Added RemoveResource extender. (#34)

* Added `RemoveResource` extender.

* Added docs for removing a resource to README

README.md

@@ -53,6 +53,8 @@ _Best for larger forums, starting at 50.000 items._
 
 ## Extending
 
+### Register a new Resource
+
 In order to register your own resource, create a class that implements `FoF\Sitemap\Resources\Resource`. Make sure
 to implement all abstract methods, check other implementations for examples. After this, register your 
 
@@ -63,6 +65,15 @@ return [
 ```
 That's it.
 
+### Remove a Resource
+
+In a very similar way, you can also remove resources from the sitemap:
+```php
+return [
+    new \FoF\Sitemap\Extend\RemoveResource(User::class)
+];
+```
+
 ## Scheduling
 
 If the size of your forum requires one of the cache modes - either in-memory or disk, consider setting up the Flarum scheduler. Read more information about this [here](https://discuss.flarum.org/d/24118)

src/Extend/RemoveResource.php

@@ -0,0 +1,49 @@
+<?php
+
+/*
+ * This file is part of fof/sitemap.
+ *
+ * Copyright (c) FriendsOfFlarum.
+ *
+ *  For the full copyright and license information, please view the LICENSE
+ *  file that was distributed with this source code.
+ *
+ */
+
+namespace FoF\Sitemap\Extend;
+
+use Flarum\Extend\ExtenderInterface;
+use Flarum\Extension\Extension;
+use FoF\Sitemap\Resources\Resource;
+use Illuminate\Contracts\Container\Container;
+use InvalidArgumentException;
+
+class RemoveResource implements ExtenderInterface
+{
+    /**
+     * @var string
+     */
+    private $resource;
+
+    public function __construct(string $resource)
+    {
+        $this->resource = $resource;
+    }
+
+    public function extend(Container $container, Extension $extension = null)
+    {
+        $container->extend('fof.sitemap.resources', function (array $resources) use ($container) {
+            $resource = $container->make($this->resource);
+
+            if ($resource instanceof Resource) {
+                $resources = array_filter($resources, function ($res) {
+                    return get_class($res) !== $this->resource;
+                });
+            } else {
+                throw new InvalidArgumentException("{$this->resource} has to extend ".Resource::class);
+            }
+
+            return $resources;
+        });
+    }
+}