CaseRegister

Provide a design pattern to manage switch statements by mapping each case to a method which is dynamically defined in advance.

Supports

Ruby 1.8 ~ 2.7

Installation
Usage
Use cases
- Refactor case statements
- Fallthrough
- Code Reuse
- Isolate methods
- Check invalid cases
- Command-line Games
Development
Contributing
License

Installation

gem 'case_register'

And then execute:

$ bundle

Or install it yourself as:

$ gem install case_register

Usage

Define your cases

First, include CaseRegister in your class. Then register some cases for it by calling register_case.

class MyClass
  include CaseRegister

  register_case('Foo'){ 'foo' }
  register_case('Bar'){ 'bar' }
end

instance = MyClass.new

invoke the case

Use invoke_case method to call the registerd cases.

instance.invoke_case('Foo')
# => foo

instance.invoke_case('Bar')
# => bar

instance.invoke_case('FooBar')
# => RuntimeError (Undefined case `FooBar` for MyClass)

Check available cases

Use may_invoke_case to check if a case exists.

instance.may_invoke_case?('Foo')
# => true

instance.may_invoke_case?('Bar')
# => true

instance.may_invoke_case?('FooBar')
# => false

Use cases

Refactor case statements

Take the examples from this article. Let you have a method like this:

def output(data, format)
  case format
  when :html
    return "<p>#{data}</p>"
  when :text
    return data
  when :pdf
    return "<pdf>#{data}</pdf>" # pseudocode -- obviously not valid PDF output
  else
    raise ArgumentError, "Invalid format (#{format})."
  end
end

output('Hi', :html)
# => "<p>Hi</p>"

You can refactor it by using a hash table to map the input, like what the article says.

Or use CaseRegister to register cases. Then, you are able to invoke the case directly without using switch-statements.

class MyFormatter
  include CaseRegister

  def initialize(text)
    @text = text
  end

  register_case(:pdf){ "<pdf>#{@text}</pdf>" }
  register_case(:text){ @text }
  register_case(:html){ "<p>#{@text}</p>" }
end

MyFormatter.new('Hi').invoke_case(:html)
# => "<p>Hi</p>"

Fallthrough

Case-statement in ruby doesn't have fallthrough behavior. There are many other ways you can do to simulate fallthrough behavior.

CaseRegister provides another way to simulate the behavior.

// Fallthrough in javascript
switch(type){
  case 'A': console.log("a"); break;
  case 'B': console.log("b"); // fallthrough
  case 'C': console.log("c"); // fallthrough
  case 'D': console.log("d"); break;
}

class MyClass
  include CaseRegister

  register_case('A'){ p 'a' }
  register_case('B'){ p 'b'; invoke_case('C') }
  register_case('C'){ p 'c'; invoke_case('D') }
  register_case('D'){ p 'd' }
end

MyClass.new.invoke_case('B')
# "b"
# "c"
# "d"

Code Reuse

In case-statement, it is impossible to exec the code in other cases since there is no way to jump out of current case. We will have to move the code out and define a method, then call it in cases.

With CaseRegister, every case is a method, We can call it directly in one case.

For example, if we want to define three kinds of skill: Shock, Flame Shock and Thunder Shock. All of them are Shock, the difference is that the Flame Shock is a Shock with fire effect and the Thunder Shock is a Shock with thunder effect. We would like to reuse the code in Shock and set fire effect for Flame Shock (Like Inheritance). In this case, we can reuse the code in Shock by calling invoke_case('Shock').

class Skill
  include CaseRegister

  register_case 'Shock' do
    @debuff = Debuff::Shock.new(duration: 5.seconds)
  end

  register_case 'Flame Shock' do
    invoke_case('Shock')

    @atk_effect.set_fire_effect!
  end

  register_case 'Thunder Shock' do
    invoke_case('Shock')
    
    @atk_effect.set_thunder_effect!
  end
end

Isolate methods

Let you have an api that allows the frontend to pass params to determetine which information it wants.

