Added interfaces, traits, and tests for the Visitor pattern

Author: jeremeamia

src/Jeremeamia/PhpPatterns/Behavioral/VisitableInterface.php

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+<?php
+
+namespace Jeremeamia\PhpPatterns\Behavioral;
+
+interface VisitableInterface
+{
+    public function accept(VisitorInterface $visitor);
+}

src/Jeremeamia/PhpPatterns/Behavioral/VisitableTrait.php

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+<?php
+
+namespace Jeremeamia\PhpPatterns\Behavioral;
+
+use Jeremeamia\PhpPatterns\Structural\Collection\TraversableTrait;
+
+trait VisitableTrait
+{
+    use TraversableTrait;
+
+    public function accept(VisitorInterface $visitor)
+    {
+        foreach ($this->getIterator() as $item) {
+            $visitor->visit($item);
+        }
+
+        return $this;
+    }
+}

src/Jeremeamia/PhpPatterns/Behavioral/VisitorInterface.php

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+<?php
+
+namespace Jeremeamia\PhpPatterns\Behavioral;
+
+interface VisitorInterface
+{
+    public function visit($node);
+}

test/Jeremeamia/PhpPatterns/Test/Behavioral/VisitableFixture.php

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+<?php
+
+namespace Jeremeamia\PhpPatterns\Test\Behavioral;
+
+use Jeremeamia\PhpPatterns\Behavioral\VisitableInterface;
+use Jeremeamia\PhpPatterns\Behavioral\VisitableTrait;
+use Jeremeamia\PhpPatterns\Structural\Collection\GetDataTrait;
+
+class VisitableFixture implements VisitableInterface, \IteratorAggregate
+{
+    use GetDataTrait, VisitableTrait;
+
+    public function __construct(array $data)
+    {
+        $this->data = $data;
+    }
+}

test/Jeremeamia/PhpPatterns/Test/Behavioral/VisitableTraitTest.php

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+<?php
+
+namespace Jeremeamia\PhpPatterns\Test\Behavioral;
+
+/**
+ * @covers Jeremeamia\PhpPatterns\Behavioral\VisitableTrait
+ */
+class VisitableTraitTest extends \PHPUnit_Framework_TestCase
+{
+    public function testAcceptVisitsNodes()
+    {
+        $visitor = new VisitorFixture();
+        $visitable = new VisitableFixture([2, 4, 6, 8]);
+        $visitable->accept($visitor);
+        $this->assertEquals(20, $visitor->getSum());
+    }
+}

test/Jeremeamia/PhpPatterns/Test/Behavioral/VisitorFixture.php

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+<?php
+
+namespace Jeremeamia\PhpPatterns\Test\Behavioral;
+
+use Jeremeamia\PhpPatterns\Behavioral\VisitorInterface;
+
+class VisitorFixture implements VisitorInterface
+{
+    protected $sum = 0;
+
+    public function getSum()
+    {
+        return $this->sum;
+    }
+
+    public function visit($node)
+    {
+        $this->sum += (int) $node;
+    }
+}