Category Archives: Elgg

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One of the more hidden features of Elgg is the Export interface.

In a nutshell this interface provides an export view for Entities, Metadata, Annotations and relationships which can provide a convenient way of accessing data objects in a machine readable form.

Endpoints

The export url is constructed in different ways for entities, relationships and metadata.

All endpoints begin with:

http://yoursite.com/export/[VIEW]/

Where [VIEW] is the format you want the data exported in – e.g. json, opendd, php or default.

Entities
To export a GUID simply add it to the end:

http://yoursite.com/export/[VIEW]/[GUID]/

Annotations & Metadata

Metadata and annotation can be exported by providing the type (‘annotation’ or ‘metadata’) and the appropriate ID.

http://yoursite.com/export/[VIEW]/[GUID]/[annotation|metadata]/[annotation_id|metadata_id]/

Relationships
Follows the same format as above, but with [GUID] being the first guid in the relationship – in essence the entity to which the relationship “belongs”.

http://yoursite.com/export/[VIEW]/[GUID]/relationship/[relationship_id]/

Security

Some items of data (for example user passwords) are restricted from this export view. Exactly what is output by an output view is governed by $object->getExportableValues(); which returns a list of exportable fields in the entity.

In addition, access permissions on the object are respected – meaning that if you can’t see an item in Elgg, you will not be able to see it in the export view either.

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Trac_Logo_512x512There are a number of ways that an Elgg plugin developer can manipulate views via the powerful Elgg views system.

Most Elgg programmers are by now familiar with extending or replacing existing views, or providing new views for new objects etc.

But what if you just wanted to make a simple tweak to an existing view – for example to replace all instances of rude words in a feed article, or to turn passive links into active ones?

Well, fortunately Elgg provides a post processing hook for views which can do just that.

After every view has been generated, the framework will trigger a plugin hook called “‘display’, ‘view'”. This hook is passed a parameter ‘view’ which contains the name of the view being processed (eg. object/blog).

The contents of the view are passed in the $returnvalue variable which you can perform any processing on before returning it from the hook.

I have just uploaded a Trac tags plugin to the Elgg community site which provides a good example of this.

The Trac tags plugin is a tiny plugin which uses the views post processing hook to turn Trac links (e.g. #xxxxx for tickets and [xxxxx] for changesets) into active links into your repository, and here’s how – first we register the hook:

function tractags_init()
{
....
// Register our post processing hook
register_plugin_hook('display', 'view', 'tractags_rewrite');


// define views we want to rewrite codes on (means we don't have to process *everything*)
$CONFIG->tractags_views = array(
'object/thewire',
'object/blog'
);

....
}

Then in our handler looks something like this:

function tractags_rewrite($hook, $entity_type, $returnvalue, $params)
{
global $CONFIG;

$view = $params['view'];

if (($view) && (in_array($view, $CONFIG->tractags_views)))
{
// Search and replace ticket numbers
$returnvalue = preg_replace_callback('/(#)([0-9]+)/i',
create_function(
'$matches',
'
global $CONFIG;

return "<a href=\"{$CONFIG->trac_baseurl}ticket/{$matches[2]}\">{$matches[0]}</a>";
'
), $returnvalue);

// Search and replace changesets
$returnvalue = preg_replace_callback('/(\[)([0-9]+)(\])/i',
create_function(
'$matches',
'
global $CONFIG;

return "<a href=\"{$CONFIG->trac_baseurl}changeset/{$matches[2]}\">{$matches[0]}</a>";
'
), $returnvalue);

return $returnvalue;
}
}

I’m sure you will be able to come up with some much more interesting uses!

Image from the Trac project.

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A new CAPTCHA approachOne thing we try and do when working on a new Elgg feature is – where we can – couple things together as loosely as possible and provide hooks for third party developers to extend Elgg and fill in any blanks.

A good example of where this has been done is the newly introduced Captcha functionality available in the latest nightly testing builds of Elgg.

The Captcha functionality is provided by a module which extends a view called “input/captcha“. This view is blank by default but is used in several places such as user registration and the lost password form.

This means two things; firstly that if a Captcha module isn’t installed or enabled then forms behave normally, and secondly it becomes a trivial matter for third party modules to provide their own Captcha functionality.

This same mechanism is how the URL shortener module works by the way.

Next, the Captcha module extends a number of actions to require a correctly validated Captcha code. This list itself is the product of a plugin hook which returns an array of actions which require Captcha validation:

$actions = array();
$actions = trigger_plugin_hook('actionlist', 'captcha', null, $actions);

...

function captcha_actionlist_hook($hook, $entity_type, $returnvalue, $params)
{
if (!is_array($returnvalue))
$returnvalue = array();

$returnvalue[] = 'register';
$returnvalue[] = 'user/requestnewpassword';

return $returnvalue;
}

The reason why the list of actions is provided this way is twofold, firstly it lets modules use Captcha functionality in their own code through a generic interface, and secondly it is harder to spoof than looking for some marker in the form code.

The Captcha itself injects a server generated token into the form, which together with the user’s response to the characters generated in an image are used to validate that the user is indeed human.

As we can see, Elgg asks to be provided with a Captcha if one is available by including a specific view, but is agnostic as to where (or indeed if) this functionality is supplied.

By using the techniques available to an Elgg programmer I was able to loosely couple the Captcha system to Elgg in such a way that a third party can easily use the same techniques to provide a more advanced module.

Happy coding!

Image “A new Captcha approach” by XKCD