Cuprum

An opinionated implementation of the Command pattern for Ruby applications. Cuprum wraps your business logic in a consistent, object-oriented interface and features status and error management, composability and control flow management.

It defines the following concepts:

Commands - A function-like object that responds to #call and returns a Result.
Operations - A stateful Command that wraps and delegates to its most recent Result.
Results - A data object with a #value, an #errors object, and #success? and #failure? status methods.

About

Traditional frameworks such as Rails focus on the objects of your application - the "nouns" such as User, Post, or Item. Using Cuprum or a similar library allows you the developer to make your business logic - the "verbs" such as Create User, Update Post or Ship Item - a first-class citizen of your project. This provides several advantages:

Consistency: Use the same Commands to underlie controller actions, worker processes and test factories.
Encapsulation: Each Command is defined and run in isolation, and dependencies must be explicitly provided to the command when it is initialized or run. This makes it easier to reason about the command's behavior and keep it insulated from changes elsewhere in the code.
Testability: Because the logic is extracted from unnecessary context, testing its behavior is much cleaner and easier.
Composability: Complex logic such as "find the object with this ID, update it with these attributes, and log the transaction to the reporting service" can be extracted into a series of simple Commands and composed together. The Chaining feature allows for complex control flows.
Reusability: Logic common to multiple data models or instances in your code, such as "persist an object to the database" or "find all records with a given user and created in a date range" can be refactored into parameterized commands.

Alternatives

If you want to extract your logic but Cuprum is not the right solution for you, here are several alternatives:

Service objects. A common pattern used when first refactoring an application that has outgrown its abstractions. Service objects are simple and let you group related functionality, but they are harder to compose and require firm conventions to tame.
The Interactor gem. Provides an Action module to implement logic and an Organizer module to manage control flow. Supports before, around, and after hooks.
The Waterfall gem. Focused more on control flow.
Trailblazer Operations. A pipeline-based approach to control flow, and can integrate tightly with other Trailblazer elements.

Compatibility

Cuprum is tested against Ruby (MRI) 2.3 through 2.5.

Documentation

Method and class documentation is available courtesy of RubyDoc.

Documentation is generated using YARD, and can be generated locally using the yard gem.

License

Cuprum is released under the MIT License.

Contribute

The canonical repository for this gem is located at https://github.com/sleepingkingstudios/cuprum.

To report a bug or submit a feature request, please use the Issue Tracker.

To contribute code, please fork the repository, make the desired updates, and then provide a Pull Request. Pull requests must include appropriate tests for consideration, and all code must be properly formatted.

Credits

Hi, I'm Rob Smith, a Ruby Engineer and the developer of this library. I use these tools every day, but they're not just written for me. If you find this project helpful in your own work, or if you have any questions, suggestions or critiques, please feel free to get in touch! I can be reached on GitHub or via email. I look forward to hearing from you!

Concepts

Commands

require 'cuprum'

Commands are the core feature of Cuprum. In a nutshell, each Cuprum::Command is a functional object that encapsulates a business logic operation. A Command provides a consistent interface and tracking of result value and status. This minimizes boilerplate and allows for interchangeability between different implementations or strategies for managing your data and processes.

Each Command implements a #call method that wraps your defined business logic and returns an instance of Cuprum::Result. The result wraps the returned data (with the #value method), any #errors generated when running the Command, and the overall status with the #success? and #failure methods. For more details about Cuprum::Result, see below.

Class Documentation

Defining Commands

The recommended way to define commands is to create a subclass of Cuprum::Command and override the #process method.

class BuildBookCommand < Cuprum::Command
  def process attributes
    Book.new(attributes)
  end # method process
end # class

command = BuildPostCommand.new
result  = command.call(:title => 'The Hobbit')
result.class #=> Cuprum::Result

book = result.value
book.class #=> Book
book.title #=> 'The Hobbit'

There are several takeaways from this example. First, we are defining a custom command class that inherits from Cuprum::Command. We are defining the #process method, which takes a single attributes parameter and returns an instance of Book. Then, we are creating an instance of the command, and invoking the #call method with an attributes hash. These attributes are passed to our #process implementation. Invoking #call returns a result, and the #value of the result is our new Book.

Because a command is just a Ruby object, we can also pass values to the constructor.

class SaveBookCommand < Cuprum::Command
  def initialize repository
    @repository = repository
  end # constructor

  def process book
    @repository.persist(book)
  end # method process
end # class

books = [
  Book.new(:title => 'The Fellowship of the Ring'),
  Book.new(:title => 'The Two Towers'),
  Book.new(:title => 'The Return of the King')
]
command = SaveBookCommand.new(books_repository)
books.each { |book| command.call(book) }

Here, we are reusing the same command three times, rather than creating a new save command for each book. Each book is persisted to the books_repository. This is also an example of how using commands can simplify code - notice that nothing about the SaveBookCommand is specific to the Book model. Thus, we could refactor this into a generic SaveModelCommand.

A command can also be defined by passing block to Cuprum::Command.new.

increment_command = Cuprum::Command.new { |int| int + 1 }

increment_command.call(2).value #=> 3

If the command is wrapping a method on the receiver, the syntax is even simpler:

inspect_command = Cuprum::Command.new { |obj| obj.inspect }
inspect_command = Cuprum::Command.new(&:inspect) # Equivalent to above.

Commands defined using Cuprum::Command.new are quick to use, but more difficult to read and to reuse. Defining your own command class is recommended if a command definition takes up more than one line, or if the command will be used in more than one place.

