Add custom attribute to standard element from plugin

Question

Developing plugin for Craft 3, I want to add extra property to Assets (data being pulled from other table)

I am able to pull in the data by extending element query:

Event::on(
            ElementQuery::class,
            ElementQuery::EVENT_AFTER_PREPARE,
            function(CancelableEvent $e) {
                if ($e->sender->elementType == 'craft\\elements\\Asset')
                {
                    $e->sender->query->addSelect('linkstats.downloads');

                    $subQuery = (new Query())
                        ->select('assetId, SUM(downloads) AS downloads')
                        ->from('{{%my_plugin_table}}')
                        ->groupBy('assetId');

                    $e->sender->query->leftJoin(
                        ['linkstats' => $subQuery], 'assets.id = linkstats.assetId'
                    );
                }
            }
        );

However when trying to display the asset I get error:

Setting unknown property: craft\elements\Asset::downloads

Question, how do I register this new property/attribute for Assets? I cannot find any event to do that, is it possible at all?

Answer 1

Another approach is to use the following event to attach a behavior

Event::on(Asset::class, Asset::EVENT_DEFINE_BEHAVIORS, 
function(DefineBehaviorsEvent $event) {
    $event->behaviors[] = MyBehavior::class;
});

The behavior could look like

class MyBehavior extends Behavior { 
    public $downloads; 
}

This way you won't receive an exception and can access your property via

$asset = Assets::find()->all();
$asset->downloads;

You can as well manipulate the value of downloads in the Behaviors init function in order to return the file or something like this

Answer 2

You can use the asArray() function in order to gain the raw query result and populate the entry on your own.

Just take a look at the ElementQuery->populate() function, copy it to you own service and attach your custom Behavior you can see how this works here

class MyBehavior extends Behavior { 

    public $prop1; 
    private $_prop2;

    public function getProp2() {
        return $this->_prop2; 
    } 

    public function setProp2($value) { 
         $this->_prop2 = $value;
    }
 }

Just include all values for the element in your custom behavior.

The key is to change this piece of code from

$element = new $class($row);

To

$element = new $class();
$element->attachBehavior('myBehavior1', new MyBehavior);
Craft::configure($element, $row);

In your custom service.