ruby 35 lines · 9 steps

Delegation in Ruby with Forwardable

How Forwardable and Enumerable let a ShoppingCart borrow behavior from the objects it wraps instead of reimplementing it.

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Intermediate delegation composition enumerable structs mixins
1require "forwardable"
2 
3class ShoppingCart
4 extend Forwardable
5 
6 def_delegators :@items, :each, :size, :empty?, :map, :sum
7 def_delegator :@customer, :name, :customer_name
8 def_delegator :@customer, :email, :customer_email
9 
10 include Enumerable
11 
12 def initialize(customer)
13 @customer = customer
14 @items = []
15 end
16 
17 def add(product, quantity: 1)
18 @items << LineItem.new(product, quantity)
19 self
20 end
21 
22 def total
23 sum(&:subtotal)
24 end
25 
26 def to_s
27 "Cart for #{customer_name} (#{size} items, $#{format('%.2f', total)})"
28 end
29 
30 LineItem = Struct.new(:product, :quantity) do
31 def subtotal
32 product.price * quantity
33 end
34 end
35end
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Forwardable lets a class expose methods of its collaborators without hand-writing wrapper methods.
  2. 2Delegating iteration methods and mixing in Enumerable gives you dozens of collection methods for free.
  3. 3Composing objects and forwarding messages keeps a class focused on its own responsibilities.

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