This document discusses monkey patching in Ruby. It defines monkey patching as modifying an existing class at runtime to modify or extend its functionality. It provides examples of monkey patching classes like Person, Hash, Array, and DateTime to add new methods. It also discusses the Axlsx and AxlsxStyler gems, where AxlsxStyler patches the Array class to add style formatting methods to simplify styling Excel spreadsheets generated with Axlsx.

1. Monkey Patching
in Ruby
by Anton Sakovich
@ Ottawa Ruby
Feb 24, 2015

2. What is monkey patching?
Modification of an existing class at runtime with an intent to
modify or extend existing functionality.

3. Guerilla Patching

4. Gorilla Patching

5. Monkey Patching

6. Reopening classes
class Person
attr_accessor :children
def initialize(name)
@name = name
@children = []
end
end
# some code omitted
class Person
def <<(child)
children << child
end
end

7. bob = Person.new('Bob')
paul = Person.new('Paul')
julie = Person.new('Julie')
bob << paul
bob << julie
puts bob.children.count
# 2

8. Example: merging nested hashes
h1 = {
first_name: "Bob",
personal_data: { age: 30 }
}
h2 = {
last_name: "Smith",
personal_data: { weight: 160 }
}
Age data is lost on simple merge
puts h1.merge(h2)
# { first_name: "Bob",
# personal_data: { weight: 160 },
# last_name: "Smith"
# }

9. ActiveSupport gem adds a method to merge nested hashes
require 'active_support/core_ext/hash/deep_merge'
puts h1.deep_merge(h2)
# { first_name: "Bob",
# personal_data: { age: 30, weight: 160 },
# last_name: "Smith"
# }
The new method is added by the Hash classreopening
class Hash
def deep_merge(other_hash, &block)
# some code
end
def deep_merge!(other_hash, &block)
# some code
end
end

10. Example: custom extensions - Rails
# lib/core_extensions.rb
class DateTime
def weekday?
!sunday? && !saturday?
end
end

11. The patch can be wrapped in a module
# lib/core_extensions/date_time/business_days.rb
module CoreExtensions
module DateTime
module BusinessDays
def weekday?
!sunday? && !saturday?
end
end
end
end
and the module gets mixed in
DateTime.include CoreExtensions::DateTime::BusinessDays

12. Example: overriding methods
class Array
alias_method :old_to_s, :to_s
def to_s
stars = "n" << '*' * 40 << "n"
"#{stars}#{old_to_s}#{stars}"
end
end
array = [1, 2, 3]
puts array.to_s
# ****************************************
# [1, 2, 3]
# ****************************************

13. Example: scoped patching in Ruby 2+
module Starrable
refine Array do
alias_method :old_to_s, :to_s
def to_s
stars = "n" << '*' * 40 << "n"
"#{stars}#{old_to_s}#{stars}"
end
end
end
class Logger
using Starrable
def self.debug(x)
puts x.to_s
end
end
Logger.debug [4, 5, 6]
# ****************************************
# [4, 5, 6]
# ****************************************

14. and gems for Excel
spreadsheets
Axlsx AxlsxStyler

15. Sample worksheet

16. With Axlsx

17. With Axlsx + AxlsxStyler
require 'axlsx_styler'
axlsx = Axlsx::Package.new
workbook = axlsx.workbook
workbook.add_worksheet do |sheet|
sheet.add_row
sheet.add_row ["", "Product", "Category", "Price"]
sheet.add_row ["", "Butter", "Dairy", 4.99 ]
sheet.add_row ["", "Bread", "Baked Goods", 3.45 ]
sheet.add_row ["", "Broccoli", "Produce", 2.99 ]
sheet.column_widths 5, 20, 20, 20
# using AxlsxStyler DSL
sheet["B2:D2"].add_style b: true
sheet["B2:B5"].add_style b: true
sheet["B2:D2"].add_style bg_color: "95AFBA"
sheet["B3:D5"].add_style bg_color: "E2F89C"
sheet["D3:D5"].add_style alignment: { horizontal: :left }
sheet["B2:D5"].add_border
sheet["B3:D3"].add_border [:top]
end
workbook.apply_styles
axlsx.serialize "grocery.xlsx"

18. Axlsx allows to retrieve an array of cells with
sheet["B2:D5"]
This allows to for easy implementation of style applications,
such as
sheet["B2:D2"].add_style b: true
sheet["B2:D5"].add_border
sheet["B3:D3"].add_border [:top]

19. AxlsxStyler: a patch to the Array (for now)
module AxlsxStyler
module Array
def add_style(style)
validate_cells
each do |cell|
cell.add_style(style)
end
end
def add_border(edges = :all)
validate_cells
selected_edges(edges).each do |edge|
add_border_at(edge)
end
end
# ...
end
end

20. require 'axlsx'
require 'axlsx_styler/version'
require 'axlsx_styler/array'
require 'axlsx_styler/axlsx_extensions'
# adding the patches
Array.send :include, AxlsxStyler::Array
Axlsx::Workbook.send :include, AxlsxStyler::Axlsx::Workbook
Axlsx::Cell.send :include, AxlsxStyler::Axlsx::Cell

21. Credits
Russ Olsen, Eloquent Ruby, Addison-Wesley 2011
David A. Black, The Well-Grounded Rubyist, 2nd ed.,
Manning 2014
http://www.justinweiss.com/blog/2015/01/20/3-ways-to-
monkey-patch-without-making-a-mess

22. Thank you!