observer.rst

Observer

Intent

According to the Gang of Four, the Observer pattern defines "a one-to-many dependency between objects so that when one object [the subject] changes state, all its dependents [the observers] are notified and updated automatically" (Design Patterns: Elements of Reusable Object-Oriented Software, 2013, p. 293).

Traditionally observers will be notified by calling one of their methods (like update() in the examples below). To illustrate it, I have decided to write code where observers want to monitor products' prices. Each time the price evolves, the observers will be notified!

When to use it?

The Observer pattern allows you to apply the Open Closed Principle (OCP) which states that you should be able to extend classes without modifying them (thanks to Bertrand Meyer for that principle).

Indeed you can register an unlimited number of observers without the need to change the class of the Subject (you may simply don't have any idea of how many Observers objects will be linked to the Subject).

In a nutshell, you should use the Observer pattern when you don't want your objects to be tightly coupled (here, only one class, the Observer, is directly aware about the existence of the subject).

Diagram

Created using PhpStorm and yFiles.

Using Standard PHP Library (SPL)

Without Standard PHP Library (SPL)

Implementation

Using Standard PHP Library (SPL)

Product.php

.. literalinclude:: ../../src/Behavioral/Observer/UsingSPL/Product.php
    :linenos:
    :language: php

ProductPriceObserver.php

.. literalinclude:: ../../src/Behavioral/Observer/UsingSPL/ProductPriceObserver.php
    :linenos:
    :language: php

Without Standard PHP Library (SPL)

SubjectInterface.php

.. literalinclude:: ../../src/Behavioral/Observer/WithoutSPL/SubjectInterface.php
    :linenos:
    :language: php

ObserverInterface.php

.. literalinclude:: ../../src/Behavioral/Observer/WithoutSPL/ObserverInterface.php
    :linenos:
    :language: php

Product.php

.. literalinclude:: ../../src/Behavioral/Observer/WithoutSPL/Product.php
    :linenos:
    :language: php

ProductPriceObserver.php

.. literalinclude:: ../../src/Behavioral/Observer/WithoutSPL/ProductPriceObserver.php
    :linenos:
    :language: php

Tests

Using Standard PHP Library (SPL)

ProductTest.php

.. literalinclude:: ../../tests/Behavioral/Observer/UsingSPL/ProductTest.php
    :linenos:
    :language: php

ProductPriceObserverTest.php

.. literalinclude:: ../../tests/Behavioral/Observer/UsingSPL/ProductPriceObserverTest.php
    :linenos:
    :language: php

Without Standard PHP Library (SPL)

ProductTest.php

.. literalinclude:: ../../tests/Behavioral/Observer/WithoutSPL/ProductTest.php
    :linenos:
    :language: php

ProductPriceObserverTest.php

.. literalinclude:: ../../tests/Behavioral/Observer/WithoutSPL/ProductPriceObserverTest.php
    :linenos:
    :language: php