Implement ArrayAccess

Solves symbiote#45. Note that this is meant for the older version 3 of this module. I didn't start from the master branch because I still have SilverStripe 3 so I wouln't be able to test with SS4. But I guess this can be easily adapted to the newer version.

code/fields/MultiValueField.php

@@ -6,7 +6,7 @@
  *
  * @author Marcus Nyeholt <[email protected]>
  */
-class MultiValueField extends DBField implements CompositeDBField
+class MultiValueField extends DBField implements CompositeDBField, ArrayAccess
 {
 
     /**
@@ -211,4 +211,72 @@ protected function restoreValueFromDb($value)
         }
         return $value ?: array();
     }
+
+    /**
+	 * Whether a offset exists
+	 * @link http://php.net/manual/en/arrayaccess.offsetexists.php
+	 * @param mixed $offset <p>
+	 * An offset to check for.
+	 * </p>
+	 * @return boolean true on success or false on failure.
+	 * </p>
+	 * <p>
+	 * The return value will be casted to boolean if non-boolean was returned.
+	 * @since 5.0.0
+	 */
+	public function offsetExists($offset)
+	{
+		$field_values = $this->getValue();
+		return isset($field_values[$offset]);
+	}
+
+	/**
+	 * Offset to retrieve
+	 * @link http://php.net/manual/en/arrayaccess.offsetget.php
+	 * @param mixed $offset <p>
+	 * The offset to retrieve.
+	 * </p>
+	 * @return mixed Can return all value types.
+	 * @since 5.0.0
+	 */
+	public function offsetGet($offset)
+	{
+		$field_values = $this->getValue();
+		return $field_values[$offset];
+	}
+
+	/**
+	 * Offset to set
+	 * @link http://php.net/manual/en/arrayaccess.offsetset.php
+	 * @param mixed $offset <p>
+	 * The offset to assign the value to.
+	 * </p>
+	 * @param mixed $value <p>
+	 * The value to set.
+	 * </p>
+	 * @return void
+	 * @since 5.0.0
+	 */
+	public function offsetSet($offset, $value)
+	{
+		$field_values = $this->getValue();
+		$field_values[$offset] = $value;
+		$this->setValue($field_values);
+	}
+
+	/**
+	 * Offset to unset
+	 * @link http://php.net/manual/en/arrayaccess.offsetunset.php
+	 * @param mixed $offset <p>
+	 * The offset to unset.
+	 * </p>
+	 * @return void
+	 * @since 5.0.0
+	 */
+	public function offsetUnset($offset)
+	{
+		$field_values = $this->getValue();
+		unset($field_values[$offset]);
+		$this->setValue($field_values);
+	}
 }