Result Values

Calling the #call method on a Cuprum::Command instance will always return an instance of Cuprum::Result. The result's #value property is determined by the object returned by the #process method (if the command is defined as a class) or the block (if the command is defined by passing a block to Cuprum::Command.new).

The #value depends on whether or not the returned object is a result or is compatible with the result interface. Specifically, any object that responds to the methods #to_result, #value, and #success? is considered to be a result.

If the object returned is not a result, then the #value of the returned result is set to the object.

command = Cuprum::Command.new { 'Greetings, programs!' }
result  = command.call
result.class #=> Cuprum::Result
result.value #=> 'Greetings, programs!'

If the object returned is the command's own result object, then the #value of the returned result is unchanged. For convenience, methods to set the result status or mark the result as halted will return the result.

command = Cuprum::Command.new { result.failure! }
result  = command.call
result.class    #=> Cuprum::Result
result.value    #=> nil
result.success? #=> false

If the object returned is another result or compatible object, then #call will call the #to_result method on the result and return the resulting object.

command = Cuprum::Command.new { |value| Cuprum::Result.new(value) }
result  = command.call('Greetings, starfighter!')
result.class #=> Cuprum::Result
result.value #=> 'Greetings, starfighter!'

In some cases, returning a result directly will discard information on the command's own result object. When this occurs, Cuprum will display a warning.

command =
  Cuprum::Command.new do
    result.errors << 'Oops! We are throwing away this result.'

    Cuprum::Result.new
  end

#=> This calls Kernel#warn with a warning message.
result = command.call
result.class    #=> Cuprum::Result
result.value    #=> nil
result.success? #=> true
result.errors   #=> []

Success, Failure, and Errors

Whether defined with a block or in the #process method, the Command implementation can access an #errors object while in the #call method. Any errors added to the errors object will be exposed by the #errors method on the result object.

class PublishBookCommand < Cuprum::Command
  private

  def process book
    if book.cover.nil?
      errors << 'This book does not have a cover.'

      return
    end # if

    book.published = true

    book
  end # method process
end # class

In addition, the result object defines #success? and #failure? predicates. If the result has no errors, then #success? will return true and #failure? will return false.

book = Book.new(:title => 'The Silmarillion', :cover => Cover.new)
book.published? #=> false

result = PublishBookCommand.new.call(book)
result.errors   #=> []
result.success? #=> true
result.failure? #=> false
result.value    #=> book
book.published? #=> true

If the result does have errors, #success? will return false and #failure? will return true.

book = Book.new(:title => 'The Silmarillion', :cover => nil)
book.published? #=> false

result = PublishBookCommand.new.call(book)
result.errors   #=> ['This book does not have a cover.']
result.success? #=> false
result.failure? #=> true
result.value    #=> book
book.published? #=> false

Composing Commands

Because Cuprum::Command instances are proper objects, they can be composed like any other object. For example, we could define some basic mathematical operations by composing commands:

increment_command = Cuprum::Command.new { |i| i + 1 }
increment_command.call(1).value #=> 2
increment_command.call(2).value #=> 3
increment_command.call(3).value #=> 4

add_command = Cuprum::Command.new do |addend, i|
  addend.times { i = increment_command(i).value }

  i
end # command

add_command.call(1, 1).value #=> 2
add_command.call(1, 2).value #=> 3
add_command.call(2, 1).value #=> 3
add_command.call(2, 2).value #=> 4

This can also be done using command classes.

class IncrementCommand < Cuprum::Command
  private

  def process i
    i + 1
  end
end

class AddCommand < Cuprum::Command
  def initialize addend
    @addend = addend
  end

  private

  def process i
    addend.times { i = IncrementCommand.new.call(i).value }

    i
  end
end

add_two_command = AddCommand.new(2)
add_two_command.call(0).value #=> 2
add_two_command.call(1).value #=> 3
add_two_command.call(8).value #=> 10

Chaining Commands

Cuprum::Command also defines methods for chaining commands together. When a chain of commands is called, each command in the chain is called in sequence and passed the value of the previous command. The result of the last command in the chain is returned from the chained call.

name_command       = Cuprum::Command.new { |klass| klass.name }
pluralize_command  = Cuprum::Command.new { |str| str.pluralize }
underscore_command = Cuprum::Command.new { |str| str.underscore }

table_name_command =
  name_command
    .chain(pluralize_command)
    .chain(underscore_command)

result = table_name_command.call(LibraryCard)
result.class #=> Cuprum::Result
result.value #=> 'library_cards'

When the table_name_command is called, the class (in our case LibraryCard) is passed to the first command in the chain, which is the name_command. This produces a Result with a value of 'LibraryCard'. This value is then passed to pluralize_command, which returns a Result with a value of 'LibraryCards'. Finally, underscore_command is called and returns a Result with a value of 'library_cards'. Since there are no more commands in the chain, this final result is then returned.

Chained commands can also be defined with a block. This creates an anonymous command, equivalent to Cuprum::Command.new {}. Thus, the table_name_command could have been defined as either of these:

table_name_command =
  Cuprum::Command.new { |klass| klass.name }
    .chain { |str| str.pluralize }
    .chain { |str| str.underscore }

table_name_command =
  Cuprum::Command.new(&:name).chain(&:pluralize).chain(&:underscore)

Chaining Details

The #chain method is defined for instances of Cuprum::Command (or object that extend Cuprum::Chaining). Calling #chain on a command will always create a copy of the command. The given command is then added to the command chain for the copied command. The original command is unchanged. Thus:

first_command = Cuprum::Command.new { puts 'First command!' }
first_command.call #=> Outputs 'First command!' to STDOUT.

second_command = first_command.chain { puts 'Second command!' }
second_command.call #=> Outputs 'First command!' then 'Second command!'.

