Design Patterns - State Pattern

Purpose

The purpose of the design pattern

Encapsulate varying behaviour for the same routine based on an object's state. This can be a cleaner way for an object to change its behaviour at runtime without resorting to large monolithic conditional statements.

Examples

Examples of how the design pattern can be used

UML

UML design pattern diagram

Code

Code snippets

State

State interface defining method signature for proceedToNext and toString.


namespace DesignPatterns\Behavioral\State;

interface State
{
    public function proceedToNext(OrderContext $context);
    public function toString(): string;
}

StateCreated

StateCreated implements State. The proceedToNext method changes the state to StateShipped. The toString method outputs ‘created’.


namespace DesignPatterns\Behavioral\State;

class StateCreated implements State
{
    public function proceedToNext(OrderContext $context)
    {
        $context->setState(new StateShipped());
    }

    public function toString(): string
    {
        return 'created';
    }
}

StateShipped

StateShipped implements State. The proceedToNext method changes the state to StateDone. The toString method outputs ‘shipped’.


namespace DesignPatterns\Behavioral\State;

class StateShipped implements State
{
    public function proceedToNext(OrderContext $context)
    {
        $context->setState(new StateDone());
    }

    public function toString(): string
    {
        return 'shipped';
    }
}

StateDone

StateDone implements State. The proceedToNext method doesn’t do anything, as there’s nothing left to do. The toString method outputs ‘done’.


namespace DesignPatterns\Behavioral\State;

class StateDone implements State
{
    public function proceedToNext(OrderContext $context)
    {
        // there is nothing more to do
    }

    public function toString(): string
    {
        return 'done';
    }
}

OrderContext

OrderContext manages and holds a state. When OrderContent is created the state is set to StateCreated.


namespace DesignPatterns\Behavioral\State;

class OrderContext
{
    private State $state;

    public static function create(): OrderContext
    {
        $order = new self();
        $order->state = new StateCreated();

        return $order;
    }

    public function setState(State $state)
    {
        $this->state = $state;
    }

    public function proceedToNext()
    {
        $this->state->proceedToNext($this);
    }

    public function toString()
    {
        return $this->state->toString();
    }
}

StateTest

Quick unit test showing the design pattern in action. Shows how the order state can be changed via a method call to create initially, then proceedToNext.


namespace DesignPatterns\Behavioral\State\Tests;

use DesignPatterns\Behavioral\State\OrderContext;
use PHPUnit\Framework\TestCase;

class StateTest extends TestCase
{
    public function testIsCreatedWithStateCreated()
    {
        $orderContext = OrderContext::create();

        $this->assertSame('created', $orderContext->toString());
    }

    public function testCanProceedToStateShipped()
    {
        $contextOrder = OrderContext::create();
        $contextOrder->proceedToNext();

        $this->assertSame('shipped', $contextOrder->toString());
    }

    public function testCanProceedToStateDone()
    {
        $contextOrder = OrderContext::create();
        $contextOrder->proceedToNext();
        $contextOrder->proceedToNext();

        $this->assertSame('done', $contextOrder->toString());
    }

    public function testStateDoneIsTheLastPossibleState()
    {
        $contextOrder = OrderContext::create();
        $contextOrder->proceedToNext();
        $contextOrder->proceedToNext();
        $contextOrder->proceedToNext();

        $this->assertSame('done', $contextOrder->toString());
    }
}