Classes and utilities for working with object factories.

BocoFactory = require 'boco-factory'
assert = require 'assert'

Provides a basic implementation of the [factory pattern] where objects are constructed based off of their type.

TypeFactory = BocoFactory.TypeFactory
factory = new TypeFactory()

Each factory should encapsulate the constructors for concrete classes it maintains. Let's create a couple simple classes:

class User
  constructor: (properties = {}) ->
    @id = properties.id
    @name = properties.name

class Post
  constructor: (properties = {}) ->
    @id = properties.id
    @title = properties.title

To register them with our factory, we call the register method with an collection of key/value pairs. The key and value represent the name for our constructor and the constructor function itself, respectively.

factory.register User: User, Post: Post

Now that we have registered some constructors, we can use our factory to create instances of those classes.

user = factory.construct 'User', name: "john.doe"
assert user instanceof User
assert.equal "john.doe", user.name

ConstructorUndefined = BocoFactory.Errors.ConstructorUndefined

If you attempt to construct an object using an unregistered name, a ConstructorUndefined error will be thrown.

shouldThrow = ->
  factory.construct 'Foo'

isCorrectError = (error) ->
  error instanceof ConstructorUndefined

assert.throws shouldThrow, isCorrectError

You may want to extend the Factory class to perform custom construction of your objects. Let's create a new Factory class that generates an identity for each object it constructs.

The decorate method is called after the object is constructed. By default, it does nothing. We can override it to set the identity on objects we construct.

class IdentifyingFactory extends TypeFactory

  generateId: ->
    @lastId = if @lastId? then @lastId + 1 else 1

  decorate: (object) ->
    object.id = @generateId() unless object.id?
    return object

Now, let's create an instance of this factory class and register our constructors.

factory = new IdentifyingFactory()
factory.register Post: Post

You can define a single constructor by passing in the typeName and constructor:

factory.define "User", User

If we construct a User and Post, they should now have an id assigned by our factory.

post = factory.construct 'Post', title: "My first post"
user = factory.construct 'User', name: "john.doe"

assert.equal 1, post.id
assert.equal 2, user.id