# The original command is unchanged.
first_command.call #=> Outputs 'First command!' to STDOUT.

When a chained command is called, the original command is called with whatever parameters are passed in to the #call method, and the command executes the #process method as normal, generating a Cuprum::Result and assigning it a value and optionally errors or a status. Rather than returning this result, however, it is passed on to the next command in the chain. In the context of a chained command, this means two things.

First, the next command is called. If the next command does not take any arguments, it is not passed any arguments. If the next command takes one or more arguments, it is passed the #value of that previous result.

double_command    = Cuprum::Command.new { |i| 2 * i }
increment_command = Cuprum::Command.new { |i| 1 + i }
square_command    = Cuprum::Command.new { |i| i * i }
chained_command   =
  double_command
    .chain(increment_command)
    .chain(square_command)

# First, the double_commmand is called with 2. This returns a Cuprum::Result
# with a value of 4.
#
# Next, the increment_command is called with 4, returning a result with value 5.
#
# Finally, the square_command is called with 5, returning a result with a value
# of 25. This final result is returned by #call.
result = chained_command.call(2)
result.class #=> Cuprum::Result
result.value #=> 25

Second, the previous result is set as the result of the next command (before it is evaluated). This makes it available to the next command during execution as the #result method, and makes available the value, errors, and status of the previous command (and avoids unnecessary object allocation). The value of the result will be updated to reflect the return value of the next command execution.

validate_command =
  Cuprum::Command.new do |object|
    result.errors << 'Object is invalid!' unless object.valid?

    object
  end
persist_command =
  Cuprum::Command.new do |object|
    object.save if result.success?

    object
  end
chained_command = validate_command.chain(persist_command)

# First, validate_command is called with a valid object. This creates a result
# with no errors and whose value is the valid object.
#
# Then, persist_command is called with the object, and its result is assigned to
# the previous result. Since there are no errors on the result, the object is
# saved. Finally, the value of the result is set to the object, and the result
# is returned.
result = chained_command.call(a_valid_object) #=> Saves the object.
result.value              #=> a_valid_object
result.errors             #=> []
a_valid_object.persisted? #=> true

# First, validate_command is called with an invalid object. This creates a
# result whose value is the invalid object, and with errors 'Object is
# invalid!'.
#
# Then, persist_command is called with the object, and its result is assigned to
# the previous result. Since the result has an error, the object is not saved.
# Finally, the value of the result is set to the object, and the result
# is returned.
result = chained_command.call(an_invalid_object) #=> Does not save the object.
result.value                 #=> an_invalid_object
result.errors                #=> ['Object is invalid!']
an_invalid_object.persisted? #=> false

Conditional Chaining

The #chain method can be passed an optional :on => value keyword. This keyword determines whether or not the chained command will execute, based on the previous result status. Possible values are :success, :failure, :always, or nil. The default value is nil.

If the :on keyword is omitted, the chained command will always be executed after the previous command unless the result is halted (see Halting A Command Chain)

If the command is chained with :on => :success, then the chained command will only execute if the previous result is passing, e.g. the #success? method returns true and the command is not halted. A result is passing if there are no errors, or if the status is set to :success.

If the command is chained with :on => :failure, then the chained command will only execute if the previous result is failing, e.g. the #success? method returns false and the command is not halted. A result is failing if the errors object is not empty, or if the status is set to :failure.

If the command is chained with :on => always, then the chained command will always be executed, even if the previous result is halted.

find_command =
  Cuprum::Command.new do |attributes|
    book = Book.where(:id => attributes[:id]).first

    result.errors << 'Book not found' unless book

    book
  end
create_command =
  Cuprum::Command.new do |attributes|
    book = Book.new(attributes)

    if book.save
      result.success!
    else
      book.errors.full_messages.each { |message| result.errors << message }
    end

    book
  end

find_or_create_command = find_command.chain(create_command, :on => :failure)

# With a book that exists in the database, the find_command is called and
# returns a result with no errors and a value of the found book. The
# create_command is not called.
hsh    = { id: 0, title: 'Journey to the West' }
result = find_or_create_command.call(hsh)
book   = result.value
book.id         #=> 0
book.title      #=> 'Journey to the West'
result.success? #=> true
result.errors   #=> []

# With a book that does not exist but with valid attributes, the find command
# returns a failing result with a value of nil. The create_command is called and
# creates a new book with the attributes, returning a passing result but
# preserving the errors.
hsh    = { id: 1, title: 'The Ramayana' }
result = find_or_create_command.call(hsh)
book   = result.value
book.id         #=> 1
book.title      #=> 'The Ramayana'
result.success? #=> true
result.errors   #=> ['Book not found']

# With a book that does not exist and with invalid attributes, the find command
# returns a failing result with a value of nil. The create_command is called and
# is unable to create a new book with the attributes, returning the
# (non-persisted) book and adding the validation errors.
hsh    = { id: 2, title: nil }
result = find_or_create_command.call(hsh)
book   = result.value
book.id         #=> 2
book.title      #=> nil
result.success? #=> false
result.errors   #=> ['Book not found', "Title can't be blank"]

The #success method can be used as shorthand for chain(command, :on => :success). Likewise, the #failure method can be used in place of chain(command, :on => :failure).

