使用count()计算实现ArrayAccess的对象的元素？

class MyObject implements ArrayAccess {
    private $data = array();

    public function offsetExists($offset) {
        return isset($this->data[$offset]);
    }

    public function offsetGet($offset) {
        return $this->data[$offset];
    }

    public function offsetSet($offset, $value) {
        $this->data[$offset] = $value;
    }

    public function offsetUnset($offset) {
        unset($this->data[$offset]);
    }

    public function count() {
        return count($this->data);
    }
}

$obj = new MyObject();
$obj['key1'] = 'value1';
$obj['key2'] = 'value2';

echo count($obj);  // 输出 2