Allow varying LinkGeneratorFactory behavior

This patch adds a new optional constructor argument to the
`LinkGeneratorFactory`, a string `$urlGeneratorServiceName`. By default,
it uses `Zend\Expressive\Hal\LinkGenerator\UrlGeneratorInterface` for
the value. Users can, however, specify an alternate service class. This
allows users to vary the `LinkGenerator` per module or path-segregated
middleware pipeline:

```php
// In configuration:
return [
    'dependencies' => [
        'factories' => [
            \Api\Router::class => FastRouteRouterFactory::class,
            \Api\RouteMiddleware::class => new RouteMiddlewareFactory(\Api\Router::class),
            \Api\UrlHelper::class => new UrlHelperFactory('/api', \Api\Router::class),
            \Api\UrlHelperMiddleware::class => new UrlHelperMiddlewareFactory(\Api\UrlHelper::class),
            \Api\UrlGenerator::class => new ExpressiveUrlGeneratorFactory(\Api\UrlHelper::class),
            \Api\LinkGenerator::class => new LinkGeneratorFactory(\Api\UrlGenerator::class),
        ],
    ],
],
```

The factory implements `__set_state()`, allowing serialization via
`var_export()` (the mechanism used by the default expressive
configuration caching mechanism).

Author: weierophinney

src/LinkGeneratorFactory.php

@@ -11,10 +11,31 @@
 
 class LinkGeneratorFactory
 {
+    /** @var string */
+    private $urlGeneratorServiceName;
+
+    /**
+     * Allow serialization
+     */
+    public static function __set_state(array $data) : self
+    {
+        return new self(
+            $data['urlGeneratorServiceName'] ?? LinkGenerator\UrlGeneratorInterface::class
+        );
+    }
+
+    /**
+     * Allow varying behavior based on URL generator service name.
+     */
+    public function __construct(string $urlGeneratorServiceName = LinkGenerator\UrlGeneratorInterface::class)
+    {
+        $this->urlGeneratorServiceName = $urlGeneratorServiceName;
+    }
+
     public function __invoke(ContainerInterface $container) : LinkGenerator
     {
         return new LinkGenerator(
-            $container->get(LinkGenerator\UrlGeneratorInterface::class)
+            $container->get($this->urlGeneratorServiceName)
         );
     }
 }

test/LinkGeneratorFactoryTest.php

@@ -27,4 +27,26 @@ public function testReturnsLinkGeneratorInstance() : void
         self::assertInstanceOf(LinkGenerator::class, $instance);
         self::assertAttributeSame($urlGenerator, 'urlGenerator', $instance);
     }
+
+    public function testConstructorAllowsSpecifyingUrlGeneratorServiceName()
+    {
+        $urlGenerator = $this->prophesize(LinkGenerator\UrlGeneratorInterface::class)->reveal();
+
+        $container = $this->prophesize(ContainerInterface::class);
+        $container->get(UrlGenerator::class)->willReturn($urlGenerator);
+
+        $instance = (new LinkGeneratorFactory(UrlGenerator::class))($container->reveal());
+        self::assertInstanceOf(LinkGenerator::class, $instance);
+        self::assertAttributeSame($urlGenerator, 'urlGenerator', $instance);
+    }
+
+    public function testFactoryIsSerializable()
+    {
+        $factory = LinkGeneratorFactory::__set_state([
+            'urlGeneratorServiceName' => UrlGenerator::class,
+        ]);
+
+        $this->assertInstanceOf(LinkGeneratorFactory::class, $factory);
+        $this->assertAttributeSame(UrlGenerator::class, 'urlGeneratorServiceName', $factory);
+    }
 }