Development:Events API

Overview

The Events API is a core system in Moodle to allow communication between modules.

An event is when something "interesting" happens in Moodle that is worth alerting the system about.

Any Moodle modules can trigger new events (with attached data), and other modules can elect to handle those events with custom functions that operate on the given data.

Example

Let's look at an example of how events are used to implement Messaging in Moodle 2.0. In the messaging system, textual messages are generated for users by different modules, and the user can decide how certain types of messages are displayed.

Triggering an event

When a messaging event occurs, the module should trigger a "message_send" event. In this example let's pretend someone just posted to a forum.

The forum module needs to create an object with the data that this event needs. This may vary completely for different types of events, it's just a data object.

$eventdata = new object();
$eventdata->component         = 'mod/forum';    // path in Moodle
$eventdata->name              = 'posts';        // type of message from that module (as module defines it)
$eventdata->userfrom          = $userfrom;      // user object
$eventdata->userto            = $userto;        // user object
$eventdata->subject           = "Hi there";     // short one-line subject
$eventdata->fullmessage       = "Here is the full message";      // raw text
$eventdata->fullmessageformat = FORMAT_PLAIN;   // text format
$eventdata->fullmessagehtml   = "Here is the <b>full</b> message";    // html rendered version   (optional)
$eventdata->smallmessage      = "Here is the truncated message";      // useful for plugins like sms or twitter  (optional)

Then we post the object as an event and forget about it:

events_trigger('message_send', $eventdata);

Handling an event

Modules or core code can define an events.php in the db directory which defines events they want to be notified about, and describes which of their functions or class methods should be notified. For this case, there is this definition of a handler in lib/db/events.php

$handlers = array (
    'message_send' => array (
         'handlerfile'      => '/lib/messagelib.php',
         'handlerfunction'  => 'message_send_handler',
         'schedule'         => 'instant'
     )
);

These events.php files are parsed during install / upgrade and stored in a simple database table.

Now, when a message_send event happens, all the registered handlers functions for that event will be called something like this (but with more error handling):

         include_once($CFG->dirroot.$handlers['message_send']['handlerfile']);
         call_user_func($handlers['message_send']['handlerfunction'], $eventdata);

Any code can hook into any events this way.

The handler function accepts one parameter (the event data object) and should return a boolean. Returning false indicates that there was an error and the event will be left in the event queue.

   function message_send_handler($eventdata) {
       // handle event 
       // ...
       return true;
   }

Database structure

There are 3 core tables for events. Note that if a handler is queued, and yet to be processed or processing failed, then all subsequent calls on that handler must be queued.

events_handlers

This table is for storing which components requests what type of event, and the location of the responsible handler functions.

These entries are created by parsing events.php files in all the modules, and can be rebuilt any time (during an upgrade, say).

Field	Type	Info
id	int(10)	auto increment identifier
eventname	varchar(255)	name of the event, e.g. 'message_send'
handlermodule	varchar(255)	e.g. moodle, mod/forum, block/rss_client
handlerfile	varchar(255)	path to the file of the function, eg /lib/messagelib.php
handlerfunction	text	serialized string or array describing function, suitable to be passed to call_user_func()
schedule	varchar(255)	'cron' or 'instant'.
status	int(10)	number of failed attempts to process this handler

events_queue

This table is for storing queued events. It stores only one copy of the eventdata here, and entries from this table are being references by the events_queue_handlers table.

Field	Type	Info
id	int(10)	auto increment identifier
eventdata	longtext	serialized version of the data object passed to the event handler.
stackdump	text	serialized debug_backtrace showing where the event was fired from
userid	int(10)	$USER->id when the event was fired
timecreated	int(10)	time stamp of the first time this was added

events_queue_handlers

This is the list of queued handlers for processing. The event object is retrieved from the events_queue table. When no further reference is made to the events_queue table, the corresponding entry in the events_queue table should be deleted. Entry should get deleted (?) after a successful event processing by the specified handler. The status field keeps track of failures, after it gets to a certain number (eg 10?) it should trigger an "event failed" event (that could result in admin being emailed etc, or perhaps even the originating module taking care of it or rolling something back etc).

