messaging

Let’s look at the definition in light of group messaging.

There are six people on my contact list: Mom, Dad, Uncle Bob, Cousin Nick, Aunt Julia, and Amy Kinser. The Parents group is composed of Mom and Dad, whereas the Uncle Bob’s Family group is composed of Uncle Bob, Cousin Nick, and Aunt Julia.

In this example, Contacts is represented in a tree structure made of nodes. A node is either a group of people or a single person. If a node is a group of people, like Parents, then it contains other nodes. If a node is a single person, like Mom, then it’s a leaf node.

Thanks to the tree structure, Contacts can be arbitrarily complex. For example, I can add another node, CollegeFriends, containing four friends, two of which are grouped into the Roommates node.

We want to message a group of people the same way we message a single person — parents.message(“I’m coming home this weekend :D”) and mom.message(‘I miss you, mom. ❤’).

We can achieve this by having all nodes, both leaf-nodes and non-leaf-nodes, support the same interface: node.message(text).

The code for leaf-node is straightforward:

class PersonContact
  attr_reader :title, :number
  
  def initialize(title, number)
    @title = title
    @number = number
  end
  
  def message(text)
    puts "Messaging #{title}(#{number}): "
    puts "  #{text}"
  end
end

Let’s send a message to mom:

The code for non-leaf node implements the same interface:

class GroupContact
  attr_reader :group
  def initialize(people_in_group)
    @group = people_in_group
  end
  
  def message(text)
    group.each do |person|
      person.message(text)
    end
  end
end

Let me message my parents:

Woohoo, that’s the Composite pattern!

Takeaways

Question: may I add you to my contact list? 🙂

Related Posts

• Refactoring Legacy Code V.S. Staying Focused On The Current Project

• First Impression of Elixir, from a Rubyist’s Perspective

• Moving Millions of Dollars with Ruby While Still Able to Sleep at Night

• How to Contribute to Ruby

• Ouch! That Code Hurts My Brain.