Re: [SMW-devel] autoload SMWWikiPageValue?

[Markus K. <ma...@ai...>]

On Samstag, 22. März 2008, Matt Williamson wrote:
> Devs,
>
> Quick question: I was trying to instantiate a SMWWikiPageValue object,
> and got a class not found error. Come to find, that it's not added to
> $wgAutoloadClasses in SMW_GlobalFunctions.php like most of the others,
> but instead in SMWDataValueFactory::initDatatypes(). From there, I
> traced it back a bit and perhaps it is supposed to get called by some
> smwInitDatatypes hook? Then I got lost.
>
> So, what is the "proper" way to make sure this function is called? Am
> I supposed to manually call initDatatypes() before directly
> instantiating any DV objects? Or was that supposed to already happen
> through some prerequisite step that I missed?

Extensions can overwrite any of SMW's datatypes (which are identified by 
internal type IDs, not by class names), or add new such datatypes. This is 
done in the smwInitDatatypes hook, where it is assumed that any such 
overwrites preserve the public interface of the original implementation 
(which extends SMWDataValue in some cases, e.g., SMWWikipageValue has a 
method getDBkey()). 

To make this overwriting possible, the proper way of creating datavalue 
objects is via the static methods from the SMWDataValueFactory:

SMWDataValueFactory::newTypeIDValue($typeid, $value=false, $caption=false, 
$propertyname=false)

Examples:
* Make a date value container that is not holding a value yet:
  SMWDataValueFactory::newTypeIDValue('_dat')
  // the value's setUserValue() can be used to set a value later
* Make a numerical value representing the input "1234.560":
  SMWDataValueFactory::newTypeIDValue('_num', '1234.560')
* Make a wikipage value container and initialise it from known title data:
  $value = SMWDataValueFactory::newTypeIDValue('_wpg');
  $value->setValues('DB_Key', NS_MAIN, 123);
  // 123 is the optional page id; 
  // performance improvement if the ID is known but not cached by MediaWiki

The propertyname is needed only to create things like the search links shown 
in the Factbox, the caption is the alternative string given by the user, and 
the value is always a user-supplied string (i.e. its format may depend on the 
wiki language -- setXSDValue() can be used if this should be a problem).

The type ids are defined in SMWDataValueFactory::initDatatypes(). IDs of 
built-in types start with one underscore if wiki users can directly use those 
types for properties (i.e. they have a translated label in the wiki 
language), and with two underscores if the datatype is used internally by SMW 
only (e.g. for special properties). Extension datatypes should use no 
underscores in order to prevent name clashes with future SMW extensions.

The SMWDatavalueFactory also has methods for making datavalue objects from 
SMWTypesValue-objects (the container object for the values of SMW's 
property "has type"), and some methods for making datavalues for properties 
(described by their name or by their Title object). The property-related 
interfaces might change slightly in the future, but the methods 
newTypeIDValue and newTypeObjectValue are rather stable.

-- Markus

>
> -Matt
>
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-- 
Markus Krötzsch
Institut AIFB, Universität Karlsruhe (TH), 76128 Karlsruhe
phone +49 (0)721 608 7362          fax +49 (0)721 608 5998
ma...@ai...          www  http://korrekt.org