Test Double Generator

Generates appropriate test doubles based on testing needs.

Test Double Types

Type	Purpose	Behavior	Verification
Stub	Provide canned answers	Returns predefined values	No
Mock	Verify interactions	Configurable	Yes (expectations)
Fake	Working implementation	Real logic, simplified	No
Spy	Record interactions	Passes through	Yes (inspection)

Decision Matrix

Need to verify method was called?
├── Yes → Need to record actual calls?
│   ├── Yes → Spy
│   └── No → Mock
└── No → Need real behavior?
    ├── Yes → Fake
    └── No → Stub

When to Use Each

Scenario	Double Type	Example
External API response	Stub	HTTP client returns fixed JSON
Verify email sent	Mock	Assert send() was called
Repository for domain tests	Fake	InMemory implementation
Audit logging	Spy	Record all log calls
Time-dependent code	Fake	FrozenClock

Stub Examples

PHPUnit Stub

use PHPUnit\Framework\TestCase;

final class OrderServiceTest extends TestCase
{
    public function test_calculates_order_total(): void
    {
        // Create stub
        $taxCalculator = $this->createStub(TaxCalculatorInterface::class);
        $taxCalculator->method('calculate')
            ->willReturn(Money::EUR(100));

        $service = new OrderService($taxCalculator);

        $total = $service->calculateTotal($order);

        self::assertEquals(Money::EUR(1100), $total);
    }
}

Stub with Different Returns

public function test_handles_multiple_calls(): void
{
    $repository = $this->createStub(UserRepositoryInterface::class);

    // Return different values based on argument
    $repository->method('findById')
        ->willReturnCallback(function (UserId $id) {
            return match ($id->toString()) {
                'user-1' => UserMother::john(),
                'user-2' => UserMother::jane(),
                default => null,
            };
        });

    // Or return sequence
    $repository->method('findById')
        ->willReturnOnConsecutiveCalls(
            UserMother::john(),
            UserMother::jane(),
            null
        );
}

Manual Stub Class

<?php

declare(strict_types=1);

namespace Tests\Stub;

use App\Infrastructure\Http\HttpClientInterface;
use App\Infrastructure\Http\HttpResponse;

final class FixedHttpClientStub implements HttpClientInterface
{
    public function __construct(
        private readonly HttpResponse $response
    ) {}

    public function get(string $url, array $headers = []): HttpResponse
    {
        return $this->response;
    }

    public function post(string $url, array $data, array $headers = []): HttpResponse
    {
        return $this->response;
    }

    public static function returning(int $status, array $body): self
    {
        return new self(new HttpResponse($status, json_encode($body)));
    }

    public static function ok(array $body): self
    {
        return self::returning(200, $body);
    }

    public static function error(int $status = 500): self
    {
        return self::returning($status, ['error' => 'Server error']);
    }
}

Mock Examples

PHPUnit Mock

public function test_sends_notification_on_order_placed(): void
{
    // Create mock with expectations
    $notifier = $this->createMock(NotifierInterface::class);
    $notifier->expects($this->once())
        ->method('notify')
        ->with(
            $this->isInstanceOf(OrderPlacedNotification::class)
        );

    $service = new OrderService($this->repository, $notifier);

    $service->placeOrder($command);

    // Expectations verified automatically
}

Mock with Argument Matching

public function test_logs_with_correct_context(): void
{
    $logger = $this->createMock(LoggerInterface::class);
    $logger->expects($this->once())
        ->method('info')
        ->with(
            'Order placed',
            $this->callback(function (array $context) {
                return isset($context['orderId'])
                    && isset($context['customerId'])
                    && $context['amount'] > 0;
            })
        );

    $service = new OrderService($this->repository, $logger);

    $service->placeOrder($command);
}

Mock Method Call Count

public function test_retries_on_failure(): void
{
    $client = $this->createMock(HttpClientInterface::class);
    $client->expects($this->exactly(3))  // Called exactly 3 times
        ->method('post')
        ->willThrowException(new ConnectionException());

    $service = new PaymentService($client, maxRetries: 3);

    $this->expectException(PaymentFailedException::class);

    $service->charge($payment);
}

Fake Examples

InMemory Repository (Fake)

<?php

declare(strict_types=1);

namespace Tests\Fake;

use App\Domain\User\User;
use App\Domain\User\UserId;
use App\Domain\User\UserRepositoryInterface;

final class InMemoryUserRepository implements UserRepositoryInterface
{
    /** @var array<string, User> */
    private array $users = [];

    public function save(User $user): void
    {
        $this->users[$user->id()->toString()] = $user;
    }

    public function findById(UserId $id): ?User
    {
        return $this->users[$id->toString()] ?? null;
    }

    public function delete(User $user): void
    {
        unset($this->users[$user->id()->toString()]);
    }

