feat: add Less custom function extender, is-extension-enabled function (#3190)

Co-authored-by: luceos <luceos@users.noreply.github.com>
Co-authored-by: Sami Mazouz <sychocouldy@gmail.com>

src/Extend/Theme.php

@@ -12,11 +12,13 @@ namespace Flarum\Extend;
 use Flarum\Extension\Extension;
 use Flarum\Frontend\Assets;
 use Illuminate\Contracts\Container\Container;
+use RuntimeException;
 
 class Theme implements ExtenderInterface
 {
     private $lessImportOverrides = [];
     private $fileSourceOverrides = [];
+    private $customFunctions = [];
 
     /**
      * This can be used to override LESS files that are imported within the code.
@@ -52,8 +54,58 @@ class Theme implements ExtenderInterface
         return $this;
     }
 
+    /**
+     * This method allows you to add custom Less functions.
+     *
+     * All custom Less functions may only return numbers, strings or booleans.
+     *
+     * **Example usage:**
+     * ```php
+     * (new Extend\Theme)
+     *     ->addCustomLessFunction('is-flarum', function (mixed $text) {
+     *         return strtolower($text) === 'flarum'
+     *     }),
+     * ```
+     *
+     * @see https://leafo.net/lessphp/docs/#custom_functions
+     *
+     * @param string $functionName Name of the function identifier.
+     * @param callable $callable The PHP function to run when the Less function is called.
+     * @return self
+     */
+    public function addCustomLessFunction(string $functionName, callable $callable): self
+    {
+        $this->customFunctions[$functionName] = function (...$args) use ($callable, $functionName) {
+            $argVals = array_map(function ($arg) {
+                return $arg->value;
+            }, $args);
+
+            $return = $callable(...$argVals);
+
+            if (is_bool($return)) {
+                return new \Less_Tree_Quoted('', $return ? 'true' : 'false');
+            }
+
+            if (is_string($return)) {
+                return new \Less_Tree_Quoted('', $return);
+            }
+
+            if (is_numeric($return)) {
+                return new \Less_Tree_Dimension($return);
+            }
+
+            throw new RuntimeException('Custom Less function `'.$functionName.'` must only return a string, number or boolean.');
+        };
+
+        return $this;
+    }
+
     public function extend(Container $container, Extension $extension = null)
     {
+        $container->extend('flarum.frontend.custom_less_functions', function (array $customFunctions) {
+            return array_merge($customFunctions, $this->customFunctions);
+        });
+
         $container->extend('flarum.assets.factory', function (callable $factory) {
             return function (...$args) use ($factory) {
                 /** @var Assets $assets */