From: sera@...
Date: 2005-07-05T01:55:44+09:00
Subject: ANN: ContextualService 0.1.0

Here's another library that comes from Lafcadio: ContextualService.

The ContextualService library makes it easy to manage and set services
to a single, global resource such as a database or file system. By
defining a service to be a child of ContextualService::Service, you
* lock off access to instantiation, so all access have to go through
  the service's get method,
* and provide a simple way to set the instance, most likely for
  testing.

For example, Lafcadio ( http://lafcadio.rubyforge.org ) uses
ContextualService to manage access to the ObjectStore, which controls
access to a database.

  class Lafcadio::ObjectStore < ContextualService::Service
    def initialize
      ...
    end
  end

ObjectStore.new will raise an error. To get an instance of ObjectStore,
you call ObjectStore.get_object_store. If no instance has been
previously set, this will instantiate the ObjectStore and save the
instance for future accesses. If a mock instance has been previously
set
with set_object_store, it will simply return that mock instance.

Since access to the global service is attached to the class, there's no
need to pass in a mock service as an argument to any method that needs
it. This can simplify testing considerably if you've got highly
decomposed code that needs to access a global service from many places.
Also, since the Context object is hidden, clients of the service don't
have to be conscious of it.

  def some_method
    os = Lafcadio::ObjectStore.get_object_store
    # do something with the object store here
    some_other_method
  end

  def some_other_method
    os = Lafcadio::ObjectStore.get_object_store
    # do something with the object store here, too
  end

  class TestSomeMethod < Test::Unit::TestCase
    def test1
      mock_object_store = Lafcadio::MockObjectStore.new
      Lafcadio::ObjectStore.set_object_store mock_object_store
      some_method
      # assert something happened
    end
  end

The project page can be found at
http://rubyforge.org/projects/contxtlservice .