Validate plugin setting

Here is how the current form is created:
rma.php

function rma_init() {
    $plugin = new Plugin(); // Create container
    $plugin['path'] = realpath(plugin_dir_path(__FILE__)) . DIRECTORY_SEPARATOR;
    $plugin['url'] = plugin_dir_url(__FILE__);
    $plugin['version'] = '0.1.0';
    $plugin['properties'] = [
        'parent_slug' => 'options-general.php',
        'page_title' => '<h2>Remote Member Authentication</h2>',
        'menu_title' => 'Remote Member Authentication',
        'capability' => 'manage_options',
        'menu_slug' => 'rma-settings',
        'option_group' => 'rma_option_group',
        'section_heading' => '',
        'submit_label' => 'Save stuff',
        'options' => [
            ['fieldName' => 'rma_user_data_uri',
                'label' => 'User data URI',],
            ['fieldName' => 'rma_auth_type',
                'label' => 'Authentication type'],
            ['fieldName' => 'rma_auth_type_none',
                'label' => ''],
            ['fieldName' => 'rma_auth_type_api_key',
                'label' => 'API key'],
            ['fieldName' => 'rma_auth_type_basic_username',
                'label' => 'Username'],
            ['fieldName' => 'rma_auth_type_basic_password',
                'label' => 'Password'],
        ],
    ];
    $plugin['settings_page'] = function ( $plugin ) {
        static $object;

        if (null !== $object) {
            return $object;
        }
        return new SettingsPage($plugin['properties']);
    };
    $templates = [
        './Templates/member-content.php' => 'Restricted Member Content',
    ];
    //initialization functions
    $templater = new Rma\PageTemplater($templates);
    add_action('init', ['Rma\Tools', 'member_password_check']);
    add_action( 'admin_enqueue_scripts', 'rmaQueueScripts' );
    add_action('plugins_loaded', array('Rma\PageTemplater', 'get_instance'));
    add_shortcode('member_sign_in', ['Rma\Shortcodes\SigninForm', 'createSignInForm']);
    $plugin->run();
}

SettingsPage.php

namespace Rma;

class SettingsPage extends WpSubPage
{

    public function render_settings_page() {
        $option_group = $this->properties['option_group'];
        $page = $this->properties['menu_slug'];
        $submit = $this->properties['submit_label'];
        echo $this->properties['page_title'];
        ?>
        <div class="wrap">
            <form method="post" action="options.php">
                <?php
                settings_fields($option_group);
                do_settings_sections($page);
                submit_button($submit);
                ?>
            </form>
        </div>
        <?php
    }
}

WpSubPage.php

namespace Rma;

abstract class WpSubPage
{

    protected $properties;

    public function __construct($properties) {
        $this->properties = $properties;
    }

    public function run() {
        add_action('admin_menu', array($this, 'add_menu_and_page'));
        add_action('admin_init', array($this, 'register_settings'));
    }

    public function add_menu_and_page() {

        add_submenu_page(
                $this->properties['parent_slug'], $this->properties['page_title'], $this->properties['menu_title'], $this->properties['capability'], $this->properties['menu_slug'], array($this, 'render_settings_page')
        );
    }

    public function register_settings() {
        $group = $this->properties['option_group'];
        $options = $this->properties['options'];
        $page = $this->properties['menu_slug'];
        add_settings_section($page . '_main', '', function() {
            echo $this->properties['section_heading'];
        }, $page);
        foreach ($options as $option) {
            $fieldName = $option['fieldName'];
            if (method_exists('Rma\Tools', 'validate_' . $fieldName)) {
                register_setting($group, $fieldName, ['Rma\Tools', 'validate_' . $fieldName]);
            } else {
                register_setting($group, $fieldName);
            }
            add_settings_field($fieldName, $option['label'], ['Rma\Fields', 'fieldHtml'], $page, $page . '_main', ['fieldName' => $fieldName]);
        }
    }

    public function render_settings_page() {
        
    }

}

Thanks for your reply. Having read this I got the impression that WP_Customize_Settings::validate() was a step toward true full-form validation. The register_setting() callback argument appears to be intended to tidy up settings rather than perform validation. Also, the form I’m trying to validate is a plugin’s settings form.

It appears that to validate a form by requiring all inputs to be valid before saving any values requires coding the form from scratch. So be it.

I think you’re right about add_settings_error() as the missing link. I’d not seen that before; it’ll be much easier to add to existing code than starting over again. Thanks.