Design Patterns - Bridge Pattern

Purpose

The purpose of the design pattern

Decouple an abstraction from its implementation so that the two can vary independently.

UML

UML design pattern diagram

Code

Code snippets

Formatter

Formatter Interface


namespace DesignPatterns\Structural\Bridge;

interface Formatter
{
    public function format(string $text): string;
}

Service

The purpose of this Service class is to eliminate boilerplate code over the concrete service classes (HelloWorldService and PingService - which extend it). A class which implements the Formatter Interface is passed into its construct which adjusts the behaviour of the service.


namespace DesignPatterns\Structural\Bridge;

abstract class Service
{
    public function __construct(protected Formatter $implementation)
    {
    }

    final public function setImplementation(Formatter $printer)
    {
        $this->implementation = $printer;
    }

    abstract public function get(): string;
}

HelloWorldService

The HelloWorldService extends the abstract class Service.


namespace DesignPatterns\Structural\Bridge;

class HelloWorldService extends Service
{
    public function get(): string
    {
        return $this->implementation->format('Hello World');
    }
}

PingService

The PingService class extends the abstract class Service.


namespace DesignPatterns\Structural\Bridge;

class PingService extends Service
{
    public function get(): string
    {
        return $this->implementation->format('pong');
    }
}

PlainTextFormatter

Adjusts the service output via formatting with plain text.


namespace DesignPatterns\Structural\Bridge;

class PlainTextFormatter implements Formatter
{
    public function format(string $text): string
    {
        return $text;
    }
}

HtmlFormatter

Adjusts the service output via formatting with HTML.


namespace DesignPatterns\Structural\Bridge;

class HtmlFormatter implements Formatter
{
    public function format(string $text): string
    {
        return sprintf('%s', $text);
    }
}

Tests

When a service is instantiated, a formatter is passed into its construct which adjusts the behaviour. i.e - Output Hello World in Plain Text and Output Hello World wrapped in HTML tags.


public function testCanPrintUsingThePlainTextFormatter()
{
    $service = new HelloWorldService(new PlainTextFormatter());

    $this->assertSame('Hello World', $service->get());
}

public function testCanPrintUsingTheHtmlFormatter()
{
    $service = new HelloWorldService(new HtmlFormatter());

    $this->assertSame('Hello World', $service->get());
}