Field	Type	Info
id	int(10)	auto increment identifier
queuedeventid	int(10)	foreign key id corresponding to the id of the event_queues table
handlerid	int(10)	foreign key id corresponding to the id of the event_handlers table
status	int(10)	number of failed attempts to process this handler
errormessage	text	if an error happened last time we tried to process this event, record it here.
timemodified	int(10)	time stamp of the last attempt to run this from the queue

Standards for naming events

All event names should follow a consistent naming pattern, such as modulename_noun_verb

If the event is being fired after the action has taken place (as in most cases) then use the past tense for the verb (created / deleted / updated / sent).

If the event is the action, then use the present tense (create / delete / update / send).

Events which exist

When we add new events to core we should always add them here too.

Under each event, list the data sent as part of the event.

Users

• user_created
  • full new record from 'user' table
• user_deleted
  • record from 'user' table before marked as deleted
• user_updated
  • full new record from 'user' table

Roles

• role_assigned (full new record from 'role_assignments' table)
• role_unassigned (record from 'role_assignments', course context only)

Courses

• course_created
  • full course record
• course_updated
  • full course record
• course_deleted
  • full course record
• course_category_deleted
  • full category record

Groups

• groups_member_added
  • groupid
  • userid
• groups_member_removed
  • groupid
  • userid
• groups_group_created
  • id
  • courseid
  • name
  • description
  • timecreated
  • timemodified
  • picture
• groups_group_updated
  • id
  • courseid
  • name
  • description
  • timecreated
  • timemodified
  • picture
• groups_group_deleted
  • id
  • courseid
  • name
  • description
  • timecreated
  • timemodified
  • picture
• groups_grouping_created
  • id
  • courseid
  • name
  • timecreated
  • timemodified
• groups_grouping_updated
  • id
  • courseid
  • name
  • timecreated
  • timemodified
• groups_grouping_deleted
  • id
  • courseid
  • name
  • timecreated
  • timemodified
• groups_members_removed (user deleted from all groups in a course)
  • courseid
  • userid
• groups_groupings_groups_removed (remove all groups from all groupings in a course)
  • courseid (as plain integer, not object)
• groups_groups_deleted (delete all groups in a course)
  • courseid (as plain integer, not object)
• groups_groupings_deleted (delete all groupings in a course)
  • courseid (as plain integer, not object)

Messaging

• message_send
  • component = 'mod/forum': path in Moodle
  • name = 'posts': type of message from that module (as module defines it)
  • userfrom = $userfrom: a user object to send from
  • userto = $userto: a user object to send to
  • subject = 'subject line': a short text line
  • fullmessage = 'full plain text': raw text as entered by user
  • fullmessageformat = FORMAT_PLAIN|FORMAT_HTML|FORMAT_MOODLE|FORMAT_MARKDOWN: the format of this text
  • fullmessagehtml = 'long html text'; html rendered version (optional)
  • smallmessage = 'short text': useful for plugins like sms or twitter (optional)

Portfolio

• portfolio_send
  • id : recordid in portfolio_tempdata table, used for itemid in file storage

Events wishlist

List of events which it would be nice to have. Please add to this list if what you want is not shown here.

• mform_print_form -- this for all types of form e.g. admin settings, user profile, module updating, + some sort of standard way of discriminiating between them e.g. if ($form->name == 'user_profile') {}. This would be better triggered at the end of the form generation process so that new bits can be inserted at any point, or existing bits could be removed.
• module_installed
• module_removed
• grade_update
• assignment_submitted
• course completed
• course module completion state changed (completed / not completed)

Provide event trigger hooks for the modules, similar to what is done for the cron service which checks the LOCAL directory for a cron file. It is already possible to define an event trigger but the core/module code must be modified to actually make use of it.

See also

• General Developer Forum thread for discussing this proposal.
• Development:Messaging_2.0