poner un contador en un gridview con SerialColumn

<?= GridView::widget([
'dataProvider' => $dataProvider,
'filterModel' => $searchModel,
'columns' => [
['class' => 'yii\grid\SerialColumn'],
'id',
'user.username',
[
'attribute' => 'live',
'header' => 'Live?',
'value'=>function($data) {
return ($data->live == 1) ? "Live" : "Draft";
},
'filter' => ['0' => 'Draft', '1' => 'Live'],
],
'title',
[
'header' => 'Snippet',
'value'=>function($data) {
return substr($data->content, 0, 100);
290CHAPTER 12. WORKING WITH WIDGETS
},
'format'=>'html'
],
// 'date_updated',
// 'date_published',
['class' => 'yii\grid\ActionColumn'],
],
]); ?>