silverstripe-subsites/code/forms/GridFieldSubsiteDetailForm.php

<?php
class GridFieldSubsiteDetailForm extends GridFieldDetailForm
{
    protected $itemRequestClass='GridFieldSubsiteDetailForm_ItemRequest';
}

class GridFieldSubsiteDetailForm_ItemRequest extends GridFieldDetailForm_ItemRequest
{
    private static $allowed_actions = array(
        'ItemEditForm',
    );

    /**
     * Builds an item edit form.  The arguments to getCMSFields() are the popupController and
     * popupFormName, however this is an experimental API and may change.
     * 
     * @todo In the future, we will probably need to come up with a tigher object representing a partially
     * complete controller with gaps for extra functionality.  This, for example, would be a better way
     * of letting Security/login put its log-in form inside a UI specified elsewhere.
     * 
     * @return Form
     * @see GridFieldDetailForm_ItemRequest::ItemEditForm()
     */
    public function ItemEditForm()
    {
        $form=parent::ItemEditForm();
        
        if ($this->record->ID == 0) {
            $templates = Subsite::get()->sort('Title');
            $templateArray = array();
            if ($templates) {
                $templateArray = $templates->map('ID', 'Title');
            }

            $templateDropdown = new DropdownField('TemplateID', _t('Subsite.COPYSTRUCTURE', 'Copy structure from:'), $templateArray);
            $templateDropdown->setEmptyString('(' . _t('Subsite.NOTEMPLATE', 'No template') . ')');
            $form->Fields()->addFieldToTab('Root.Configuration', $templateDropdown);
        }
        
        return $form;
    }
    
    public function doSave($data, $form)
    {
        $new_record = $this->record->ID == 0;
        if ($new_record && isset($data['TemplateID']) && !empty($data['TemplateID'])) {
            $template = Subsite::get()->byID(intval($data['TemplateID']));
            if ($template) {
                $this->record = $template->duplicate();
            }
        }

        return parent::doSave($data, $form);
    }
}