Using shared_context and shared_examples in Rspec with optional arguments. Answer to SO 24872199

rspec-shared-context-examples-optional.rb

#-------------------------- #
#
# @author Ashley Engelund (ashley@ashleycaroline.com  weedySeaDragon @ github)
# @date   1/13/17
#
# @desc   This is my answer to the Stack Overflow (SO) question:
#         How can I use a default method to create something in a shared_example,
#         sometimes overriding it with some other method and parameters for it?
#
#         @see http://stackoverflow.com/questions/24872199/rspec-with-reusable-method-for-shared-examples-context
#
#         The example in the SO question refers to the classes as
#         'associations'. That's why those terms are used in the code.
#         The classes in this example aren't "associations" in any way.
#
#
#         Note that I don't think doing that is a good idea. It's a clue that
#         your specifications need to be refactored / redesigned. 
#         It also makes your tests error prone (what if you mistype a method name?)
#
#         But I did this as an exersize to show that this is all just Ruby.
#         You can check to see if a parameter is nil and work with it just
#         as you can in 'regular' Ruby code.
#
#         Repetitive code and 'puts' are used to make the results clear
#         and things explicit so that novices can understand it.
#         No error checking is done; that's not the point of this example.
#
#
# @usage  Run this with 'rspec'
#         It will use puts statements to show which objects were created and
#         used in the shared_examples.
#
#--------------------------


#---------------------------------------------------

# Classes used in the RSpec code below.
#
# The names of the classes are from the StackOverflow (SO) question.
#
# These have different numbers of attributes and so the methods used
# to instantiate (create) them require different numbers of arguments.
# I did this to make it clear that number of arguments can vary.
#
# The methods that create new objects set the attributes such that it's
# clear that we have used the arguments passed to the methods, and so that
# it's clear which method was used to create the object.
#
class AddLink

  attr_accessor :user_symbol, :arg1

  def initialize(user_symbol = nil, param1 = nil)
    @user_symbol = user_symbol
    @arg1 = param1
  end

  def self.create_and_repeat_arg1(user_symbol, param1)
    new_link = new
    new_link.user_symbol = "created with method '#{__method__}' #{user_symbol}"
    new_link.arg1 = "#{param1}-#{param1}"
    new_link
  end

end


class DeleteLink

  attr_accessor :user_symbol, :arg1, :arg2

  def self.create_and_repeat_args(user_symbol, param1, param2)
    new_link =new
    new_link.user_symbol = "created with method '#{__method__}' #{user_symbol}"
    new_link.arg1 = "#{param1}-#{param1}"
    new_link.arg2 = "#{param2}-#{param2}"
    new_link
  end
end


#---------------------------------------------------

# Shared context and example
#

RSpec.shared_context 'assoc created with default method, parameters' do |default_class, default_method, parameters|

  # you can use 'default_assoc' anywhere you've included this shared context

  let(:default_assoc) {
    default_class.send(default_method, *parameters)
  }
end


RSpec.shared_examples 'update association service' do |optional_class, optional_method, parameters|

  # The name of this is from the SO question

  # warning: no error checking is done to see if default_assoc.nil? or even defined,
  #  or if optional_class.respond_to? optional_method

  let(:assoc) {
    if optional_class.nil? || optional_method.nil?
      default_assoc
    else
      optional_class.send(optional_method, *parameters)
    end
  }

  it 'used this association:' do
    puts_example_desc_and_association(assoc)
  end

end

#---------------------------------------------------

# helper methods just to show things
def puts_example_desc_and_association(association)
  puts "\n  #{@__inspect_output}\n      association = #{association.inspect}\n"
end


def puts_spec_and_association(association)
  puts "\n- - -\n#{self.class.description}"
  puts_example_desc_and_association(association) #"\n  #{@__inspect_output}\n    association = #{default_assoc.inspect}"
end


#---------------------------------------------------

RSpec.describe 'Links' do

  describe 'AddLink (use the default assocation, which = use the default method for creating it)' do

    # The shared_context 'assoc created with default method, parameters'
    # is how we set the default way to create an association within this example group.
    # Instead of trying to memoize the class, method, and parameter, we
    # go ahead and memoize the assocation created and call it 'default_assoc'.
    #
    # In other words:
    # In this example group, the default method (and class) to create an association
    #  is DeleteLink.create_and_repeat_args('userSymbol', 'firstArg', 'secondArg', 'thirdArg')
    #
    # The result of this is stored in 'default_assoc' (default association).
    # Any example that comes after this can refer to (use)' default_assoc'

    include_context 'assoc created with default method, parameters', AddLink, :create_and_repeat_arg1, ['userSymbol', 'firstArg']


    # This example shows what 'default_assoc' is.
    #  If this is the first time 'default_assoc' is used, then it will
    #  execute the let(:default_assoc) block that is in the
    #  shared_context 'assoc created with default method, parameters' that is
    #  included above.
    it 'created default association' do
      puts_spec_and_association(default_assoc)
    end


    # Since no arguments are passed to this shared example,
    # it will use 'default_assoc'. (See the code in the shared_example.)
    it_behaves_like 'update association service'

  end


  describe 'DeleteLink (override default association)' do

    # This is how 'default_assoc' is created:
    include_context 'assoc created with default method, parameters', DeleteLink, :create_and_repeat_args, ['userSymbol', 'firstArg', 'secondArg']

    # Show what 'default_assoc' is:
    it 'created default association' do
      puts_spec_and_association(default_assoc)
    end


    # This is how you effectively override the default method to create an 'assocation'
    # (or whatever class you want).
    #
    # In this example group, the default method (and class) to create an association
    #  is DeleteLink.create_and_repeat_args('userSymbol', 'firstArg', 'secondArg')
    #
    # The result of this is stored in 'default_assoc'
    #
    # Instead of using 'default_assoc', this will create an association
    # with the DeleteLink class, 'new' method, and an empty argument list

    it_behaves_like 'update association service', DeleteLink, :new, []

  end


  describe "BlorfLink (use 'AddLink.new('userSymbol', 'firstArg')' to create the default association)" do

    # This just shows that you can use the 'new' method to
    # create the default association ('default_assoc').

    include_context 'assoc created with default method, parameters', AddLink, :new, ['userSymbol', 'firstArg']

    it 'created default association' do
      puts_spec_and_association(default_assoc)
    end

  end

end