Refactor frontend code to allow for extension of assets

- Simpler class naming:
    Frontend\CompilerFactory → Frontend\Assets
    Frontend\HtmlDocumentFactory → Frontend\Frontend
    Frontend\HtmlDocument → Frontend\Document

- Remove AssetInterface and simply collect callbacks in Frontend\Assets
  instead

- Remove ContentInterface because it serves no purpose (never type-
  hinted or type-checked)

- Commit and add asset URLs to the Document via a content callback
  instead of in the Document factory class itself

- Add translations and locale assets to Assets separate to the assets
  factory, as non-forum/admin asset bundles probably won't want them

- Update Frontend Extender to allow the creation of new asset bundles

- Make custom LESS validation listener a standalone class instead of
  extending RecompileFrontendAssets

src/Frontend/Assets.php

@@ -0,0 +1,195 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Flarum\Frontend;
+
+use Flarum\Frontend\Compiler\CompilerInterface;
+use Flarum\Frontend\Compiler\JsCompiler;
+use Flarum\Frontend\Compiler\LessCompiler;
+use Flarum\Frontend\Compiler\Source\SourceCollector;
+use Illuminate\Contracts\Filesystem\Filesystem;
+
+/**
+ * A factory class for creating frontend asset compilers.
+ */
+class Assets
+{
+    /**
+     * @var array
+     */
+    public $sources = [
+        'js' => [],
+        'css' => [],
+        'localeJs' => [],
+        'localeCss' => []
+    ];
+
+    /**
+     * @var string
+     */
+    protected $name;
+
+    /**
+     * @var Filesystem
+     */
+    protected $assetsDir;
+
+    /**
+     * @var string
+     */
+    protected $cacheDir;
+
+    /**
+     * @var array
+     */
+    protected $lessImportDirs;
+
+    public function __construct(string $name, Filesystem $assetsDir, string $cacheDir = null, array $lessImportDirs = null)
+    {
+        $this->name = $name;
+        $this->assetsDir = $assetsDir;
+        $this->cacheDir = $cacheDir;
+        $this->lessImportDirs = $lessImportDirs;
+    }
+
+    public function js($sources)
+    {
+        $this->addSources('js', $sources);
+
+        return $this;
+    }
+
+    public function css($callback)
+    {
+        $this->addSources('css', $callback);
+
+        return $this;
+    }
+
+    public function localeJs($callback)
+    {
+        $this->addSources('localeJs', $callback);
+
+        return $this;
+    }
+
+    public function localeCss($callback)
+    {
+        $this->addSources('localeCss', $callback);
+
+        return $this;
+    }
+
+    private function addSources($type, $callback)
+    {
+        $this->sources[$type][] = $callback;
+    }
+
+    private function populate(CompilerInterface $compiler, string $type, string $locale = null)
+    {
+        $compiler->addSources(function (SourceCollector $sources) use ($type, $locale) {
+            foreach ($this->sources[$type] as $callback) {
+                $callback($sources, $locale);
+            }
+        });
+    }
+
+    public function makeJs(): JsCompiler
+    {
+        $compiler = new JsCompiler($this->assetsDir, $this->name.'.js');
+
+        $this->populate($compiler, 'js');
+
+        return $compiler;
+    }
+
+    public function makeCss(): LessCompiler
+    {
+        $compiler = $this->makeLessCompiler($this->name.'.css');
+
+        $this->populate($compiler, 'css');
+
+        return $compiler;
+    }
+
+    public function makeLocaleJs(string $locale): JsCompiler
+    {
+        $compiler = new JsCompiler($this->assetsDir, $this->name.'-'.$locale.'.js');
+
+        $this->populate($compiler, 'localeJs', $locale);
+
+        return $compiler;
+    }
+
+    public function makeLocaleCss(string $locale): LessCompiler
+    {
+        $compiler = $this->makeLessCompiler($this->name.'-'.$locale.'.css');
+
+        $this->populate($compiler, 'localeCss', $locale);
+
+        return $compiler;
+    }
+
+    protected function makeLessCompiler(string $filename): LessCompiler
+    {
+        $compiler = new LessCompiler($this->assetsDir, $filename);
+
+        if ($this->cacheDir) {
+            $compiler->setCacheDir($this->cacheDir.'/less');
+        }
+
+        if ($this->lessImportDirs) {
+            $compiler->setImportDirs($this->lessImportDirs);
+        }
+
+        return $compiler;
+    }
+
+    public function getName(): string
+    {
+        return $this->name;
+    }
+
+    public function setName(string $name)
+    {
+        $this->name = $name;
+    }
+
+    public function getAssetsDir(): Filesystem
+    {
+        return $this->assetsDir;
+    }
+
+    public function setAssetsDir(Filesystem $assetsDir)
+    {
+        $this->assetsDir = $assetsDir;
+    }
+
+    public function getCacheDir(): ?string
+    {
+        return $this->cacheDir;
+    }
+
+    public function setCacheDir(?string $cacheDir)
+    {
+        $this->cacheDir = $cacheDir;
+    }
+
+    public function getLessImportDirs(): array
+    {
+        return $this->lessImportDirs;
+    }
+
+    public function setLessImportDirs(array $lessImportDirs)
+    {
+        $this->lessImportDirs = $lessImportDirs;
+    }
+}