Retrieve values from a custom field in Joomla 3.8

Question

excuse for my English. I try to retrieve my value from a custom field in com_categories. I added this field by a plugin. My value is correctly saved but i don't why, i can't retrieve my value from the field in blog_item.php file. (Of course, i ovverided this component) I used this tutorial to make the plugin : https://zunostudios.com/blog/development/203-how-to-add-custom-fields-to-articles-in-joomla

Here's my code :

zfield.php

<?php
// no direct access
defined ( '_JEXEC' ) or die ( 'Restricted access' );

class plgContentZfields extends JPlugin {

        /**
         * Load the language file on instantiation.
         * Note this is only available in Joomla 3.1 and higher.
         * If you want to support 3.0 series you must override the constructor
         *
         * @var boolean
         * @since 3.1
         */

        protected $autoloadLanguage = true;

        function onContentPrepareForm($form, $data) {

                $app = JFactory::getApplication();
                $option = $app->input->get('option');

                switch($option) {

                    case 'com_categories':
                        if ($app->isAdmin()) {

                                JForm::addFormPath(__DIR__ . '/forms');

                                //Show specific forms based on categories
                                $form->loadFile('content', false); 

                        }
                        return true;

                }

                return true;

        }

}
?>

content.xml

<form>
    <fields name="params" >
        <fieldset name="params" >

            <field name="backgroundcolor"
                type="color"
                label="Color of the category"
                /> 
        </fieldset>
    </fields>
</form>

autoblog_item.php (overrided file)

.
.
.
<?php // Todo Not that elegant would be nice to group the params ?>
<?php $useDefList = ($params->get('show_modify_date') || $params->get('show_publish_date') || $params->get('show_create_date')
    || $params->get('show_hits') || $params->get('show_category') || $params->get('show_parent_category') || $params->get('show_author') || $assocParam); ?>

<?php if ($useDefList && ($info == 0 || $info == 2)) : ?>
    <?php // Todo: for Joomla4 joomla.content.info_block.block can be changed to joomla.content.info_block 
$attributes = json_decode($this->item->params);
echo "background".$attributes->backgroundcolor;
    ?>
    <div>
    <?php echo JLayoutHelper::render('joomla.content.info_block.block', array('item' => $this->item, 'params' => $params, 'position' => 'above')); ?>
</div>
<?php endif; ?>
<?php if ($info == 0 && $params->get('show_tags', 1) && !empty($this->item->tags->itemTags)) : ?>
    <?php echo JLayoutHelper::render('joomla.content.tags', $this->item->tags->itemTags); ?>
<?php endif; ?>

.
.
.

My final goal is to put a background-color to each category. I want to use the value in my custom field to define the background-color. Any ideas please ?

Answer 1

The category object is $this->category, not $this->item and params are a Registry object so no need for json_decode().

echo $this->category->params->get('backgroundcolor');

UPDATE: To show category params for each article based on that article's category, you need retrieve the category for each article:

$categoryParams = JCategories::getInstance('Content')->get($this->item->catid)->getParams();
echo $categoryParams->get('backgroundcolor');