J4 Plugin example - Table of Contents

From Joomla! Documentation

The J4xdemostoc Example Plugin Tutorial

Background

MediaWiki automatically generates a nice Table of Contents (ToC). There are plugins for Joomla 3.x that do the same Job but nothing yet for Joomla 4. So for demonstration purposes I decided to create a ToC Plugin and document the process as a tutorial. The plugin code and working zip file are available on github:

There is also a helpful Joomla 4 tutorial here:

The zip file structure

Zip file structures vary a little because the destination of individual files is decided in the manifest file. For example, if the two ini language files shown here are at one higher level in the zip folder structure and the en-GB folder does not exist here, then the destination en-GB folder must be set in the manifest file. The ToC installation zip file is structured like this:

plg_j4xdemost.zip
     plg_j4xdemos_toc
          language
               en-GB
                    en-GB.plg_content_j4xdemostc.ini
                    en-GB.plg_content_j4xdemostc.sys.ini
          j4xdemostoc.php
          j4xdemostoc.xml

The manifest file

The manifest file contains extension data and it tells the install code what goes where. As type plugin in the content group the plugin files will be installed in site_root/plugins/content/j4xdemostoc. The languages destination folder is site_root/administrator/language. The config section is used to set parameters in the plugin administration interface (described below). By convention the <name> translation key is in lower case. All other translation keys are upper case and include the group after the type code, in this case PLG_CONTENT_J4XDEMOSTOC_usage.

    <?xml version="1.0" encoding="UTF-8"?>
    <extension type="plugin" version="4.0" method="upgrade" group="content">
        <name>plg_content_j4xdemostoc</name>
        <creationDate>August 2019</creationDate>
        <author>Clifford E Ford</author>
        <authorEmail>cliff@ford.myzen.co.uk</authorEmail>
        <authorUrl>http://www.fford.me.uk/</authorUrl>
        <copyright>Copyright (C) 2019 Clifford E Ford, All rights reserved.</copyright>
        <license>GNU/GPLv3 http://www.gnu.org/licenses/gpl-3.0.html</license>
        <!--  The version string is recorded in the components table -->
        <version>0.1.0</version>
        <!-- The description is optional and defaults to the name -->
        <description>PLG_CONTENT_J4XDEMOSTOC_XML_DESCRIPTION</description>
    
        <files>
            <filename plugin="j4xdemostoc">j4xdemostoc.php</filename>
        </files>
        
        <languages folder="administrator/language">
            <language tag="en-GB">language/en-GB/en-GB.plg_content_j4xdemostoc.ini</language>
            <language tag="en-GB">language/en-GB/en-GB.plg_content_j4xdemostoc.sys.ini</language>
        </languages>
    
        <config>
            <fields name="params">
                <fieldset name="basic">
                    <field
                        name="help"
                        type="text"
                        label="PLG_CONTENT_J4XDEMOSTOC_PARAMS_APPLIES_LABEL"
                        description="PLG_CONTENT_J4XDEMOSTOC_PARAMS_APPLIES_DESC"
                        default="PLG_CONTENT_J4XDEMOSTOC_PARAMS_APPLIES_DEFAULT"
                        readonly="readonly"
                    />
    
                    <field 
                        name="toc_class"
                        type="list"
                        label="PLG_CONTENT_J4XDEMOSTOC_PARAMS_CARD_CLASS_LABEL"
                        description="PLG_CONTENT_J4XDEMOSTOC_PARAMS_CARD_CLASS_DESC"
                        default="light"
                    >
                        <option value="light">PLG_CONTENT_J4XDEMOSTOC_PARAMS_CARD_CLASS_OPTION_LIGHT</option>
                        <option value="info">PLG_CONTENT_J4XDEMOSTOC_PARAMS_CARD_CLASS_OPTION_INFO</option>
                        <option value="primary">PLG_CONTENT_J4XDEMOSTOC_PARAMS_CARD_CLASS_OPTION_PRIMARY</option>
                        <option value="secondary">PLG_CONTENT_J4XDEMOSTOC_PARAMS_CARD_CLASS_OPTION_SECONDARY</option>
                        <option value="success">PLG_CONTENT_J4XDEMOSTOC_PARAMS_CARD_CLASS_OPTION_SUCCESS</option>
                        <option value="danger">PLG_CONTENT_J4XDEMOSTOC_PARAMS_CARD_CLASS_OPTION_DANGER</option>
                        <option value="warning">PLG_CONTENT_J4XDEMOSTOC_PARAMS_CARD_CLASS_OPTION_WARNING</option>
                    </field>
    
