public function beginHeader()
    {
        echo Html::beginTag('div', $this->headerOptions) . "\n";
        if ($this->title !== null) {
            echo Html::tag('h3', $this->title, ['class' => 'box-title']);
        }
        if ($this->collapsable) {
            echo '<div class="box-tools pull-right">
                <button class="btn btn-box-tool" data-widget="collapse">
                    <i class="fa fa-' . ($this->collapsed ? 'plus' : 'minus') . '"></i>
                </button>
            </div>';
        }
    }