Hooks API

Since 4.3

This page describes the Hooks API which is a replacement for some of the lib.php based one-to-many plugin callbacks implementing on PSR-14.

The most common use case for hooks is to allow customisation of standard plugins or core functionality through hook callbacks in local plugins. For example adding a custom institution password policy that applies to all enabled authentication plugins through a new local plugin.

Hook Policies

New hooks added to Moodle from Moodle 4.4 onwards must meet the following rules:

• When describing a hook which happens before or after, the following modifiers are allowed:
  • before
  • after
• When a modifier is used, it should be used as a prefix
• Hooks belonging to a core subsystem should be located within that subsystem, and not in the core subsystem
• Hook descriptions should be in English, and should not use language strings or support translation

Examples of valid hook names

• \core_course\hook\before_course_visibility_changed
• \core_form\hook\before_form_validation
• \core_form\hook\after_form_validation
• \core\hook\navigation\before_render

General concepts

Mapping to PSR-14

Moodle does not allow camelCase for naming of classes and method and Moodle already has events, however the PSR-14 adherence has higher priority here.

PSR-14	Hooks
Event	Hook
Listener	Hook callback
Emitter	Hook emitter
Dispatcher	Hook dispatcher (implemented in Hook manager)
Listener Provider	Hook callback provider (implemented in Hook manager)

Hook emitter

A Hook emitter is a place in code where core or a plugin needs to send or receive information to/from any other plugins. The exact type of information flow facilitated by hooks is not defined.

Hook instance

Information passed between subsystem and plugins is encapsulated in arbitrary PHP class instances. These can be in any namespace, but generally speaking they should be placed in the some_component\hook\* namespace.

Hooks are encouraged to describe themselves and to provide relevant metadata to make them easier to use and discover. There are two ways to describe a hook:

• implement the \core\hook\described_hook interface, which has two methods:
  • get_description(): string;
  • get_tags(): array;
• add an instance of the following attributes to the class:
  • \core\attribute\label(string $description)
  • \core\attribute\tags($a, $set, $of, $tags, ...)
  • \core\attribute\hook\replaces_callbacks('a_list_of_legacy_callbacks', 'that_this_hook_replaces')

Hook callback

The code executing a hook does not know in advance which plugin is going to react to a hook.

Moodle maintains an ordered list of callbacks for each class of hook. Any plugin is free to register its own hook callbacks by creating a db/hooks.php file. The specified plugin callback method is called whenever a relevant hook is dispatched.

Hooks overview page

The Hooks overview page lists all hooks that may be triggered in the system together with all registered callbacks. It can be accessed by developers and administrators from the Site administration menu.

This page is useful especially when adding custom local plugins with hook callbacks that modify standard Moodle behaviour.

In special cases administrators may override default hook callback priorities or disable some unwanted callbacks completely:

/config.php

$CFG->hooks_callback_overrides = [
    \mod_activity\hook\installation_finished::class => [
        'test_otherplugin\\callbacks::activity_installation_finished' => ['disabled' => true],
    ],
];

The hooks overview page will automatically list any hook which is placed inside any *\hook\* namespace within any Moodle component. If you define a hook which is not in this namespace then you must also define a new \core\hook\discovery_agent implementation in [component]\hooks.

Adding new hooks

1. Developer first identifies a place where they need to ask or inform other plugins about something.
2. Depending on the location a new class implementing core\hook\described_hook is added to core\hook\* or some_plugin\hook\* namespace as appropriate.
3. Optionally the developer may wish to allow the callback to stop any subsequent callbacks from receiving the object. If so, then the object should implement the Psr\EventDispatcher\StoppableEventInterface interface.
4. Optionally if any data needs to be sent to hook callbacks, the developer may add internal hook constructor, some instance properties for data storage and public methods for data access from callbacks.

Hook classes may be any class you like. When designing a new Hook, you should think about how consumers may wish to change the data they are passed.

All hook classes should be defined as final, if needed traits can help with code reuse in similar hooks.

Hooks not located in standard locations

If you define a hook which is not in the [component]\hook\* namespace then you must also define a new \core\hook\discovery_agent implementation in [component]\hooks.

/mod/example/classes/hooks.php

<?php

namespace mod_example;

class hooks implements \core\hook\hook_discovery_agent {
    public static function discover_hooks(): array {
        return [
            [
                'class' => \mod_example\local\entitychanges\create_example::class,
                'description' => 'A hook fired when an example was created',
            ],
        ];
    }
}

Example of hook creation

Imagine mod_activity plugin wants to notify other plugins that it finished installation, then mod_activity plugin developer adds a new hook and calls it at the end of plugin installation process.