Halting A Command Chain

If the #halt method is called as part of a Command block or #process method, the command chain is halted. Any subsequent chained commands will not be called unless they were chained with the :on => :always option. This allows you to terminate a Command chain early without having to raise and rescue an exception.

double_command  = Cuprum::Command.new { |i| 2 * i }
halt_command    = Cuprum::Command.new { |value| value.tap { result.halt! } }
add_one_command = Cuprum::Command.new { |i| i + 1 }

chained_command =
  double_command
    .chain(halt_command)
    .chain(add_one_command)
    .chain(:on => :always) { |count| "There are #{count} lights!" }

# First, double_command is called with 2, returning a result with no errors and
# a value of 4. Then, halt_command is called, which marks the result as halted.
# Because the result is now halted, the add_one_command is not called. Finally,
# the last command is called (even though the result is halted, the command is
# chained :on => :always), which returns a result with the string value. The
# result is passing, but is still halted.
result = chained_command.call(2)
result.value    #=> 'There are 4 lights!'
result.success? #=> true
result.halted?  #=> true

Advanced Chaining

The #tap_result and #yield_result methods provide advanced control over the flow of chained commands.

Tap Result

The #tap_result method allows you to insert arbitrary code into a command chain without affecting later commands. The method takes a block and yields the previous result, which is then returned and passed to the next command, or returned by #call if #tap_result is the last item in the chain.

command =
  Cuprum::Command.new do
    result.errors << 'Example error'

    'Example value'
  end
chained_command =
  command
    .tap_result do |result|
      puts "The result value was #{result.inspect}"
    end

# Prints 'The result value was "Example value"' to STDOUT.
result = chained_command.call
result.class    #=> Cuprum::Result
result.value    #=> 'Example value'
result.success? #=> false
result.errors   #=> ['Example error']

Like #chain, #tap_result can be given an :on => value keyword.

find_book_command =
  Cuprum::Command.new do |book_id|
    book = Book.where(:id => book_id).first

    result.errors << "Unable to find book with id #{book_id}" unless book

    book
  end

chained_command =
  find_book_command
    .tap_result(:on => :success) do |result|
      render :show, :locals => { :book => result.value }
    end
    .tap_result(:on => :failure) do
      redirect_to books_path
    end

# Calls find_book_command with the id, which queries for the book and returns a
# result with a value of the book and no errors. Then, the first tap_result
# block is evaluated, calling the render method and passing it the book via the
# result.value method. Because the result is passing, the next block is skipped.
# Finally, the result of find_book_command is returned unchanged.
result = chained_command.call(valid_id)
result.class    #=> Cuprum::Result
result.value    #=> an instance of Book
result.success? #=> true
result.errors   #=> []

# Calls find_book_command with the id, which queries for the book and returns a
# result with a value of nil and the error message. Because the result is
# failing, the first block is skipped. The second block is then evaluated,
# calling the redirect_to method. Finally, the result of find_book_command is
# returned unchanged.
result = chained_command.call(invalid_id)
result.class    #=> Cuprum::Result
result.value    #=> nil
result.success? #=> false
result.errors   #=> ['Unable to find book with id invalid_id']

Yield Result

The #yield_result method offers advanced control over a chained command step. The method takes a block and yields the previous result. If the object returned by the block is not a result, a new result is created with a value equal to the returned object. In either case, the result is returned and passed to the next command, or returned by #call if #tap_result is the last item in the chain.

Unlike #chain, the block in #yield_result yields the previous result, not the previous value. In addition, #yield_result does not automatically carry over any errors from the previous result, the result status, or whether the result was marked as halted.

chained_command =
  Cuprum::Command.new do
    'Example value'
  end
  .yield_result do |result|
    result.errors << 'Example error'

    result
  end
  .yield_result do |result|
    "The last result was a #{result.success? ? 'success' : 'failure'}."
  end

# The first command creates a passing result with a value of 'Example value'.
#
# This is passed to the first yield_result block, which adds the 'Example error'
# string to the result errors, and then returns the result. Because the
# yield_result block returns the previous result, it is then passed to the next
# item in the chain.
#
# Finally, the second yield_result block is called, which checks the status of
# the passed result. Since the second block does not return the previous result,
# the previous result is discarded and a new result is created with the string
# value starting with 'The last result was ...'.
result = chained_command.call
result.class    #=> Cuprum::Result
result.value    #=> 'The last result was a failure.'
result.success? #=> true
result.errors   #=> []

Like #chain, #yield_result can be given an :on => value keyword.

# The Collatz conjecture in mathematics concerns a sequence of numbers defined
# as follows. Start with any positive integer. If the number is even, then
# divide the number by two. If the number is odd, multiply by three and add one.
# These steps are repeated until the value is one or the numbers loop.
step_command =
  Cuprum::Command.new do |i|
    result.failure! unless i.even?

    i
  end
    .yield_result(:on => :success) { |result| result.value / 2 }
    .yield_result(:on => :failure) { |result| 1 + 3 * result.value }

# Because the value is even, the first command returns a passing result with a
# value of 8. The first yield_result block is then called with that result,
# returning a new result with a value of 4. The result is passing, so the second
# yield_result block is skipped and the result is returned.
result = collatz_command.call(8)
result.value    #=> 4
result.success? #=> true

# Because the value is odd, the first command returns a failing result with a
# value of 5. Because the result is failing, the first yield_result block is
# skipped. The second yield_result block is then called with the result,
# returning a new (passing) result with a value of 16.
result = collatz_command.call(5)
result.value    #=> 16
result.success? #=> true

Under the hood, both #chain and #tap_result are implemented on top of #yield_result.

