Custom Twig Extensions (#49)

* Adding the ability to load custom Twig Extensions

* Adding Twig Extension docs and example

Author: EvanLovely

README.md

@@ -319,6 +319,52 @@ This tag would be used like this in a pattern:
 {{ name }}
 ```
 
+### Adding a custom Twig Extension
+
+A [Twig Extension](https://twig.symfony.com/doc/1.x/advanced.html#creating-an-extension) is a collection of Twig functions, filters, tags, globals, and tests all as a single bundle. This approach is more advanced than adding a single function or filter using the above method, but allows greater flexibility as the whole Twig Extension can be installed in multiple environments.
+
+To add a Twig Extension using the PHP class `\MyProject\MyCustomTwigExtension`, add this to `config.yml`:
+
+```yml
+twigExtensions:
+    - '\MyProject\MyCustomTwigExtension'
+```
+
+What happens under the hood is basically this:
+
+```php
+$twig = new Twig_Environment($loader);
+
+foreach ($twigExtensions as $twigExtension) {
+    $twig->addExtension(new $twigExtension());
+}
+```
+
+If two Twig Extensions declare a function, filter, etc; the later ones override earlier ones. Any ones declared in Pattern Lab's `_twig-components` folder will override any declared using this method of custom Twig Extensions.
+
+For an example of how this works, see `ExampleTwigExtension.php` in this repo. You can enable it by adding this to your `config.yml`:
+
+```yml
+twigExtensions:
+    - '\PatternLab\PatternEngine\Twig\ExampleTwigExtension'
+```
+
+Then place this in any Twig file:
+
+```twig
+<p>Testing: {{ testPlFunction('testing...') }}</p>
+```
+
+That function declaration looks like this in `ExampleTwigExtension.php`:
+
+```php
+new Twig_SimpleFunction('testPlFunction', function($arg) {
+    return 'Thanks for testing out the Pattern Lab Example Twig Extension with this arg: ' . $arg;
+}),
+```
+
+An incredible amount of exciting possibilities are enabled with this; have fun!
+
 ### Enable `dump()`
 
 To use `dump()` set `twigDebug` in `config/config.yml` to `true`.

src/PatternLab/PatternEngine/Twig/ExampleTwigExtension.php

@@ -0,0 +1,93 @@
+<?php
+
+namespace PatternLab\PatternEngine\Twig;
+
+use Twig_Extension;
+use Twig_ExtensionInterface;
+use Twig_SimpleFilter;
+use Twig_SimpleFunction;
+
+class ExampleTwigExtension extends Twig_Extension implements Twig_ExtensionInterface {
+
+  /**
+   * Returns the name of the extension.
+   *
+   * @return string The extension name
+   *
+   * @deprecated since 1.26 (to be removed in 2.0), not used anymore internally
+   */
+  function getName() {
+    return 'Pattern Lab Twig ExampleTwigExtension';
+  }
+
+  /**
+   * Returns a list of filters to add to the existing list.
+   *
+   * @return Twig_SimpleFilter[]
+   */
+  function getFilters() {
+    return [];
+  }
+
+  /**
+   * Returns a list of functions to add to the existing list.
+   *
+   * @return Twig_SimpleFunction[]
+   */
+  function getFunctions() {
+    return [
+        new Twig_SimpleFunction('testPlFunction', function($arg) {
+          return 'Thanks for testing out the Pattern Lab Example Twig Extension with this arg: ' . $arg;
+        }),
+    ];
+  }
+
+  /**
+   * Returns a list of operators to add to the existing list.
+   *
+   * @return array<array> First array of unary operators, second array of binary operators
+   */
+  function getOperators() {
+    return [];
+  }
+
+  /**
+   * Returns a list of global variables to add to the existing list.
+   *
+   * @return array An array of global variables
+   *
+   * @deprecated since 1.23 (to be removed in 2.0), implement Twig_Extension_GlobalsInterface instead
+   */
+  function getGlobals() {
+    return [
+        'pl' => 'is awesome',
+    ];
+  }
+
+  /**
+   * Returns a list of tests to add to the existing list.
+   *
+   * @return Twig_SimpleTest[]
+   */
+  function getTests() {
+    return [];
+  }
+  /**
+   * Returns the token parser instances to add to the existing list.
+   *
+   * @return (Twig_TokenParserInterface|Twig_TokenParserBrokerInterface)[]
+   */
+  function getTokenParsers() {
+    return [];
+  }
+
+  /**
+   * Returns the node visitor instances to add to the existing list.
+   *
+   * @return Twig_NodeVisitorInterface[]
+   */
+  function getNodeVisitors() {
+    return [];
+  }
+
+}

src/PatternLab/PatternEngine/Twig/Loaders/FilesystemLoader.php

@@ -50,6 +50,7 @@ public function __construct($options = array()) {
 
 		// customize Twig
 		TwigUtil::setInstance($instance);
+		TwigUtil::loadCustomExtensions();
 		TwigUtil::loadFilters();
 		TwigUtil::loadFunctions();
 		TwigUtil::loadTags();

src/PatternLab/PatternEngine/Twig/Loaders/PatternLoader.php

@@ -92,6 +92,7 @@ public function __construct($options = array()) {
 
 		// customize Twig
 		TwigUtil::setInstance($instance);
+		TwigUtil::loadCustomExtensions();
 		TwigUtil::loadFilters();
 		TwigUtil::loadFunctions();
 		TwigUtil::loadTags();

src/PatternLab/PatternEngine/Twig/Loaders/StringLoader.php

@@ -56,6 +56,7 @@ public function __construct($options = array()) {
 
 		// customize Twig
 		TwigUtil::setInstance($instance);
+		TwigUtil::loadCustomExtensions();
 		TwigUtil::loadFilters();
 		TwigUtil::loadFunctions();
 		TwigUtil::loadTags();

src/PatternLab/PatternEngine/Twig/TwigUtil.php

@@ -50,6 +50,27 @@ public static function setInstance($instance = "") {
 
 	}
 
+  /**
+   * Load Custom Twig Extensions
+   * Looks in config for an array of strings that are classes that can be called and implements the interface `Twig_ExtensionInterface`
+   * Config example:
+   *   twigExtensions:
+   *     - '\MyProject\MyCustomTwigExtension'
+   * @link https://twig.symfony.com/doc/1.x/advanced.html#creating-an-extension
+   */
+  public static function loadCustomExtensions() {
+    $twigExtensions = Config::getOption("twigExtensions");
+    if ($twigExtensions) {
+      foreach ($twigExtensions as $twigExtension) {
+        if (class_exists($twigExtension)) {
+          self::$instance->addExtension(new $twigExtension());
+        } else {
+          Console::writeError("Your custom Twig Extension setting isn't a PHP class that exists: `" . $twigExtension . "`");
+        }
+      }
+    }
+  }
+
 	/**
 	* Get an instance of the Twig loaders
 	*