public function indexAt($index)
    {
        return ($index >= 0 && $index <= $this->count() - 1) ? $this->blockManagers[$index] : null;
    }
public function indexAt($index)
    {
        return ($index >= 0 && $index <= $this->count() - 1) ? $this->blockManagers[$index] : null;
    }
public function indexAt ( $ index ) { return ( $ index >= 0 && $ index <= $ this -> count ( ) - 1 ) ? $ this -> blockManagers [ $ index ] : null ; }
Returns the block manager at the given index.

@param  int                          $index
@return null|BlockManagerInterface

@api
Returns the block manager at the given index .