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Decorator Pattern

To dynamically add new functionality to class instances.

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Stuart Todd

•

2 years ago

•

Design Patterns: Structural

Purpose

The purpose of the design pattern

To dynamically add new functionality to class instances.

UML

UML design pattern diagram

Code

Code snippets

Booking

Interface for all the booking options.


namespace DesignPatterns\Structural\Decorator;

interface Booking
{
    public function calculatePrice(): int;
    public function getDescription(): string;
}

DoubleRoomBooking

The base option, other options will be dynamically added to this based on requirements.


namespace DesignPatterns\Structural\Decorator;

class DoubleRoomBooking implements Booking
{
    public function calculatePrice(): int
    {
        return 40;
    }

    public function getDescription(): string
    {
        return 'double room';
    }
}

BookingDecorator

Sub options extend this abstract BookingDecorator, which takes a class which implements the Booking Interface as a construct parameter.


namespace DesignPatterns\Structural\Decorator;

abstract class BookingDecorator implements Booking
{
    public function __construct(protected Booking $booking)
    {

    }
}

ExtraBed

Extra Bed is a sub option which can potentially be attached to the base option. It extends the abstract BookingDecorator.


namespace DesignPatterns\Structural\Decorator;

class ExtraBed extends BookingDecorator
{
    private const PRICE = 30;

    public function calculatePrice(): int
    {
        return $this->booking->calculatePrice() + self::PRICE;
    }

    public function getDescription(): string
    {
        return $this->booking->getDescription() . ' with extra bed';
    }
}

WiFi

WiFi is a sub option which can potentially be attached to the base option. It extends the abstract BookingDecorator.


namespace DesignPatterns\Structural\Decorator;

class WiFi extends BookingDecorator
{
    private const PRICE = 2;

    public function calculatePrice(): int
    {
        return $this->booking->calculatePrice() + self::PRICE;
    }

    public function getDescription(): string
    {
        return $this->booking->getDescription() . ' with wifi';
    }
}

Tests

Some examples of how to implement this pattern


// Customer wants a double room. Price is £40 and the description is ‘double room’.
public function testCanCalculatePriceForBasicDoubleRoomBooking()
{
    $booking = new DoubleRoomBooking();

    $this->assertSame(40, $booking->calculatePrice());
    $this->assertSame('double room', $booking->getDescription());
}

// Customer wants a double room with Wifi. Price is now £42 with the description ‘double room with wifi’.
public function testCanCalculatePriceForDoubleRoomBookingWithWiFi()
{
    $booking = new DoubleRoomBooking();
    $booking = new WiFi($booking);

    $this->assertSame(42, $booking->calculatePrice());
    $this->assertSame('double room with wifi', $booking->getDescription());
}

// Customer wants a double room with Wifi and an extra bed. Price is now £72 with the description ‘double room with wifi with extra bed’.
public function testCanCalculatePriceForDoubleRoomBookingWithWiFiAndExtraBed()
{
    $booking = new DoubleRoomBooking();
    $booking = new WiFi($booking);
    $booking = new ExtraBed($booking);

    $this->assertSame(72, $booking->calculatePrice());
    $this->assertSame('double room with wifi with extra bed', $booking->getDescription());
}