Magento. OnBlockToHtml Before-After - magento

Question

In magento there are 2 events:
core_block_abstract_to_html_before
core_block_abstract_to_html_after
They are called every time when toHtml method is called. What I want is to echo to output some valuable data, BUT I get following error:
HEADERS ALREADY SENT
So is there any way to append to output?

Answer 1

If you look at the events being fired, the first event (core_block_abstract_to_html_before) receives the block instance as a parameter. The second event (core_block_abstract_to_html_after) receives both the block and the transport object from which you can extract the rendered content (this is the major feature of that transport object, as the rendered string is a local variable to the method otherwise). You can see the transport object receiving the rendered string in the line immediately preceding the dispatching of the second event:
self::$_transportObject->setHtml($html);
How you add to the block output depends on what you are trying to do. If you need to wrap your output and your preamble needs to use the ..._before event, you should set a param on the block in that event's observer in your module, e.g.
public function coreBlockAbstractToHtmlBefore($observer)
{
$arg = 'Whatever you are doing';
$observer->getBlock()->setYourCustomParam($arg); //e.g. using Magento setter method
}
Then, in your ...after event observer, you can evaluate your custom param and prepend it to the output like so:
public function coreBlockAbstractToHtmlAfter($observer)
{
$argBefore = $observer->getBlock()->getYourCustomParam();
$argAfter = 'Whatever you are doing afterwards';
//get output from _toHtml()
$normalOutput = $observer->getTransport()->getHtml();
//change the output; assume that both args are strings for this ex....
$observer->getTransport()->setHtml( $argBefore . $normalOutput . $argAfter )
}
That said, don't forget that you have a number of options at your disposal, including $block->setFrameTag($open,$close) which can be (somewhat hackishly) used to wrap whatever you want around block output.
A caveat regarding these approaches: the output that is being added will not be cached in the block html cache.
One thing I'd like to add as well is that if you need to target your rendering to specific scopes you can always use the full-action-name-automatic event from Mage_Core_Controller_Varien_Action::renderLayout() method (Mage::dispatchEvent('controller_action_layout_render_before_'.$this->getFullActionName());). You just use the same class to observe this event, invoke it as a singleton for all events, and set a flag.
And finally, a note on your "Headers already sent" error: typical Magento rendering uses a response object, and output should be added to that response object via $response->appendBody('string');.

Answer 2

You can edit the template that it is rendering. Or add another template file and call it from the template file that is rendering (after adding the new one to the layout file).