docs/hooks.doc

   1 HOOKS.DOC
   2
   3 This document describes how event hooks work in MediaWiki; how to add
   4 hooks for an event; and how to run hooks for an event.
   5
   6 ==Glossary==
   7
   8 event
   9      Something that happens with the wiki. For example: a user logs
  10      in. A wiki page is saved. A wiki page is deleted. Often there are
  11      two events associated with a single action: one before the code
  12      is run to make the event happen, and one after. Each event has a
  13      name, preferably in CamelCase. For example, 'UserLogin',
  14      'ArticleSave', 'ArticleSaveComplete', 'ArticleDelete'.
  15
  16 hook
  17      A clump of code and data that should be run when an event
  18      happens. This can be either a function and a chunk of data, or an
  19      object and a method.
  20
  21 hook function
  22      The function part of a hook.
  23
  24 ==Rationale==
  25
  26 Hooks allow us to decouple optionally-run code from code that is run
  27 for everyone. It allows MediaWiki hackers, third-party developers and
  28 local administrators to define code that will be run at certain points
  29 in the mainline code, and to modify the data run by that mainline
  30 code. Hooks can keep mainline code simple, and make it easier to
  31 write extensions. Hooks are a principled alternative to local patches.
  32
  33 Consider, for example, two options in MediaWiki. One reverses the
  34 order of a title before displaying the article; the other converts the
  35 title to all uppercase letters. Currently, in MediaWiki code, we
  36 handle this as follows:
  37
  38     function showAnArticle($article) {
  39         global $wgReverseTitle, $wgCapitalizeTitle;
  40
  41         if ($wgReverseTitle) {
  42             wfReverseTitle($article);
  43         }
  44
  45         if ($wgCapitalizeTitle) {
  46            wfCapitalizeTitle($article);
  47         }
  48
  49         # code to actually show the article goes here
  50     }
  51
  52 An extension writer, or a local admin, will often add custom code to
  53 the function -- with or without a global variable. For example,
  54 someone wanting email notification when an article is shown may add:
  55
  56     function showAnArticle($article) {
  57         global $wgReverseTitle, $wgCapitalizeTitle;
  58
  59         if ($wgReverseTitle) {
  60             wfReverseTitle($article);
  61         }
  62
  63         if ($wgCapitalizeTitle) {
  64            wfCapitalizeTitle($article);
  65         }
  66
  67         # code to actually show the article goes here
  68
  69         if ($wgNotifyArticle) {
  70            wfNotifyArticleShow($article));
  71         }
  72     }
  73
  74 Using a hook-running strategy, we can avoid having all this
  75 option-specific stuff in our mainline code. Using hooks, the function
  76 becomes:
  77
  78     function showAnArticle($article) {
  79         if (wfRunHooks('ArticleShow', $article)) {
  80             # code to actually show the article goes here
  81             wfRunHooks('ArticleShowComplete', $article);
  82         }
  83     }
  84
  85 We've cleaned up the code here by removing clumps of weird,
  86 infrequently used code and moving them off somewhere else. It's much
  87 easier for someone working with this code to see what's _really_ going
  88 on, and make changes or fix bugs.
  89
  90 In addition, we can take all the code that deals with the little-used
  91 title-reversing options (say) and put it in one place. Instead of
  92 having a little title-reversing if-block spread all over the codebase
  93 in showAnArticle, deleteAnArticle, exportArticle, etc., we can
  94 concentrate it all in an extension file:
  95
  96     function reverseArticleTitle($article) {
  97         # ...
  98     }
  99
 100     function reverseForExport($article) {
 101         # ...
 102     }
 103
 104 The setup function for the extension just has to add its hook
 105 functions to the appropriate events:
 106
 107     setupTitleReversingExtension() {
 108         global $wgHooks;
 109
 110         $wgHooks['ArticleShow'][] = reverseArticleTitle;
 111         $wgHooks['ArticleDelete'][] = reverseArticleTitle;
 112         $wgHooks['ArticleExport'][] = reverseForExport;
 113     }
 114
 115 Having all this code related to the title-reversion option in one
 116 place means that it's easier to read and understand; you don't have to
 117 do a grep-find to see where the $wgReverseTitle variable is used, say.
 118
 119 If the code is well enough isolated, it can even be excluded when not
 120 used -- making for some slight savings in memory and time at runtime.