                    <field 
                        name="toc_head_class"
                        type="list"
                        label="PLG_CONTENT_J4XDEMOSTOC_PARAMS_CARD_HEAD_CLASS_LABEL"
                        description="PLG_CONTENT_J4XDEMOSTOC_PARAMS_CARD_HEAD_CLASS_DESC"
                        default="center"
                    >
                        <option value="center">center</option>
                        <option value="left">left</option>
                    </field>
    
                    <field 
                        name="list_indent"
                        type="list"
                        label="PLG_CONTENT_J4XDEMOSTOC_PARAMS_CONTENTS_INDENT_LABEL"
                        description="PLG_CONTENT_J4XDEMOSTOC_PARAMS_CONTENTS_INDENT_DESC"
                        default="1"
                    >
                        <option value="1">1rem</option>
                        <option value="2">2rem</option>
                        <option value="3">3rem</option>
                    </field>
    
                </fieldset>
            </fields>
        </config>
    </extension>

The language Files

In case you were wondering, the sys.ini file is used by the system during installation and when listing plugins. The .ini file is used when displaying the plugin parameters form. And if any strings appear in the site interface they can be located here too. That is unusual but it provides for possible alternative titles, such as Contents or Index. Those terms could be included in the placeholder but would then be untranslatable.

The en-GB.plg_content_j4xdemostoc.sys.ini file:

PLG_CONTENT_J4XDEMOSTOC="Content J4xdemos ToC"
PLG_CONTENT_J4XDEMOSTOC_XML_DESCRIPTION="Render Article Table of Contents."

The en-GB.plg_content_j4xdemostoc.ini file:

PLG_CONTENT_J4XDEMOSTOC_CONTENTS="Contents"

PLG_CONTENT_J4XDEMOSTOC_PARAMS_APPLIES_LABEL="Applies to"
PLG_CONTENT_J4XDEMOSTOC_PARAMS_APPLIES_DESC="Usage: {ToC} or {toc} in the article where the rable of contents is to appear."
PLG_CONTENT_J4XDEMOSTOC_PARAMS_APPLIES_DEFAULT="Articles"

PLG_CONTENT_J4XDEMOSTOC_PARAMS_CARD_CLASS_LABEL="Contents card class"
PLG_CONTENT_J4XDEMOSTOC_PARAMS_CARD_CLASS_DESC="For card and border colours"

PLG_CONTENT_J4XDEMOSTOC_PARAMS_CARD_CLASS_OPTION_LIGHT="Bootstrap light: Light grey"
PLG_CONTENT_J4XDEMOSTOC_PARAMS_CARD_CLASS_OPTION_INFO="Bootstrap info: Aquamarine"
PLG_CONTENT_J4XDEMOSTOC_PARAMS_CARD_CLASS_OPTION_PRIMARY="Bootstrap primary: Blue"
PLG_CONTENT_J4XDEMOSTOC_PARAMS_CARD_CLASS_OPTION_SECONDARY="Bootstrap secondary: Grey"
PLG_CONTENT_J4XDEMOSTOC_PARAMS_CARD_CLASS_OPTION_SUCCESS="Bootstrap success: Green"
PLG_CONTENT_J4XDEMOSTOC_PARAMS_CARD_CLASS_OPTION_DANGER="Bootstrap danger: Red"
PLG_CONTENT_J4XDEMOSTOC_PARAMS_CARD_CLASS_OPTION_WARNING="Bootstrap Warning: Yellow"

PLG_CONTENT_J4XDEMOSTOC_PARAMS_CARD_HEAD_CLASS_LABEL="Card heading class"
PLG_CONTENT_J4XDEMOSTOC_PARAMS_CARD_HEAD_CLASS_DESC="Should the word 'Contents' appear at the left or center?"