Protected Chaining Methods

Each Command also defines the #chain!, #tap_result!, and #yield_result! methods - note the imperative !. These methods behave identically to their non-imperative counterparts, but they modify the current command directly instead of creating a clone. They are also protected methods, so they cannot be called from outside the command itself. These methods are designed for use when defining commands.

# We subclass the build command, which will be executed first.
class CreateCommentCommand < BuildCommentCommand
  include Cuprum::Chaining
  include Cuprum::Processing  #

  def initialize    # After the build step is run, we validate the comment.

    chain!(ValidateCommentCommand.new)  #
    # If the validation passes, we then save the comment.

    chain!(SaveCommentCommand.new, on: :success)
  end
end

Comment.count #=> 0

body   = 'Why do hot dogs come in packages of ten, and hot dog buns come in ' \
         'packages of eight?'
result = CreateCommentCommand.new.call({ user_id: '12345', body: body })

result.value    #=> an instance of Comment with the given user_id and body.
result.success? #=> true
Comment.count   #=> 1; the comment was added to the database

result = CreateCommentCommand.new.call({ user_id: nil, body: body })

result.value    #=> an instance of Comment with the given user_id and body.
result.success? #=> false
result.errors   #=> ["User id can't be blank"]
Comment.count   #=> 1; the comment was not added to the database

Results

require 'cuprum'

Class Documentation

A Cuprum::Result is a data object that encapsulates the result of calling a Cuprum command. Each result has a #value, an #errors object (defaults to an Array), and status methods #success?, #failure?, and #halted?.

value  = 'A result value'.freeze
result = Cuprum::Result.new(value)

result.value    #=> 'A result value'
result.errors   #=> []
result.success? #=> true
result.failure? #=> false
result.halted?  #=> false

Adding errors to the #errors object will change the status of the result.

result.errors << "I'm sorry, something went wrong."
result.success? #=> false
result.failure? #=> true

The status can also be overriden with the #success! and #failure! methods, which will set the status regardless of the presence or absence of errors.

result.success!
result.errors   #=> ["I'm sorry, something went wrong."]
result.success? #=> true
result.failure? #=> false

Operations

require 'cuprum'

Class Documentation

An Operation is like a Command, but with two key differences. First, an Operation retains a reference to the result object from the most recent time the operation was called, and delegates the methods defined by Cuprum::Result to the most recent result. This allows a called Operation to replace a Cuprum::Result in any code that expects or returns a result. Second, the #call method returns the operation instance, rather than the result itself.

These two features allow developers to simplify logic around calling and using the results of operations, and reduce the need for boilerplate code (particularly when using an operation as part of an existing framework, such as inside of an asynchronous worker or a Rails controller action).

class CreateBookOperation < Cuprum::Operation
  def process    # Implementation here.

  end # method process
end # class

# Defining a controller action using an operation.
def create
  operation = CreateBookOperation.new.call(book_params)

  if operation.success?
    redirect_to(operation.value)
  else
    @book = operation.value

    render :new
  end # if-else
end # create

Like a Command, an Operation can be defined directly by passing an implementation block to the constructor or by creating a subclass that overwrites the #process method.

An operation inherits the #call method from Cuprum::Command (see above), and delegates the #value, #errors, #success?, and #failure methods to the most recent result. If the operation has not been called, these methods will return default values.

The Operation Mixin

Module Documentation

The implementation of Cuprum::Operation is defined by the Cuprum::Operation::Mixin module, which provides the methods defined above. Any command class or instance can be converted to an operation by including (for a class) or extending (for an instance) the operation mixin.

Finally, the result can be halted using the #halt method, which indicates that further chained commands should not be executed.

result.halt!
result.halted? #=> true

Command Factories

Class Documentation

Commands are powerful and flexible objects, but they do have a few disadvantages compared to traditional service objects which allow the developer to group together related functionality and shared implementation details. To bridge this gap, Cuprum implements the CommandFactory class. Command factories provide a DSL to quickly group together related commands and create context-specific command classes or instances.

For example, consider a basic entity command:

class Book
  def initialize(attributes = {})
    @title  = attributes[:title]
    @author = attributes[:author]
  end

  attr_accessor :author, :publisher, :title
end

class BuildBookCommand < Cuprum::Command
  private

  def process(attributes = {})
    Book.new(attributes)
  end
end

class BookFactory < Cuprum::CommandFactory
  command :build, BuildBookCommand
end

Our factory is defined by subclassing Cuprum::CommandFactory, and then we map the individual commands with the ::command or ::command_class class methods. In this case, we've defined a Book factory with the build command. The build command can be accessed on a factory instance in one of two ways.

First, the command class can be accessed directly as a constant on the factory instance.

factory = BookFactory.new
factory::Build #=> BuildBookCommand

Second, the factory instance now defines a #build method, which returns an instance of our defined command class. This command instance can be called like any command, or returned or passed around like any other object.

factory = BookFactory.new

attrs   = { title: 'A Wizard of Earthsea', author: 'Ursula K. Le Guin' }
command = factory.build()     #=> an instance of BuildBookCommand
result  = command.call(attrs) #=> an instance of Cuprum::Result
book    = result.value        #=> an instance of Book

book.title     #=> 'A Wizard of Earthsea'
book.author    #=> 'Ursula K. Le Guin'
book.publisher #=> nil

The ::command Method And A Command Class

The first way to define a command for a factory is by calling the ::command method and passing it the name of the command and a command class:

class BookFactory < Cuprum::CommandFactory
  command :build, BuildBookCommand
end

This makes the command class available on a factory instance as ::Build, and generates the #build method which returns an instance of BuildBookCommand.

