Improve symbol element parsing

This change adds a separate Symbol tag class since the element should be treated slightly different from a Group, namely in that a vewBox can be applied. Additionally, a Symbol is like a def in that it should not be rendered until referenced by a Use element.

fixes #58

src/Svg/Document.php

@@ -23,6 +23,7 @@
 use Svg\Tag\Polyline;
 use Svg\Tag\Rect;
 use Svg\Tag\Stop;
+use Svg\Tag\Symbol;
 use Svg\Tag\Text;
 use Svg\Tag\StyleTag;
 use Svg\Tag\UseTag;
@@ -323,10 +324,14 @@ private function _tagStart($parser, $name, $attributes)
                 break;
 
             case 'g':
-            case 'symbol':
                 $tag = new Group($this, $name);
                 break;
 
+            case 'symbol':
+                $this->inDefs = true;
+                $tag = new Symbol($this, $name);
+                break;
+
             case 'clippath':
                 $tag = new ClipPath($this, $name);
                 break;
@@ -379,6 +384,11 @@ function _tagEnd($parser, $name)
                 $this->inDefs = false;
                 return;
 
+            case 'symbol':
+                $this->inDefs = false;
+                $tag = array_pop($this->stack);
+                break;
+
             case 'svg':
             case 'path':
             case 'rect':
@@ -394,7 +404,6 @@ function _tagEnd($parser, $name)
             case 'style':
             case 'text':
             case 'g':
-            case 'symbol':
             case 'clippath':
             case 'use':
             case 'a':

src/Svg/Tag/Symbol.php

@@ -0,0 +1,34 @@
+<?php
+/**
+ * @package php-svg-lib
+ * @link    http://github.com/PhenX/php-svg-lib
+ * @author  Fabien Ménager <fabien.menager@gmail.com>
+ * @license GNU LGPLv3+ http://www.gnu.org/copyleft/lesser.html
+ */
+
+namespace Svg\Tag;
+
+use Svg\Style;
+
+class Symbol extends AbstractTag
+{
+    protected function before($attributes)
+    {
+        $surface = $this->document->getSurface();
+
+        $surface->save();
+
+        $style = $this->makeStyle($attributes);
+
+        $this->setStyle($style);
+        $surface->setStyle($style);
+
+        $this->applyViewbox($attributes);
+        $this->applyTransform($attributes);
+    }
+
+    protected function after()
+    {
+        $this->document->getSurface()->restore();
+    }
+}