WordPress Actions

WordPress Application Structure

WordPress' architecture is "event-driven"

From receiving the request to sending the response, WordPress mostly consists of a series of events, and functions that "hook" into them

Image from Tom McFarlin: https://tommcfarlin.com/wordpress-and-mvc/

WordPress Application Structure

WordPress' actions implement the "Publish-Subscribe" (or Pub Sub) design pattern, a specific kind of event-driven programming.

In Pub Sub, the "publishers" and "subscribers" don't know about each other.

More about Pub Sub here

WordPress Application Structure

You've seen Pub Sub before:

jQuery(document).on('ready', function($){
    // subscribing to the document ready event

    // the code that fired the event doesn't 
    // know about our subscribing code

    // our subscribing code doesn't know anything
    // about the code that fired the event
});

WordPress Application Structure

In WordPress, there are two types of events: actions and filters.

Actions are a typical Pub Sub implementation: events are fired and subscribers listen for them.

Filters are a little different: they give some data to the subscriber, and then expect a return value.

WordPress Actions

Defining an action (firing an event)

do_action( 'foo' ); // Just making an announcement

Hooking into an action (responding to an event)

add_action( 'foo', 'some_function');

function some_function() {
    echo "do some stuff";
}

WordPress Actions

Actions with data

do_action( 'foo', 'a', 'b', 'c' );

add_action( 'foo', 'some_function', 10, 3);

function some_function($x, $y, $z) {
    echo $x; // => 'a'
    echo $y; // => 'b'
    echo $z; // => 'c'
}

WordPress Actions

the add_action() function

add_action( $tag, $callback_function, $priority, $accepted_args );

• $tag - the action name
• $callback_function - the function to run
• $priority - tells WP what order to run actions in
• $accepted_args - how many arguments to accept

WordPress Actions

The callback function can be any PHP "callable".

1. Function name: 'my_great_function' 
2. Method of an object
   • array($some_obj, 'method_name')
   • array($this, 'method_name')
3. Static method of a class
   • array('ClassName', 'method_name')
   • 'ClassName::method_name'
4. Closure / anonymous function
   1. ​function() {echo "do stuff";}

WordPress Actions

Example: do something after all plugins are loaded, inside a class

<?php
class Foo {
    public function __construct() {
        add_action( 'plugins_loaded', array( $this, 'do_stuff' ) );
    }
    public function do_stuff() {
        echo "do stuff";
    }
}
$foo = new Foo();

WordPress Filters

Filters are like actions, but they expect data back. This example lets you allow SVG files through the media uploader.

<?php
// example adapted from CSS Tricks
function allow_svg_uploads($mimes) {
    $mimes['svg'] = 'image/svg+xml';
    return $mimes; // sending a value back!
}

add_filter('upload_mimes', 'allow_svg_uploads');

Listener (your code)

From WP core:

<?php
return apply_filters( 'upload_mimes', $t, $user );