WV305 WordPress Plugin Development

Session 2: Hooks

What is a hook?

In WordPress parlance, a hook is something that lets your plugin (or theme) interact with WordPress in a different way than functions and classes allow.

These interactions might let you modify how WordPress behaves, or change some value somewhere within WordPress.

Hundreds of these hooks are present throughout WordPress.

How do they work?

• A core developer adds a hook to WordPress during develoment.
• At some point, another developer "hooks" something to the hook.
• When WordPress encounters the hook added by the core developer, it "fires" the hook, executing whatever is hooked to it.

Convoluted explanation, no?

Think of these as similar to Javascript events.

Examples

• init
• after_setup_theme
• the_title
• the_author

Types of Hooks

• Actions – let you do something
  • Register sidebars
  • Add a custom post type or custom taxonomy
  • Enqueue a script or stylesheet
• Filters – let you modify the value of something
  • Modify post titles
  • Append something to post content
  • Add data to a row in a list table

Actions

Comprised of two functions:

do_action( $tag, $arg = '' )

add_action( $tag, $function_to_add, $priority = 10, $accepted_args = 1 )

do_action()

• This is the function that fires this action.
• Arguments:
  • $tag – the name of the action that other code can hook to.
  • $arg – datum made available to the function that executes when this action is called.
• Important! – a hook can accept multiple arguments.

Example:

do_action( 'after_setup_theme' );

add_action()

• This is how you tell WordPress what to fire.
• Arguments:
  • $tag – the name of the action to hook to.
  • $function_to_add – the name of the function to execute when the action fires.
  • $priority – an integer indicating this function's priority. Default is 10. Lower numbers execute sooner.
  • $accepted_args – how many arguments is your function expecting. Hint: can't be more than the action provides.

Example:

/**
 * Tell WordPress to run twentyeleven_setup() when the 'after_setup_theme' hook is run.
 */
add_action( 'after_setup_theme', 'twentyeleven_setup' );

Filters

Comprised of two functions:

apply_filters( $tag, $value )

add_filter( $tag, $function_to_add, $priority = 10, $accepted_args = 1 )

Look familiar?

apply_filters()

• This is the function that applies the filters to the value.
• Arguments:
  • $tag – the name of the filter to hook to.
  • $value – the value to be filtered.
• Important! A filter can provide more arguments, which contain additional data useful to the filter.

Example:

$excerpt_length = apply_filters('excerpt_length', 55);

add_filter()

• This is how you tell WordPress what to fire.
• Arguments:
  • $tag – the name of the filter to hook to.
  • $function_to_add – the name of the function to execute when the filter fires.
  • $priority – an integer indicating this function's priority. Default is 10. Lower numbers execute sooner.
  • $accepted_args – how many arguments is your function expecting. Hint: can't be more than the filter provides.

Example:

add_filter( 'excerpt_length', 'twentyeleven_excerpt_length' );

Actions vs Filters

Guess what? That last slide lied.

Before I explain why, any questions?

Resources

• http://codex.wordpress.org/Plugin_API
• http://adambrown.info/p/wp_hooks