Described by attribute

/mod/activity/classes/hook/installation_finished.php

<?php
namespace mod_activity\hook;

#[\core\attribute\label('Hook dispatched at the very end of installation of mod_activity plugin.')]
#[\core\attribute\tags('installation')]
class installation_finished implements \core\hook\described_hook {
    public function __construct(
        public function string $version,
    ) {
    }
}

Described using Interface

/mod/activity/classes/hook/installation_finished.php

<?php
namespace mod_activity\hook;

class installation_finished implements \core\hook\described_hook {
    public function __construct(
        public function string $version,
    ) {
    }

    public static function get_hook_description(): string {
        return 'Hook dispatched at the very end of installation of mod_activity plugin.';
    }

    public static function get_hook_tags(): array {
        return ['installation'];
    }
}

/mod/activity/db/install.php

<?php
function xmldb_activity_install() {
    $hook = new \mod_activity\hook\installation_finished();
    \core\di::get(\core\hook\manager::class())->dispatch($hook);
}

Dispatching hooks

Once a hook has been created, it needs to be dispatched. The dispatcher is responsible for ordering all listeners and calling them with the hook data.

The hook manager is responsible for dispatching hook instances using the dispatch(object $hook) method.

The dispatch method is an instance method and requires an instance of the hook manager.

From Moodle 4.4 you should make use of the Dependency Injection system, for example:

Dispatching a hook with DI

\core\di::get(\core\hook\manager::class())->dispatch($hook);

Using DI for dependency injection has the benefit that the hook manager can use fixture callbacks to test a range of behaviours, for example:

Mocking a hook listener

// Unit test.
public function test_before_standard_footer_html_hooked(): void {
    // Load the callback classes.
    require_once(__DIR__ . '/fixtures/core_renderer/before_standard_footer_html_callbacks.php');

    // Replace the version of the manager in the DI container with a phpunit one.
    \core\di::set(
        \core\hook\manager::class,
        \core\hook\manager::phpunit_get_instance([
            // Load a list of hooks for `test_plugin1` from the fixture file.
            'test_plugin1' => __DIR__ .
                '/fixtures/core_renderer/before_standard_footer_html_hooks.php',
        ]),
    );

    $page = new moodle_page();
    $renderer = new core_renderer($page, RENDERER_TARGET_GENERAL);

    $html = $renderer->standard_footer_html();
    $this->assertIsString($html);
    $this->assertStringContainsString('A heading can be added', $html);
}

// fixtures/core_renderer/before_standard_footer_html_callbacks.php
final class before_standard_footer_html_callbacks {
    public static function before_standard_footer_html(
        \core\hook\output\before_standard_footer_html $hook,
    ): void {
        $hook->add_html("<h1>A heading can be added</h1>");
    }
}

// fixtures/core_renderer/before_standard_footer_html_hooks.php
$callbacks = [
    [
        'hook' => \core\hook\output\before_standard_footer_html::class,
        'callback' => [
            \test_fixtures\core_renderer\before_standard_footer_html_callbacks::class,
            'before_standard_footer_html',
        ],
    ],
];

note

Prior to Moodle 4.3 the only way to dispatch a hook is by accessing the manager instance:

\core\hook\manager::get_instance()->dispatch($hook);

This approach is harder to test in situ.

Registering of hook callbacks

Any plugin is free to register callbacks for all core and plugin hooks. The registration is done by adding a db/hooks.php file to plugin. Callbacks may be be provided as a PHP callable in either:

• string notation, in the form of some\class\name::static_method; or
• array notation, in the form of [\some\class\name::class, 'static_method'].

Use of array notation

Since 4.4

Support for Array notated callbacks was introduced in Moodle 4.4. If you are writing a callback for a Moodle 4.3 site, you must use the string notation.

Hook callbacks are executed in the order of their priority from highest to lowest. Any guidelines for callback priority should be described in hook descriptions if necessary.

caution

Callbacks are executed during system installation and all upgrades, the callback methods must verify the plugin is in correct state. Often the easies way is to use function during_initial_install() or version string from the plugin configuration.

Example of hook callback registration

First developer needs to add a new static method to some class that accepts instance of a hook as parameter.

/local/stuff/classes/local/hook_callbacks.php

<?php
namespace local_stuff\local;
use \mod_activity\hook\installation_finished;

class hook_callbacks {
    public static function activity_installation_finished(installation_finished $hook): void {
        if (during_initial_install()) {
            return;
        }
        if (!get_config('local_stuff', 'version') {
            return;
        }
        // Do something...
    }
}

Then the developer has to register this new method as the hook callback by adding it to the db/hooks.php file.

/local/stuff/db/hooks.php

<?php
$callbacks = [
    [
        'hook' => mod_activity\hook\installation_finished::class,
        'callback' => [\local_stuff\local\hook_callbacks::class, 'activity_installation_finished'],
        'priority' => 500,
    ],
];

Callback registrations are cached, so developers should to either increment the version number for the component they place the hook into. During development it is also possible to purge caches.

In this particular example, the developer would probably also add some code to db/install.php to perform the necessary action in case the hook gets called before the local_stuff plugin is installed.

Deprecation of legacy lib.php callbacks

Hooks are a direct replacement for one-to-many lib.php callback functions that were implemented using the get_plugins_with_function(), plugin_callback(), or component_callback() functions.

