WordPress plugin #2

WordPress Plugin #2
May 14th, 2015
Changwoo

Today’s Topics
1.Hooks in Nutshell
2.Database Tables in Nutshell

Recap
- Your first plugin
- Hello Dolly plugin
- action, and callback

Recap
Plugin Header:
/*
Plugin Name: Hello Dolly
Plugin URI: http://wordpress.org/plugins/hello-dolly/
Description: This is not just a plugin, it symbolizes the hope and enthusiasm of an
entire generation summed up in two words sung most famously by Louis Armstrong: Hello,
Dolly. When activated you will randomly see a lyric from <cite>Hello, Dolly</cite> in
the upper right of your admin screen on every page.
Author: Matt Mullenweg
Version: 1.6
Author URI: http://ma.tt/
*/

Recap
Callback Mechanism:
add_action( ‘admin_notices’, ‘hello_dolly’ );
function hello_dolly() {
...
}
Name of hook
Callback function
WP function for hook

Recap
Planning your own plugin, and ...
submission!

Hook: an Event
Plugin programming:
- Hook-driven programming.
- A hook means an event.

Hello dolly’s case
2 actions: admin_notices, admin_head
● admin_notices codex
● admin_head codex

Hello dolly’s case
2 actions: admin_notices, admin_head
Let’s find source code.
$ fgrep -Rn do_action( 'admin_notices' ) *
wp-admin/admin-header.php:238: do_action( 'admin_notices' );
$ fgrep -Rn do_action( 'admin_head' ) *
wp-admin/includes/media.php:491: do_action( 'admin_head' );
wp-admin/includes/template.php:1627:do_action( 'admin_head' );
wp-admin/includes/class-wp-press-this.php:1267: do_action( 'admin_head' );
wp-admin/admin-header.php:125:do_action( 'admin_head' );
show source codes

Hook interface
wp-includes/plugin.php
What is action?
What is filter?
● add_filter()
● has_filter()
● apply_filters()
● add_action()
● has_action()
● do_action()

Task Separation
Themes: visual part
Plugins: functional part
Actions: structural part
Filters: information part

Task Separation
Q. I want to add extra contents to every post to be
displayed. Which one do I have to use?
Q. I want to a add menu page to admin page. Which one?
Q. I want to customize the table columns of pages screen.
Q. I want to do something when a post is updated.

Task Separation (solution)
Q. I want to add extra contents to every post to be
displayed. Which one do I have to use?
Q. I want to a add menu page to admin page. Which one?
Q. I want to customize the table columns of pages screen.
Q. I want to do something when a post is updated.
A. filter. ‘the_content’ hook.
A. action. ‘admin_menu’ hook.
A. filter. ‘manage_pages_columns’ hook.
A. action. ‘post_updated’ hook.

Task separation
Do not have to remember them all.
Moreover, actually,
add_action == add_filter

Action, Filter definition
wp-includes/plugin.php
function add_action($tag, $function_to_add, $priority = 10, $accepted_args = 1) {
return add_filter($tag, $function_to_add, $priority, $accepted_args);
}
add_action() is mere an alias of add_filter()
Then, why add_action() is created?
Because they want to get more detailed code.
They just wanted detailed conception. That’s cool.

do_action, add_action feedback
do_action:
fires registered action
add_action:
loads any actions
No restriction. you can define your own
hooks.

callback functions...
....
..
.
$wp_filter
add_action(
$tag,
$callback,
$priority,
$num_args
);

....
..
.
$wp_filter
add_action(
‘my_hook’,
‘my_callback’,
10,
1
);

....
..
.
my_hook - my_callback
$wp_filter
add_action(
‘my_hook’,
‘my_callback’,
10,
1
);

....
..
.
$wp_filter
add_action(
‘my_hook’,
‘my_callback’,
10,
1
);
somewhere in your code
...
do_action(
‘my_hook’
);
...