The ::command Method And A Block

By calling the ::command method with a block, you can define a command with additional control over how the generated command. The block must return an instance of a subclass of Cuprum::Command.

class PublishBookCommand < Cuprum::Command
  def initialize(publisher:)
    @publisher = publisher
  end

  attr_reader :publisher

  private

  def process(book)
    book.publisher = publisher

    book
  end
end

class BookFactory < Cuprum::CommandFactory
  command :publish do |publisher|
    PublishBookCommand.new(publisher: publisher)
  end
end

This defines the #publish method on an instance of the factory. The method takes one argument (the publisher), which is then passed on to the constructor for PublishBookCommand by our block. Finally, the block returns an instance of the publish command, which is then returned by #publish.

factory = BookFactory.new
book    = Book.new(title: 'The Tombs of Atuan', author: 'Ursula K. Le Guin')
book.publisher #=> nil

command = factory.publish('Harper & Row') #=> an instance of PublishBookCommand
result  = command.call(book)              #=> an instance of Cuprum::Result
book.publisher #=> 'Harper & Row'

Note that unlike when ::command is called with a command class, calling ::command with a block will not set a constant on the factory instance. In this case, trying to access the PublishBookCommand at factory::Publish will raise a NameError.

The block is evaluated in the context of the factory instance. This means that instance variables or methods are available to the block, allowing you to create commands with instance-specific configuration.

class PublishedBooksCommand < Cuprum::Command
  def initialize(collection = [])
    @collection = collection
  end

  attr_reader :collection

  private

  def process
    books.reject { |book| book.publisher.nil? }
  end
end

class BookFactory < Cuprum::CommandFactory
  command :published do
    PublishedBooksCommand.new(books_collection)
  end

  def initialize(books)
    @books_collection = books
  end

  attr_reader :books_collection
end

This defines the #published method on an instance of the factory. The method takes no arguments, but grabs the books collection from the factory instance. The block returns an instance of PublishedBooksCommand, which is then returned by #published.

books   = [Book.new, Book.new(publisher: 'Baen'), Book.new(publisher: 'Tor')]
factory = BookFactory.new(books)
factory.books_collection #=> the books array

command = factory.published #=> an instance of PublishedBooksCommand
result  = command.call      #=> an instance of Cuprum::Result
ary     = result.value      #=> an array with the published books

ary.count                                    #=> 2
ary.any? { |book| book.publisher == 'Baen' } #=> true
ary.any? { |book| book.publisher.nil? }      #=> false

Simple commands can be defined directly in the block, rather than referencing an existing command class:

class BookFactory < Cuprum::CommandFactory
  command :published_by_baen do
    Cuprum::Command.new do |books|
      books.select { |book| book.publisher == 'Baen' }
    end
  end
end

books   = [Book.new, Book.new(publisher: 'Baen'), Book.new(publisher: 'Tor')]
factory = BookFactory.new(books)

command = factory.published_by_baen #=> an instance of the anonymous command
result  = command.call              #=> an instance of Cuprum::Result
ary     = result.value              #=> an array with the selected books

ary.count #=> 1

The ::command_class Method

The final way to define a command for a factory is calling the ::command_class method with the command name and a block. The block must return a subclass (not an instance) of Cuprum::Command. This offers a balance between flexibility and power.

class SelectByAuthorCommand < Cuprum::Command
  def initialize(author)
    @author = author
  end

  attr_reader :author

  private

  def process(books)
    books.select { |book| book.author == author }
  end
end

class BooksFactory < Cuprum::CommandFactory
  command_class :select_by_author do
    SelectByAuthorCommand
  end
end

The command class can be accessed directly as a constant on the factory instance:

factory = BookFactory.new
factory::SelectByAuthor #=> SelectByAuthorCommand

The factory instance now defines a #select_by_author method, which returns an instance of our defined command class. This command instance can be called like any command, or returned or passed around like any other object.

factory = BookFactory.new
books   = [
  Book.new,
  Book.new(author: 'Arthur C. Clarke'),
  Book.new(author: 'Ursula K. Le Guin')
]

command = factory.select_by_author('Ursula K. Le Guin')#=> an instance of SelectByAuthorCommand

command.author #=> 'Ursula K. Le Guin'

result = command.call(books)      #=> an instance of Cuprum::Result
ary    = result.value             #=> an array with the selected books

ary.count                                              #=> 1
ary.any? { |book| book.author == 'Ursula K. Le Guin' } #=> true
ary.any? { |book| book.author == 'Arthur C. Clarke'  } #=> false
ary.any? { |book| book.author.nil? }                   #=> false

The block is evaluated in the context of the factory instance. This means that instance variables or methods are available to the block, allowing you to create custom command subclasses with instance-specific configuration.

class SaveBookCommand < Cuprum::Command
  def initialize(collection = [])
    @collection = collection
  end

  attr_reader :collection

  private

  def process(book)
    books << book

    book
  end
end

class BookFactory < Cuprum::CommandFactory
  command :save do
    collection = self.books_collection

    Class.new(SaveBookCommand) do
      define_method(:initialize) do
        @books = collection
      end
    end
  end

  def initialize(books)
    @books_collection = books
  end

  attr_reader :books_collection
end

The custom command subclass can be accessed directly as a constant on the factory instance:

books   = [Book.new, Book.new, Book.new]
factory = BookFactory.new(books)
factory::Save #=> a subclass of SaveBookCommand

command = factory::Save.new # an instance of the command subclass
command.collection          #=> the books array
command.collection.count    #=> 3

The factory instance now defines a #save method, which returns an instance of our custom command subclass. This command instance can be called like any command, or returned or passed around like any other object.