PLG_CONTENT_J4XDEMOSTOC_PARAMS_CONTENTS_INDENT_LABEL="List item indent"
PLG_CONTENT_J4XDEMOSTOC_PARAMS_CONTENTS_INDENT_DESC="The distance to indent the content list links in rem units."

The Plugin Administration Form

This form is available via the Administrator interface in System -> Manage -> Plugins. Remember to enable the plugin for it to take effect.

The Parameters Form

The first item in the form, Applies to, is a reminder that plugin works only with articles and not featured articles or modules. or anything else.

The remaining items set the appearance of the Toc:

  • Contents card class selects one of the Bootstrap card styles from a drop-down list.
  • Card heading class selects left or center alignment of the header text.
  • List item indent selects the relative amount of indentation of each item in the contents list.

Note that for accessibility reasons it is better to show the form item descriptions in plain text rather than tooltips or popovers.

The Plugin Code

Much of the plugin code is routine php for manipulating text. Please see the code on github for the full listing. It is well commented. Here are the significant lines from j4demostoc.php:

The class statement

    defined('_JEXEC') or die;
    
    use Joomla\CMS\Plugin\CMSPlugin;
    use Joomla\CMS\Language\Text;
    
    // See https://docs.joomla.org/J4.x:Creating_a_Plugin_for_Joomla
    
    /**
     * Create and or render an article table of contents.
     *
     * @since  4.0
     */
    class PlgContentJ4xdemostoc extends CMSPlugin
    {

Do not forget to start the code with the check for loading by Joomla or die!

The use statements tell the loader which external classes are needed by this plugin. CMSPlugin is the parent class of PlgContentJ4xdemostoc and Text is used to translate the List of Contents heading.

Notice the class statement: It must begin with Plg followed by the plugin group, in this case Content, followed by the name of this plugin J4xdemostoc.

The doc blocks are important too! They are used by automated documentation tools.

The entry point

    /**
     * Look for {ToC} or {toc} and replace with Contents Panel.
     *
     * @return  void
     *
     * @since   4.0
     */
    public function onContentPrepare($context, &$article, &$params, $page = 0)
    {

Important: To invoke the entry point function a named event must have been triggered. The onContentPrepare event is generated during article creation so this plugin just needs to respond to that event. It is not necessary, even wrong, to try to trigger that named event anywhere else.

The $context variable will contain a string such as 'com_content.article' or 'com_content.featured'. That can be tested to if or how this plugin should respond.

The $article variable contains an object passed by reference. Any changes made to it in the plugin are set in the object. So the plugin does not need to return any variables.

The $params variable contains the article parameters.

Testing what action to take

        // featured article with readmore separator needs {ToC} removed
        if ($context === 'com_content.featured')
        {
            $article->text = str_ireplace('{ToC}', '', $article->text);
            return;
        }

        // the context could be something other than com_content
        // such as a module - in which case do nothing and return
        if ($context !== 'com_content.article')
        {
            return;
        }

        // return if there is no {ToC} in the article content
        if (stripos($article->text,'{ToC}') === false)
        {
            return;
        }

        // Load plugin language file only when needed
        $this->loadLanguage();

All of this is explained in the in-code comments so no further explanation is needed here. The rest of the plugin code is text manipulation: extracting the headings to create a list of contents with links and modifying the headings to add anchors.

Bugs

There are going to be bugs. Probably not for the author but probably so for anyone else. So step through the code with the debugger and look at the variables; use temporary var_dump() statements or echo and die statements; and use the browser Developer Tools for display problems. First time around I forgot to handle the {Toc} placeholder in featured articles. And the Content card display was poor.

On the Bootstrap card display: the card background color is set to one of the featured colors. The card header color is then set to black with an opacity of 0.03. This is just about perceptible on the Bootstrap site but not at all with the Cassiopeia template. An added in-line style statement upping the header background opacity to 0.1 produced a much improved distinction.

Screenshot

This is the a cropped screenshot of the outcome for a tutorial article written with Joomla 4.0 Alpha 10:

The rendered Contents Panel

The other Bootstrap colour schemes are ghastly with the Cassopeia template.

Clifford E Ford August 2019