singleton.rst

Singleton

Intent

The purpose of the Singleton pattern is to ensure that a class will only have one instance at any time during code execution. Another key feature is to provide a global access to that instance.

When to use it?

For many people (including myself), Singleton is the political and nice name for global variables on object-oriented programming and that is the main drawback (global access + global state).

Global access makes source code hard to test ! Indeed, each objects should declare its collaborators in the constructor (dependency injection) in order to be able to mock dependencies.

For more details, I suggest you the great Miško Hevery's blog (The Testability Explorer) and especially these posts:

• Singletons are Pathological Liars
• Root Cause of Singletons

Having said this, a few Singletons may be used inside your application without "harming" it. All objects that do not affect the behavior of your application (like a Logger) may be perfect examples.

Diagram

Created using PhpStorm and yFiles.

Implementation

Logger.php

.. literalinclude:: ../../src/Creational/Singleton/Logger.php
   :linenos:
   :language: php

Tests

LoggerTest.php

.. literalinclude:: ../../tests/Creational/Singleton/LoggerTest.php
   :linenos:
   :language: php