The custom command subclass can be accessed directly as a constant on the factory instance:

books   = [Book.new, Book.new, Book.new]
factory = BookFactory.new(books)
command = factory.save   # an instance of the command subclass
command.collection       #=> the books array
command.collection.count #=> 3

book   = Book.new(title: 'The Farthest Shore', author: 'Ursula K. Le Guin')
result = command.call(book) #=> an instance of Cuprum::Result

books.count          #=> 4
books.include?(book) #=> true

Built In Commands

Cuprum includes a small number of predefined commands and their equivalent operations.

IdentityCommand

require 'cuprum/built_in/identity_command'

Class Documentation

A pregenerated command that returns the value or result with which it was called.

command = Cuprum::BuiltIn::IdentityCommand.new
result  = command.call('expected value')
result.value    #=> 'expected value'
result.success? #=> true

IdentityOperation

require 'cuprum/built_in/identity_operation'

Class Documentation

A pregenerated operation that sets its result to the value or result with which it was called.

operation = Cuprum::BuiltIn::IdentityOperation.new.call('expected value')
operation.value    #=> 'expected value'
operation.success? #=> true

NullCommand

require 'cuprum/built_in/null_command'

Class Documentation

A pregenerated command that does nothing when called. Accepts any arguments.

command = Cuprum::BuiltIn::NullCommand.new
result  = command.call
result.value    #=> nil
result.success? #=> true

NullOperation

require 'cuprum/built_in/null_operation'

Class Documentation

A pregenerated operation that does nothing when called. Accepts any arguments.

operation = Cuprum::BuiltIn::NullOperation.new.call
operation.value    #=> nil
operation.success? #=> true

Reference

Cuprum::BuiltIn::IdentityCommand

require 'cuprum/built_in/identity_command'

Class Documentation

Cuprum::BuiltIn::IdentityCommand defines the following methods:

`#call`

call(value) #=> Cuprum::Result

Returns a result, whose #value is equal to the given value.

Cuprum::BuiltIn::IdentityOperation

require 'cuprum/built_in/identity_operation'

Class Documentation

Cuprum::BuiltIn::IdentityOperation defines the following methods:

`#call`

call(value) #=> Cuprum::BuiltIn::IdentityOperation

Sets the last result to a new result, whose #value is equal to the given value.

Cuprum::BuiltIn::NullCommand

require 'cuprum/built_in/null_command'

Class Documentation

Cuprum::BuiltIn::NullCommand defines the following methods:

`#call`

call(*args, **keywords) { ... } #=> Cuprum::Result

Returns a result with nil value. Any arguments or keywords are ignored.

Cuprum::BuiltIn::NullOperation

require 'cuprum/built_in/null_operation'

Class Documentation

Cuprum::BuiltIn::NullOperation defines the following methods:

`#call`

call(*args, **keywords) { ... } #=> Cuprum::BuiltIn::NullOperation

Sets the last result to a result with nil value. Any arguments or keywords are ignored.

Cuprum::Command

require 'cuprum'

Class Documentation

A Cuprum::Command defines the following methods:

`#initialize`

initialize { |*arguments, **keywords, &block| ... } #=> Cuprum::Command

Returns a new instance of Cuprum::Command. If a block is given, the #call method will wrap the block and set the result #value to the return value of the block. This overrides the implementation in #process, if any.

Method Documentation

`#build_errors`

(Private Method)

build_errors() #=> Array

Generates an empty errors object. When the command is called, the result will have its #errors property initialized to the value returned by #build_errors. By default, this is an array. If you want to use a custom errors object type, override this method in a subclass.

Method Documentation

`#call`

call(*arguments, **keywords) { ... } #=> Cuprum::Result

Executes the logic encoded in the constructor block, or the #process method if no block was passed to the constructor.

Method Documentation

`#chain`

chain(on: nil) { |result| ... } #=> Cuprum::Command

Registers a command or block to run after the current command, or after the last chained command if the current command already has one or more chained command(s). This creates and modifies a copy of the current command. See Chaining Commands, below.

chain(command, on: nil) #=> Cuprum::Command

The command will be passed the #value of the previous command result as its parameter, and the result of the chained command will be returned (or passed to the next chained command, if any).

The block will be passed the #result of the previous command as its parameter. If your use case depends on the status of the previous command or on any errors generated, use the block form of #chain.

If the block returns a Cuprum::Result (or an object responding to #value and #success?), the block result will be returned (or passed to the next chained command, if any). If the block returns any other value (including nil), the #result of the previous command will be returned or passed to the next command.

Method Documentation

`#chain!`

(Protected Method)

chain!(on: nil) { |result| ... } #=> Cuprum::Command

chain!(command, on: nil) #=> Cuprum::Command

As #chain, but modifies the current command instead of creating a clone.

Method Documentation

`#else`

else(command) #=> Cuprum::Command

Shorthand for command.chain(:on => :failure). Registers a command or block to run after the current command. The chained command will only run if the previous command was unsuccessfully run.

The command will be passed the #value of the previous command result as its parameter, and the result of the chained command will be returned (or passed to the next chained command, if any).

else() { |result| ... } #=> Cuprum::Command

The block will be passed the #result of the previous command as its parameter. If your use case depends on the status of the previous command or on any errors generated, use the block form of #chain.

Method Documentation

`#tap_result`

tap_result(on: nil) { |previous_result| } #=> Cuprum::Result

Creates a copy of the command, and then chains the block to execute after the command implementation. When #call is executed, each chained block will be yielded the previous result, and the previous result returned or yielded to the next block. The return value of the block is discarded.