    // Test helper methods
    public function clear(): void
    {
        $this->users = [];
    }

    public function count(): int
    {
        return count($this->users);
    }
}

Frozen Clock (Fake)

<?php

declare(strict_types=1);

namespace Tests\Fake;

use Psr\Clock\ClockInterface;
use DateTimeImmutable;

final class FrozenClock implements ClockInterface
{
    public function __construct(
        private DateTimeImmutable $now
    ) {}

    public function now(): DateTimeImmutable
    {
        return $this->now;
    }

    public static function at(string $datetime): self
    {
        return new self(new DateTimeImmutable($datetime));
    }

    public function advance(string $interval): void
    {
        $this->now = $this->now->modify($interval);
    }
}

// Usage
$clock = FrozenClock::at('2024-01-15 10:00:00');
$service = new SubscriptionService($clock);

$subscription = $service->create($user);
self::assertEquals('2024-01-15', $subscription->startDate()->format('Y-m-d'));

$clock->advance('+30 days');
self::assertTrue($subscription->isExpired($clock->now()));

Spy Examples

Collecting Spy

<?php

declare(strict_types=1);

namespace Tests\Spy;

use Psr\Log\LoggerInterface;
use Psr\Log\LogLevel;

final class SpyLogger implements LoggerInterface
{
    /** @var list<array{level: string, message: string, context: array}> */
    private array $logs = [];

    public function log($level, string|\Stringable $message, array $context = []): void
    {
        $this->logs[] = [
            'level' => $level,
            'message' => (string) $message,
            'context' => $context,
        ];
    }

    public function emergency(string|\Stringable $message, array $context = []): void
    {
        $this->log(LogLevel::EMERGENCY, $message, $context);
    }

    public function alert(string|\Stringable $message, array $context = []): void
    {
        $this->log(LogLevel::ALERT, $message, $context);
    }

    public function critical(string|\Stringable $message, array $context = []): void
    {
        $this->log(LogLevel::CRITICAL, $message, $context);
    }

    public function error(string|\Stringable $message, array $context = []): void
    {
        $this->log(LogLevel::ERROR, $message, $context);
    }

    public function warning(string|\Stringable $message, array $context = []): void
    {
        $this->log(LogLevel::WARNING, $message, $context);
    }

    public function notice(string|\Stringable $message, array $context = []): void
    {
        $this->log(LogLevel::NOTICE, $message, $context);
    }

    public function info(string|\Stringable $message, array $context = []): void
    {
        $this->log(LogLevel::INFO, $message, $context);
    }

    public function debug(string|\Stringable $message, array $context = []): void
    {
        $this->log(LogLevel::DEBUG, $message, $context);
    }

    // Inspection methods
    /** @return list<array{level: string, message: string, context: array}> */
    public function getLogs(): array
    {
        return $this->logs;
    }

    public function hasLogged(string $level, string $messageContains): bool
    {
        foreach ($this->logs as $log) {
            if ($log['level'] === $level && str_contains($log['message'], $messageContains)) {
                return true;
            }
        }
        return false;
    }

    public function clear(): void
    {
        $this->logs = [];
    }
}

Usage

public function test_logs_order_placement(): void
{
    $logger = new SpyLogger();
    $service = new OrderService($this->repository, $logger);

    $service->placeOrder($command);

    // Inspect recorded logs
    self::assertTrue($logger->hasLogged('info', 'Order placed'));

    $logs = $logger->getLogs();
    self::assertCount(1, $logs);
    self::assertArrayHasKey('orderId', $logs[0]['context']);
}

Generation Instructions

1. Analyze the dependency:

   • Interface or class being doubled
   • Methods that need doubling
   • Expected behavior in test

2. Choose double type:

   • Verification needed? → Mock or Spy
   • Real behavior needed? → Fake
   • Just return values? → Stub

3. Generate appropriate implementation:

   • PHPUnit built-in for simple cases
   • Custom class for complex behavior

4. File placement:

   • Stubs: tests/Stub/
   • Fakes: tests/Fake/
   • Spies: tests/Spy/
   • Or combined in tests/Double/

Best Practices

1. Mock interfaces, not classes — avoid mocking final classes
2. Prefer Fakes for repositories — more realistic behavior
3. Use Stubs for external services — HTTP, payment gateways
4. Limit to 3 mocks per test — more indicates design issue
5. Never mock Value Objects — use real instances
6. Don't mock what you don't own — wrap external libraries first