Since you have to use send to call the methods in user model dynamically, it will cause security issues if you do not use a whitelist to limit the methods it can access.

class User < ApplicationRecord
  def money_info
    { value: money, rate: gain_money_rate }
  end

  def notification_info
    { msg_count: new_msgs_count, last_received_at: last_received_at }
  end
end

class UserController
  METHOD_WHITE_LIST = [:money_info, :notification_info]
 
  def refresh
    result = params[:needs].slice(METHOD_WHITE_LIST).index_with{|method| current_user.send(method)  }
    render json: result 
  end
end

You may use if-statements or switch-statments to call the desired method. But you will find you repeat writing similiar things and it seems redundant to have a same word appearing 3 times on the same line.

class UserController
  def refresh
    result = {}
    result[:money_info] = current_user.money_info if params[:needs][:money_info]
    result[:notification_info] = current_user.notification_info if params[:needs][:notification_info]
    # ...

    render json: result 
  end
end

In this case, you can use CaseRegister to DRYing up and isolate the methods to prevent unsafely calling send, which can access all the methods defined in the model.

class RefreshHelper
  include CaseRegister

  def initialize(user)
    @user = user
  end

  register_case 'money_info' do
    { value: @user.money, rate: @user.gain_money_rate }
  end

  register_case 'notification_info' do
    { msg_count: @user.new_msgs_count, last_received_at: @user.last_received_at }
  end
end

class UserController
  def refresh
    helper = RefreshHelper.new(current_user)
    result = params[:needs].index_with{|need| helper.invoke_case(need) }
    render json: result 
  end
end

Check invalid cases

Sometimes, you may want to check if a case is valid or not, and return error message if not.

It can be accomplish by using may_invoke_case? method. See the following example:

class UserController
  def refresh
    helper = RefreshHelper.new(current_user)
    invalid_needs = params[:needs].select{|need| !helper.may_invoke_case?(need) }
    return render json: { invalid_needs: invalid_needs } if invalid_needs.any?
    # ...
  end
end

Command-line Games

CaseRegister is very suitable for creating a command-line games. It will help you get rid of large switch statements.

The following code is a little maze game, enjoy it :)

require 'case_register'

class MazeGame
  include CaseRegister

  def initialize
    @maze = <<~MAZE
      101111111111111111
      101010000000000101
      100010111110010001
      101010010010111101
      111001011010100001
      101100001000101111
      100011101111100001
      101000000000000101
      1111111111111111A1
    MAZE

    @height = @maze.count("\n")
    @width = @maze.size / @height
  end

  register_case('show') do
    puts '------ Maze ------'
    puts @maze
    puts 'Enter your command: (move up / move down / move right / move left / quit)'
  end

  register_case('move up') do
    puts 'cannot move up' if not move_to!(current_position - @width)
  end

  register_case('move down') do
    puts 'cannot move down' if not move_to!(current_position + @width)
  end

  register_case('move left') do
    puts 'cannot move left' if not move_to!(current_position - 1)
  end

  register_case('move right') do
    puts 'cannot move right' if not move_to!(current_position + 1)
  end

  register_case('cheat') do
    move_to!(20)
  end

  private

  def move_to!(new_index)
    return false if new_index < 0
    return false if @maze[new_index] != '0'

    @maze[current_position], @maze[new_index] = @maze[new_index], @maze[current_position]
    after_move
    return true
  end

  def after_move
    invoke_case('show')
    puts "You finish the maze!" if @maze.index('A') == 1
  end

  def current_position
    @maze.index('A')
  end
end

def start_game!
  game = MazeGame.new
  game.invoke_case('show')
  while (input = gets.chomp) != 'quit'
    if game.may_invoke_case?(input)
      game.invoke_case(input)
    else
      puts "invalid action: #{input}"
    end
  end
end

start_game!

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake test to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/khiav223577/case_register. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.

License

The gem is available as open source under the terms of the MIT License.

case_register

Development

CaseRegister

Supports

Table of contents

Installation

Usage

Define your cases

invoke the case

Check available cases

Use cases

Refactor case statements

Fallthrough

Code Reuse

Isolate methods

Check invalid cases

Command-line Games

Development

Contributing

License