Creating Admin Plugin - Content of a page is displayed at global scope as well

Question

I've started creating a WP plugin which should work only in Admin Area.

I've managed to create top-level menu with required sub-menu items, but when I try to print content of my page, it prints it twice, in a global scope of Admin panel and inside the page itself.

Main plugin file loads menu settings and separate page classes: require_once('settings.php');

require_once('pages/VmSettingsPage.php');
require_once('pages/VmTradersPage.php');

$settingsPage = new VmSettingsPage();
$tradersPage = new VmTradersPage();

Menu settings:

add_action('admin_menu', 'cvmAdminMenu');
function cvmAdminMenu() {
    add_menu_page('VM Tool', 'VM Tool',
        'manage_options', 'camden-vmtool', ['VmSettingsPage','pageInit']);

    add_submenu_page('camden-vmtool', 'Traders Pages', 'Traders Pages', 'manage_options',
        'camden-vmtool-traders', ['VmTradersPage','pageInit']);
    add_submenu_page('camden-vmtool', 'Style Pages', 'Style Pages', 'manage_options',
        'camden-vmtool-style', 'cvmRenderStyle');
    add_submenu_page('camden-vmtool', 'Cat Navs', 'Cat Navs', 'manage_options',
        'camden-vmtool-cat-navs', 'cvmRenderCatnavs');
}

Settings Page class:

<?php

class VmSettingsPage {
    private $options;

    public function __construct()
    {
        add_action('admin_init', [$this, 'pageInit']);
    }

    public function pageInit() {
        echo 23123;
    }
}

Question #1 - The above code will output 23123 twice, when visiting plugin page and when not. How do I make it print the content of my page only when page is the current screen? Basically: http://localhost/wp-admin/admin.php?page=camden-vmtool

Question #2 - How to preload only one page class based on currently opened page? I will have around 4 pages, and It feels bad to load classes at once, just to have the class instance initiated.

BTW, get_current_screen's base parameter != $_GET['page'], so I cannot even match those to print correct content.

Thanks

Answer 1

Question #1 - The above code will output 23123 twice, when visiting plugin page and when not. How do I make it print the content of my page only when page is the current screen? Basically: http://localhost/wp-admin/admin.php?page=camden-vmtool

When you initialize the class, you are hooking the pageInit() method into a hook that fires on all admin pages. You are then specifically calling that method in your add_menu_page() arguments.

Honestly, I don't understand this one:

Question #2 - How to preload only one page class based on currently opened page? I will have around 4 pages, and It feels bad to load classes at once, just to have the class instance initiated.

As to the first question, I don't know what PageInit() needs to do but given the limited information there is no need to instantiate the class beforehand. This works as is:

class VmSettingsPage {
    private $options;

    public function __construct()
    {
        add_action('admin_init', array($this, 'pageInit'));
    }

    public function pageInit() {
        echo 23123;
    }
}
// $settingsPage = new VmSettingsPage();

add_action('admin_menu', 'cvmAdminMenu');
function cvmAdminMenu() {
    add_menu_page('VM Tool', 'VM Tool',
        'manage_options', 'camden-vmtool', array('VmSettingsPage','pageInit'));

}

The class will be instantiated automatically. If you do need to instantiate the class then your callback should be different:

class VmSettingsPage {
    private $options;

    public function __construct()
    {
//         add_action('admin_init', array($this, 'pageInit'));
    }

    public function pageInit() {
        echo 23123;
    }
}
global $settingsPage;
$settingsPage = new VmSettingsPage();

add_action('admin_menu', 'cvmAdminMenu');
function cvmAdminMenu() {
  global $settingsPage;
    add_menu_page('VM Tool', 'VM Tool',
        'manage_options', 'camden-vmtool', array($settingsPage,'pageInit'));
}

If you do that, then you can't use admin_init but there is no apparent reason to use that admin_init hook at all anyway.

I would further suggest that you encapsulate everything into your class:

class VmSettingsPage {
    private $options;

    public function __construct()
    {
//         add_action('admin_init', array($this, 'pageInit'));
      add_action('admin_menu', array($this,'cvmAdminMenu'));

    }

    function cvmAdminMenu() {
        add_menu_page('VM Tool', 'VM Tool',
            'manage_options', 'camden-vmtool', array($this,'pageInit'));
    }

    public function pageInit() {
        echo 23123;
    }
}
$settingsPage = new VmSettingsPage();