Events and Plugin Hooks

Overview

Elgg has an event system that can be used to replace or extend core functionality.

Plugins influence the system by creating handlers (callables such as functions and methods) and registering them to handle two types of events: Elgg Events and Plugin Hooks.

When an event is triggered, a set of handlers is executed in order of priority. Each handler is passed arguments and has a chance to influence the process. After execution, the “trigger” function returns a value based on the behavior of the handlers.

Elgg Events vs. Plugin Hooks

The main differences between Elgg Events and Plugin Hooks are:

  1. Most Elgg events can be cancelled; unless the event is an “after” event, a handler that returns false can cancel the event, and no more handlers are called.
  2. Plugin hooks cannot be cancelled; all handlers are always called.
  3. Plugin hooks pass an arbitrary value through the handlers, giving each a chance to alter along the way.

Elgg Events

Elgg Events are triggered when an Elgg object is created, updated, or deleted; and at important milestones while the Elgg framework is loading. Examples: a blog post being created or a user logging in.

Unlike Plugin Hooks, most Elgg events can be cancelled, halting the execution of the handlers, and possibly cancelling an some action in the Elgg core.

Each Elgg event has a name and an object type (system, user, object, relationship name, annotation, group) describing the type of object passed to the handlers.

Before and After Events

Some events are split into “before” and “after”. This avoids confusion around the state of the system while in flux. E.g. Is the user logged in during the [login, user] event?

Before Events have names ending in ”:before” and are triggered before something happens. Like traditional events, handlers can cancel the event by returning false.

After Events, with names ending in ”:after”, are triggered after something happens. Unlike traditional events, handlers cannot cancel these events; all handlers will always be called.

Where before and after events are available, developers are encouraged to transition to them, though older events will be supported for backwards compatibility.

Elgg Event Handlers

Elgg event handlers should have the following prototype:

/**
 * @param string $event       The name of the event
 * @param string $object_type The type of $object (e.g. "user", "group")
 * @param mixed  $object      The object of the event
 *
 * @return bool if false, the handler is requesting to cancel the event
 */
function event_handler($event, $object_type, $object) {
    ...
}

If the handler returns false, the event is cancelled, preventing execution of the other handlers. All other return values are ignored.

Register to handle an Elgg Event

Register your handler to an event using elgg_register_event_handler:

elgg_register_event_handler($event, $object_type, $handler, $priority);

Parameters:

  • $event The event name.
  • $object_type The object type (e.g. “user” or “object”) or ‘all’ for all types on which the event is fired.
  • $handler The callback of the handler function.
  • $priority The priority - 0 is first and the default is 500.

Object here does not refer to an ElggObject but rather a string describing any object in the framework: system, user, object, relationship, annotation, group.

Example:

// Register the function myPlugin_handle_login() to handle the
// user login event with priority 400.
elgg_register_event_handler('login', 'user', 'myPlugin_handle_login', 400);

Trigger an Elgg Event

You can trigger a custom Elgg event using elgg_trigger_event:

if (elgg_trigger_event($event, $object_type, $object)) {
    // Proceed with doing something.
} else {
    // Event was cancelled. Roll back any progress made before the event.
}

For events with ambiguous states, like logging in a user, you should use Before and After Events by calling elgg_trigger_before_event or elgg_trigger_after_event. This makes it clear for the event handler what state to expect and which events can be cancelled.

// handlers for the user, login:before event know the user isn't logged in yet.
if (!elgg_trigger_before_event('login', 'user', $user)) {
        return false;
}

// handlers for the user, login:after event know the user is logged in.
elgg_trigger_after_event('login', 'user', $user);

Parameters:

  • $event The event name.
  • $object_type The object type (e.g. “user” or “object”).
  • $object The object (e.g. an instance of ElggUser or ElggGroup)

The function will return false if any of the selected handlers returned false and the event is stoppable, otherwise it will return true.

Plugin Hooks

Plugin Hooks provide a way for plugins to collaboratively determine or alter a value. For example, to decide whether a user has permission to edit an entity or to add additional configuration options to a plugin.

A plugin hook has a value passed into the trigger function, and each handler has an opportunity to alter the value before it’s passed to the next handler. After the last handler has completed, the final value is returned by the trigger.

Plugin Hook Handlers

Plugin hook handlers should have the following prototype:

/**
 * @param string $hook    The name of the plugin hook
 * @param string $type    The type of the plugin hook
 * @param mixed  $value   The current value of the plugin hook
 * @param mixed  $params  Data passed from the trigger
 *
 * @return mixed if not null, this will be the new value of the plugin hook
 */
function plugin_hook_handler($hook, $type, $value, $params) {
    ...
}

If the handler returns no value (or null explicitly), the plugin hook value is not altered. Otherwise the return value becomes the new value of the plugin hook. It will then be passed to the next handler as $value.

Register to handle a Plugin Hook

Register your handler to a plugin hook using elgg_register_plugin_hook_handler:

elgg_register_plugin_hook_handler($hook, $type, $handler, $priority);

Parameters:

  • $hook The name of the plugin hook.
  • $type The type of the hook or ‘all’ for all types.
  • $handler The callback of the handler function.
  • $priority The priority - 0 is first and the default is 500.

Type can vary in meaning. It may mean an Elgg entity type or something specific to the plugin hook name.

Example:

// Register the function myPlugin_hourly_job() to be called with priority 400.
elgg_register_plugin_hook_handler('cron', 'hourly', 'myPlugin_hourly_job', 400);

Trigger a Plugin Hook

You can trigger a custom plugin hook using elgg_trigger_plugin_hook:

// filter $value through the handlers
$value = elgg_trigger_plugin_hook($hook, $type, $params, $value);

Parameters:

  • $hook The name of the plugin hook.
  • $type The type of the hook or ‘all’ for all types.
  • $params Arbitrary data passed from the trigger to the handlers.
  • $value The initial value of the plugin hook.

Warning

The $params and $value arguments are reversed between the plugin hook handlers and trigger functions!

Unregister Event/Hook Handlers

The functions elgg_unregister_event_handler and elgg_unregister_plugin_hook_handler can be used to remove handlers already registered by another plugin or Elgg core. The parameters are in the same order as the registration functions, except there’s no priority parameter.

elgg_unregister_event_handler('login', 'user', 'myPlugin_handle_login');

Anonymous functions or invokable objects cannot be unregistered, but dynamic method callbacks can be unregistered by giving the static version of the callback:

$obj = new MyPlugin\Handlers();
elgg_register_plugin_hook_handler('foo', 'bar', [$obj, 'handleFoo']);

// ... elsewhere

elgg_unregister_plugin_hook_handler('foo', 'bar', 'MyPlugin\Handlers::handleFoo');

Even though the event handler references a dynamic method call, the code above will successfully remove the handler.