Added wkhtmltopdf stuff and created preview generator service

module/Common/test/Service/PreviewGeneratorTest.php

@@ -0,0 +1,60 @@
+<?php
+namespace ShlinkioTest\Shlink\Common\Service;
+
+use Doctrine\Common\Cache\ArrayCache;
+use mikehaertl\wkhtmlto\Image;
+use PHPUnit_Framework_TestCase as TestCase;
+use Prophecy\Argument;
+use Prophecy\Prophecy\ObjectProphecy;
+use Shlinkio\Shlink\Common\Service\PreviewGenerator;
+
+class PreviewGeneratorTest extends TestCase
+{
+    /**
+     * @var PreviewGenerator
+     */
+    protected $generator;
+    /**
+     * @var ObjectProphecy
+     */
+    protected $image;
+    /**
+     * @var ArrayCache
+     */
+    protected $cache;
+
+    public function setUp()
+    {
+        $this->image = $this->prophesize(Image::class);
+        $this->cache = new ArrayCache();
+        $this->generator = new PreviewGenerator($this->image->reveal(), $this->cache, 'dir');
+    }
+
+    /**
+     * @test
+     */
+    public function alreadyCachedElementsAreNotProcessed()
+    {
+        $url = 'http://foo.com';
+        $this->cache->save(sprintf('preview_%s.png', urlencode($url)), 'dir/foo.png');
+        $this->image->saveAs(Argument::cetera())->shouldBeCalledTimes(0);
+        $this->assertEquals('dir/foo.png', $this->generator->generatePreview($url));
+    }
+
+    /**
+     * @test
+     */
+    public function nonCachedElementsAreProcessedAndThenCached()
+    {
+        $url = 'http://foo.com';
+        $cacheId = sprintf('preview_%s.png', urlencode($url));
+        $expectedPath = 'dir/' . $cacheId;
+
+        $this->image->setPage($url)->shouldBeCalledTimes(1);
+        $this->image->saveAs($expectedPath)->shouldBeCalledTimes(1);
+
+        $this->assertFalse($this->cache->contains($cacheId));
+        $this->assertEquals($expectedPath, $this->generator->generatePreview($url));
+        $this->assertTrue($this->cache->contains($cacheId));
+    }
+}