If the on parameter is omitted, the block will be called if the last result is not halted. If the on parameter is set to :success, the block will be called if the last result is successful and not halted. If the on parameter is set to :failure, the block will be called if the last result is failing and not halted. Finally, if the on parameter is set to :always, the block will always be called, even if the last result is halted.

Method Documentation

`#tap_result!`

(Protected Method)

tap_result!(on: nil) { |previous_result| } #=> Cuprum::Result

As #tap_result, but modifies the current command instead of creating a clone.

Method Documentation

`#then`

then(command) #=> Cuprum::Command

Shorthand for command.chain(:on => :success). Registers a command or block to run after the current command. The chained command will only run if the previous command was successfully run.

The command will be passed the #value of the previous command result as its parameter, and the result of the chained command will be returned (or passed to the next chained command, if any).

then() { |result| ... } #=> Cuprum::Command

The block will be passed the #result of the previous command as its parameter. If your use case depends on the status of the previous command or on any errors generated, use the block form of #chain.

Method Documentation

`#yield_result`

yield_result(on: nil) { |previous_result| } #=> Cuprum::Result

Creates a copy of the command, and then chains the block to execute after the command implementation. When #call is executed, each chained block will be yielded the previous result, and the return value wrapped in a result and returned or yielded to the next block.

Method Documentation

`#yield_result!`

(Protected Method)

yield_result!(on: nil) { |previous_result| } #=> Cuprum::Result

As #yield_result, but modifies the current command instead of creating a clone.

Method Documentation

Cuprum::Operation

require 'cuprum'

Class Documentation

A Cuprum::Operation inherits the methods from Cuprum::Command (see above), and defines the following additional methods:

`#called?`

called?() #=> true, false

True if the operation has been called and there is a result available by calling #result or one of the delegated methods, otherwise false.

Method Documentation

`#reset!`

reset!()

Clears the most recent result and resets #called? to false. This frees the result and any linked data for garbage collection. It also clears any internal state from the operation.

Method Documentation

`#result`

result() #=> Cuprum::Result

The most recent result, from the previous time #call was executed for the operation.

Method Documentation

Cuprum::Result

Class Documentation

A Cuprum::Result defines the following methods:

`#==`

==(other) #=> true, false

Performs a fuzzy comparison with the other object. At a minimum, the other object must respond to #value and #success?, and the values of other.value and other.success? must be equal to the corresponding value on the result. In addition, if the #failure?, #errors, or #halted? methods are defined on the other object, then the value of each defined method is compared to the value on the result. Returns true if all values match, otherwise returns false.

`#empty?`

empty?() #=> true, false

Helper method that returns true for a new result. The method returns false if result.value is not nil, if result.errors is not empty, if the status has been manually set with #success! or #failure!, or if the result has been halted.

`#errors`

errors() #=> Array

The errors generated by the command, or an empty array if no errors were generated.

Method Documentation

`#failure!`

failure!() #=> Cuprum::Result

Marks the result as failing and returns the result. Calling #failure? will return true, even if the result has no errors.

Method Documentation

`#failure?`

failure?() #=> true, false

True if the command generated one or more errors or was marked as failing. Otherwise false.

Method Documentation

`#halt!`

halt!() #=> Cuprum::Result

Marks the result as halted and returns the result. Calling #halted? will return true.

Method Documentation

`#halted?`

halted?() #=> true, false

True if the result is halted, which prevents chained commands from executing.

`#success!`

success!() #=> Cuprum::Result

Marks the result as passing and returns the result. Calling #success? will return true, even if the result has errors.

Method Documentation

`#success?`

success?() #=> true, false

True if the command did not generate any errors, or the result has errors but was marked as passing. Otherwise false.

Method Documentation

`#value`

value() #=> Object

The value returned by the command. For example, for an increment command that added 1 to a given integer, the #value of the result object would be the incremented integer.

Method Documentation

Cuprum::Utilities::InstanceSpy

require 'cuprum/utils/instance_spy'

Class Documentation

Utility module for instrumenting calls to the #call method of any instance of a command class. This can be used to unobtrusively test the functionality of code that calls a command without providing a reference to the command instance, such as chained commands or methods that create and call a command instance.

`::clear_spies`

clear_spies() #=> nil

Retires all spies. Subsequent calls to the #call method on command instances will not be mirrored to existing spy objects. Calling this method after each test or example that uses an instance spy is recommended.

after(:example) { Cuprum::Utils::InstanceSpy.clear_spies }

Method Documentation

`::spy_on`

spy_on(command_class) #=> InstanceSpy
spy_on(command_class) { |spy| ... } #=> nil

Finds or creates a spy object for the given module or class. Each time that the #call method is called for an object of the given type, the spy's #call method will be invoked with the same arguments and block. If #spy_on is called with a block, the instance spy will be yielded to the block; otherwise, the spy will be returned.

# Observing calls to instances of a command.
spy = Cuprum::Utils::InstanceSpy.spy_on(CustomCommand)

expect(spy).to receive(:call).with(1, 2, 3, :four => '4')

CustomCommand.new.call(1, 2, 3, :four => '4')

# Observing calls to a chained command.
spy = Cuprum::Utils::InstanceSpy.spy_on(ChainedCommand)

expect(spy).to receive(:call)

Cuprum::Command.new {}.
  chain { |result| ChainedCommand.new.call(result) }.
  call

# Block syntax
Cuprum::Utils::InstanceSpy.spy_on(CustomCommand) do |spy|
  expect(spy).to receive(:call)

  CustomCommand.new.call
end # spy_on

Method Documentation