 121 Admins who want to have all the reversed titles can add:
 122
 123     require_once('extensions/ReverseTitle.php');
 124
 125 ...to their LocalSettings.php file; those of us who don't want or need
 126 it can just leave it out.
 127
 128 The extensions don't even have to be shipped with MediaWiki; they
 129 could be provided by a third-party developer or written by the admin
 130 him/herself.
 131
 132 ==Writing hooks==
 133
 134 A hook is a chunk of code run at some particular event. It consists of:
 135
 136   * a function with some optional accompanying data, or
 137   * an object with a method and some optional accompanying data.
 138
 139 Hooks are registered by adding them to the global $wgHooks array for a
 140 given event. All the following are valid ways to define hooks:
 141
 142       $wgHooks['EventName'][] = someFunction; # function, no data
 143       $wgHooks['EventName'][] = array(someFunction, $someData);
 144       $wgHooks['EventName'][] = array(someFunction); # weird, but OK
 145
 146       $wgHooks['EventName'][] = $object; # object only
 147       $wgHooks['EventName'][] = array($object, 'someMethod');
 148       $wgHooks['EventName'][] = array($object, 'someMethod', $someData);
 149       $wgHooks['EventName'][] = array($object); # weird but OK
 150
 151 When an event occurs, the function (or object method) will be called
 152 with the optional data provided as well as event-specific parameters.
 153 The above examples would result in the following code being executed
 154 when 'EventName' happened:
 155
 156       # function, no data
 157       someFunction($param1, $param2)
 158       # function with data
 159       someFunction($someData, $param1, $param2)
 160
 161       # object only
 162       $object->onEventName($param1, $param2)
 163       # object with method
 164       $object->someMethod($param1, $param2)
 165       # object with method and data
 166       $object->someMethod($someData, $param1, $param2)
 167
 168 Note that when an object is the hook, and there's no specified method,
 169 the default method called is 'onEventName'. For different events this
 170 would be different: 'onArticleSave', 'onUserLogin', etc.
 171
 172 The extra data is useful if we want to use the same function or object
 173 for different purposes. For example:
 174
 175       $wgHooks['ArticleSaveComplete'][] = array(ircNotify, 'TimStarling');
 176       $wgHooks['ArticleSaveComplete'][] = array(ircNotify, 'brion');
 177
 178 This code would result in ircNotify being run twice when an article is
 179 saved: once for 'TimStarling', and once for 'brion'.
 180
 181 Hooks can return three possible values:
 182   * true: the hook has operated successfully
 183   * "some string": an error occurred; processing should
 184           stop and the error should be shown to the user
 185   * false: the hook has successfully done the work
 186            necessary and the calling function should skip
 187
 188 The last result would be for cases where the hook function replaces
 189 the main functionality. For example, if you wanted to authenticate
 190 users to a custom system (LDAP, another PHP program, whatever), you
 191 could do:
 192
 193     $wgHooks['UserLogin'][] = array(ldapLogin, $ldapServer);
 194
 195     function ldapLogin($username, $password) {
 196         # log user into LDAP
 197         return false;
 198     }
 199
 200 Returning false makes less sense for events where the action is
 201 complete, and will probably be ignored.
 202
 203 ==Using hooks==
 204
 205 A calling function or method uses the wfRunHooks() function to run
 206 the hooks related to a particular event, like so:
 207
 208     class Article {
 209         # ...
 210         function protect() {
 211             global $wgUser;
 212             if (wfRunHooks('ArticleProtect', $this, $wgUser)) {
 213                 # protect the article
 214                 wfRunHooks('ArticleProtectComplete', $this, $wgUser);
 215             }
 216         }
 217
 218 wfRunHooks() returns true if the calling function should continue
 219 processing (the hooks ran OK, or there are no hooks to run), or false
 220 if it shouldn't (an error occurred, or one of the hooks handled the
 221 action already). Checking the return value matters more for "before"
 222 hooks than for "complete" hooks.
 223
 224 ==Events and parameters==
 225
 226 This is a list of known events and parameters; please add to it if
 227 you're going to add events to the MediaWiki code.
 228
 229 'UserLoginComplete': after a user has logged in
 230 $user: the user object that was created on login
 231
 232 'UserLogout': before a user logs out
 233 $user: the user object that is about to be logged out
 234
 235 'UserLogoutComplete': after a user has logged out
 236 $user: the user object _after_ logout (won't have name, ID, etc.)
 237
 238