....
..
.
$wp_filter
add_action(
‘my_hook’,
‘my_callback’,
10,
1
);
somewhere in your code
...
do_action(
‘my_hook’
);
...
by my_hook:
my_callback()

do_action
inside of do_action:
● retrieve the callback functions by their tags (hook)
● sort by priority
● call_user_func()
do {
foreach ( (array) current($wp_filter[$tag]) as $the_ )
if ( !is_null($the_['function']) )
call_user_func_array($the_['function'],
array_slice($args, 0, (int) $the_['accepted_args']));
} while ( next($wp_filter[$tag]) !== false );

add_action
function add_filter( $tag, $function_to_add, $priority = 10, $accepted_args = 1 ) {
global $wp_filter, $merged_filters;
$idx = _wp_filter_build_unique_id($tag, $function_to_add, $priority);
$wp_filter[$tag][$priority][$idx] = array('function' => $function_to_add,
'accepted_args' => $accepted_args);
unset( $merged_filters[ $tag ] );
return true;
}
$wp_filter is an array.

Check it out!
Build a plugin that dumps all registered
actions or filters.
Add your own hook and call it.
Download template code and edit.
http://<myip>/wp-admin/ (meetup / 1)
ftp://<myip> (meetup / 1)

Summary
● filter and action are the same. action is alias.
o structure, data separation
● In fact, this is like a fine wrapper for call_user_func.
o do_action() fires callback functions when the hook
is available
o add_action() loads callback functions.
● There are huge amount of hooks: see reference.
● You can also define your own hooks.

Database: the Root
Database: backbone of web apps.
A good app without understanding
its db structure: just an illusion.

Database description
https://codex.wordpress.org/Database_Description
● parent - meta styles
o wp_comments - wp_commentmeta
o wp_user - wp_usermeta
o wp_posts - wp_postmeta
o wp_options
● wp_terms - wp_term_taxonomy - wp_term_relationships

Meta table
Every *meta table has
● meta_key
● meta_value
closely related to “hashing”

Hashing
key - value pair.
key is short / value is often long.
key is searchable.
Hashing often called (implemented) as
“dictionary”, or “associative array”.

Meta key: benefits
Flexible, extendable
Ordinary tables can’t do these:
● Creating fields “on demands”
● This is one reason why WordPress is
a very versaitle, general, and
easy-to-customize CMS.

Meta key sample: user info
You want to add “Kakaotalk ID”
for every user:
just add “kakaotalk_id” meta key.

Does it matter?
Yes! When fresh install,
WordPress: 11+ tables
74 tables
49 tables
79 tables
Jomla:
Drupal:
XpressEngine:

Meta key: Hazards
No type checking, validation
Meta key consistency
Efficiency
Nothing is perfect!
next: terms, taxonomies, ...

Terms and Taxonomies
Terms: a word
Taxonomy: classification of terms

Terms and Taxonomies
iron
??
Home appliance Metal ???

Adding Terms in Posts
Posts have basically two taxonomies:
● tags
● categories
Adding tag/category = adding term

<terms>
term_id: 2
name: MyCategory
slug: my-category
term_group: 0

<term_taxonomy>
term_taxonomy_id: 2
term_id: 2
taxonomy: category
/ post_tag
description:
parent: 0
count: 0

Category Setting
<term_relationships>
object_id: 1
term_taxonomy_id: 2
term_order: 0

Post ID: 75 term_relationships.object_id
term_relationships.term_taxonomy_id

term_taxonomy.term_taxonomy_id
term_taxonomy.term_id
term_taxonomy.taxonomy ★

terms.term_id
terms.name ★
terms.slug
term_taxonomy.term_taxonomy_id
term_taxonomy.term_id
term_taxonomy.taxonomy ★

Initial Taxonomies
< wp-includes/taxonomy.php >
function register_taxonomy(
$taxonomy, $object_type, $args = array() ) { ... }
function create_initial_taxonomies() {
register_taxonomy( 'category', 'post', array(...) );
register_taxonomy( 'post_tag', 'post', array(...) );
... and so on ...
create_initial_taxonomies() is called in wp-settings.php

Summary
● Wordpress Datbase table uses metadata
style strategy
● meta tables have key / value fields
o flexible, easy to extend
o while it can have some disadvantages

Summary
● Taxonomy: classification of term
● You can add taxonomies (as well as terms)
● Taxonomy can be flat or hierarchical
● term_taxonomy: taxonomy -
term
● term_relationship: posts - taxonomy

Next week
Admin Menu
● Almost every plugin has its menu
● Easy WP Plugin API
Admin Post
● POST type data submission
URL Query
AJAX Data handling
Wordpress Posts in Nutshell

WordPress plugin #2

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