Design Patterns - Strategy Pattern

Purpose

The purpose of the design pattern

To separate strategies and to enable fast switching between them. Also this pattern is a good alternative to inheritance (instead of having an abstract class that is extended).

Essentially by passing an object into the construct (it’s recommended that you type hint to an interface) you can 'execute a strategy' (i.e do something) on the given object using data passed in via methods parameters.

Examples

Examples of how the design pattern can be used

Sorting a list of objects, one strategy by date, the other by id
Simplify unit testing: e.g. switching between file and in-memory storage

UML

UML design pattern diagram

Code

Code snippets

Comparator

Comparator is an Interface with a compare method signature, any class which implements Comparator must have a compare method.


namespace DesignPatterns\Behavioral\Strategy;

interface Comparator
{
  /**
  * @param mixed $a
  * @param mixed $b
  */
  public function compare($a, $b): int; 

}

DateComparator

DateComparator contains a compare method which compares two given dates. It implements Comparator to ensure it contains a compare method.


namespace DesignPatterns\Behavioral\Strategy;

use DateTime;

class DateComparator implements Comparator
{
    public function compare($a, $b): int
    {
        $aDate = new DateTime($a['date']);
        $bDate = new DateTime($b['date']);

        return $aDate <=> $bDate;
    }
}

IdComparator

IdComparator contains a compare method which compares two given ids. It implements Comparator to ensure it contains a compare method.


namespace DesignPatterns\Behavioral\Strategy;

class IdComparator implements Comparator
{
    public function compare($a, $b): int
    {
        return $a['id'] <=> $b['id'];
    }
}

Context

Context is passed an object via construct which can referenced in the executeStrategy method. In the example below, we’re using the compare method to sort an array of elements.

However, executeStrategy could potentially do anything you want with the object passed into the construct. The executeStrategy method parameters also can change to whatever is required.


namespace DesignPatterns\Behavioral\Strategy;

class Context
{
    public function __construct(private Comparator $comparator)
    {
    }

    public function executeStrategy(array $elements): array
    {
        uasort($elements, [$this->comparator, 'compare']);

        return $elements;
    }
}

Tests

Unit test examples


public function provideIntegers()
{
    return [
        [
            [['id' => 2], ['id' => 1], ['id' => 3]],
            ['id' => 1],
        ],
        [
            [['id' => 3], ['id' => 2], ['id' => 1]],
            ['id' => 1],
        ],
    ];
}

public function provideDates()
{
    return [
        [
            [['id' => 2], ['id' => 1], ['id' => 3]],
            ['id' => 1],
        ],
        [
            [['id' => 3], ['id' => 2], ['id' => 1]],
            ['id' => 1],
        ],
    ];
}

/**
* @dataProvider provideIntegers
*
* @param array $collection
* @param array $expected
*/
public function testIdComparator($collection, $expected)
{
    $obj = new Context(new IdComparator());
    $elements = $obj->executeStrategy($collection);

    $firstElement = array_shift($elements);
    $this->assertSame($expected, $firstElement);
}

/**
* @dataProvider provideDates
* 
* @param array $collection
* @param array $expected
*/
public function testDateComparator($collection, $expected)
{
    $obj = new Context(new DateComparator());
    $elements = $obj->executeStrategy($collection);

    $firstElement = array_shift($elements);
    $this->assertSame($expected, $firstElement);
}