Datawriter: 'Gets the actual existing.....' - What does it actually mean?

[TheBigK]

So far, I've been blindly following the DataWriter creation for my simpler addons; but have decided to figure out what's really happening behind the scenes. I see this explanation (comment)

/*** Gets the actual existing data out of data that was passed in. See parent for explanation.** @param mixed** @return array|false*/protected function _getExistingData($data)
{
if (!$id = $this->_getExistingPrimaryKey($data, 'note_id'))
{
return false;}

return array('scratchpad_note' => $this->getModelFromCache('Scratchpad_Model_Note')->getNoteById($id));}

The words "gets the actual existing data out of data that was passed in" confuse me.

1. Where is this $data coming from? What does it contain when being passed on to the function?
2. Why is it mandatory to define this function in any data writer?

 

[xfrocks]

The callee may use something like this

$dw = XenForo_DataWriter::create('Something');
$dw->setExistingData($data);

That is the $data that is being used in the quoted lines. It can be an array or an ID. The method "_getExistingPrimaryKey" will extract "note_id" if $data is an array and has that key. Or it will just return $data if it is a number itself.