Magento：如何在控制器中动态更改默认模板

example: a controller named ajaxhandlerController it can receive several ajax request types, each type should have separate block in the layout xml

in layout for index of ajaxhandler i would write

  <mymodule_ajaxhandler_index>
      <block some block></block>
  </mymodule_ajaxhandler_index>

How do I change from index to save for example if there is $request['type']=="save"; so I can use

  <mymodule_ajaxhandler_save>

I would forget about layout XML. An simply instantiate a Core_Block_Html_Template and assign the correct template to using the setTemplate method according the the request type received.

$html = $this->getLayout()
        ->createBlock('core/template')
        ->setTemplate('your/template.phtml')
        ->toHtml();