JPlugin properties not available in child of child class - plugins

Question

I'm writing a Joomla user plugin (Joomla 3.8 and PHP 7.0) and I'm having some trouble with inheritance. A child class can access the properties and methods of a parent using:
$this->[propertyormethod]
but ....
<?php
defined('_JEXEC') or die;
class PlgUserMyPluginParent extends JPlugin
{
public function onUserAfterLogin($options = array())
{
$MyPluginChildObj = new MyPluginChild;
}
}
class MyPluginChild extends PlgUserMyPluginParent
{
public function __construct()
{
var_dump($this->params);
die;
}
}
In the above, the method onUserAfterLogin() fires as expected when a user completes login.
Because $params is set by JPlugin and PlgUserMyPluginParent extends JPlugin, I can access $params (and other JPlugin properties) inside methods of the PlgUserMyPluginParent class thus:
$this->params
So far so good. But when I extend PlgUserMyPluginParent with MyPluginChild, $this->params is null, as are any properties I set directly in PlgUserMyPluginParent and try to access from MyPluginChild.
For example, if I declare in PlgUserMyPluginParent:
public $myParentClassProperty = "somevalue";
then try to access it from MyPluginChild using $this->myParentClassProperty, it returns null.
Can someone please shed some light on what is going on here?

Answer 1

You prohibited the initialisation of the plugin with your constructor.
If you use a constructor in a childclass, you should always call the parent's constructor explicitly.
class MyPluginChild extends PlgUserMyPluginParent
{
/**
* Constructor
*
* #param object $subject The object to observe
* #param array $config An optional associative array of configuration settings.
* Recognized key values include 'name', 'group', 'params', 'language'
* (this list is not meant to be comprehensive).
*/
public function __construct($subject, $config = [])
{
parent::construct($subject, $config);
var_dump($this->params);
die;
}
}
Then the constructor of JPlugin has a chance to populate $params.