dynamically set a radio button on a custom grid

Question

How do I dynamically set a radio button on a custom grid?

so far I have tried: (each attempt has had both value and values)

'value' => array('checked'), //not based on db value but to see if it worked

'value' => $this->getdata('is_default') // var_dump shows it is null

'value' => $this->getIdFieldName('id_default')

none checked the button below is my code if that helps

class [classname] extends Mage_Adminhtml_Block_Widget_Grid
    protected function _prepareColumns()
    {
        $this->addColumn('is_default', array(
            'header'   => $this->__('Default'),
            'filter'   => false,
            'type'     => 'radio',
            'index'    => 'is_default',

        ));

        return parent::_prepareColumns();
    }

EDIT

After further investigation into this issue I have found this class Mage_Adminhtml_Block_Widget_Grid_Column_Renderer_Radio and reading the code I believe that it is possible to set the button as checked, am I right? or have I read the code wrong? or am I looking in the wrong place?

/**
 * Renders grid column
 *
 * @param   Varien_Object $row
 * @return  string
 */
public function render(Varien_Object $row)
{
    $values = $this->getColumn()->getValues();
    $value  = $row->getData($this->getColumn()->getIndex());
    if (is_array($values)) {
        $checked = in_array($value, $values) ? ' checked="checked"' : '';
    } else {
        $checked = ($value === $this->getColumn()->getValue()) ? ' checked="checked"' : '';
    }
    $html = '<input type="radio" name="' . $this->getColumn()->getHtmlName() . '" ';
    $html .= 'value="' . $row->getId() . '" class="radio"' . $checked . '/>';
    return $html;
}

Answer 1

This is what worked for me:

$this->addColumn('column_id_here', array(
    'header'     => $this->__('Label here'),
    'header_css_class' => 'a-center',
    'type'      => 'radio',
    'name'      => 'column_id_here',
    'values'    => $this->_getSelectedValue(),
    'align'     => 'center',
    'index'     => 'column_id_here',
    'html_name' => 'column_id_here',
    'sortable'  => false,
    'filter'    => false
));

And I have a method called _getSelectedValue() that returns the value of the selected radio button.
For test purporses implement it like this:

protected function _getSelectedValue() {
    return 1; //or any other value that exists for your column id.
}