If a hook implements the core\hook\deprecated_callback_replacement interface, and if deprecated lib.php callbacks can be listed in get_deprecated_plugin_callbacks() hook method then developers needs to only add extra parameter to existing legacy callback functions and the hook manager will trigger appropriated deprecated debugging messages when it detects plugins that were not converted to new hooks yet.

Legacy fallback

Please note it is possible for plugin to contain both legacy lib.php callback and PSR-14 hook callbacks.

This allows community contributed plugins to be made compatible with multiple Moodle branches.

The legacy lib.php callbacks are automatically ignored if hook callback is present.

Example how to migrate legacy callback

This example describes migration of after_config callback from the very end of lib/setup.php.

First we need a new hook:

Described by attribute

/lib/classes/hook/after_config.php

<?php
namespace core\hook;

use core\attribute;

#[attribute\label('Hook dispatched at the very end of lib/setup.php')]
#[attribute\tags('config')]
#[attribute\hook\replaces_callbacks('after_config')]
final class after_config {
}

Described using Interface

/lib/classes/hook/after_config.php

<?php
namespace core\hook;

final class after_config implements described_hook, deprecated_callback_replacement {
    public static function get_hook_description(): string {
        return 'Hook dispatched at the very end of lib/setup.php';
    }

    public static function get_hook_tags(): array {
        return ['config'];
    }

    public static function get_deprecated_plugin_callbacks(): array {
        return ['after_config'];
    }
}

The hook needs to be emitted immediately after the current callback execution code, and an extra parameter $migratedtohook must be set to true in the call to get_plugins_with_function().

/lib/setup.php

// Allow plugins to callback as soon possible after setup.php is loaded.
$pluginswithfunction = get_plugins_with_function('after_config', 'lib.php', true, true);
foreach ($pluginswithfunction as $plugins) {
    foreach ($plugins as $function) {
        try {
            $function();
        } catch (Throwable $e) {
            debugging("Exception calling '$function'", DEBUG_DEVELOPER, $e->getTrace());
        }
    }
}
// Dispatch the new Hook implementation immediately after the legacy callback.
\core\di::get(\core\hook\manager::class())->dispatch(new \core\hook\after_config());

Hooks which contain data

It is often desirable to pass a data object when dispatching hooks.

This can be useful where you are passing code that consumers may wish to change.

Since the hook is an arbitrary PHP object, it is possible to create any range of public data and/or method you like and for the callbacks to use those methods and properties for later consumption.

/lib/classes/hook/block_delete_pre.php

<?php

namespace core\hook;

use core\attribute;

#[attribute\label('A hook dispatched just before a block instance is deleted')]
#[attribute\hook\replaces_callbacks('pre_block_delete')]
final class block_delete_pre {
    public function __construct(
        public readonly \stdClass $blockinstance,
    ) {}
}

When dispatching the hook, it behaves as any other normal PHP Object:

/lib/blocklib.php

// Allow plugins to use this block before we completely delete it.
if ($pluginsfunction = get_plugins_with_function('pre_block_delete', 'lib.php', true, true)) {
    foreach ($pluginsfunction as $plugintype => $plugins) {
            foreach ($plugins as $pluginfunction) {
                $pluginfunction($instance);
            }
        }
    }
}
$hook = new \core\hook\block_delete_pre($instance);
\core\di::get(\core\hook\manager::class())->dispatch($hook);

Hooks which can be stopped

In some situations it is desirable to allow a callback to stop execution of a hook. This can happen in situations where the hook contains that should only be set once.

The Moodle hooks implementation has support for the full PSR-14 specification, including Stoppable Events.

To make use of Stoppable events, the hook simply needs to implement the Psr\EventDispatcher\StoppableEventInterface interface.

/lib/classes/hook/block_delete_pre.php

<?php

namespace core\hook;

use core\attribute;

#[attribute\label('A hook dispatched just before a block instance is deleted')]
#[attribute\hook\replaces_callbacks('pre_block_delete')]
final class block_delete_pre implements
    Psr\EventDispatcher\StoppableEventInterface
{
    public function __construct(
        public readonly \stdClass $blockinstance,
    ) {}

    public function isPropagationStopped(): bool {
        return $this->stopped;
    }

    public function stop(): void {
        $this->stopped = true;
    }
}

A callback will only be called if the hook was not stopped before-hand. Depending on the hook implementation, it can stop he

/local/myplugin/classes/callbacks.php

<?php

namespace local_myplugin;

class callbacks {
    public static function block_pre_delete(\core\hook\block_delete_pre $hook): void {
        // ...
        $hook->stop();
    }
}

Tips and Tricks

Whilst not being formal requirements, you are encouraged to:

• describe and tag your hook as appropriate using either:
  • the \core\hook\described_hook interface; or
  • the \core\attribute\label and \core\attribute\tags attributes
• make use of constructor property promotion combined with readonly properties to reduce unnecessary boilerplate.