Thread by @igorbenic

I am embarrassed to say, but I didn't understand hooks at first.

Started fully understanding hooks when I started working on my custom #WordPress plugins.

I'll try to teach you everything about them in this thread. 👇 💻

At first, the WordPress Hook system was also called the Plugin API, because it's the core system that allowed extending WordPress.

This is the old documentation: codex.wordpress.org/Plugin_API

The new one is a little better at explaining: developer.wordpress.org/plugins/hooks/

Let's talk about actions. When you start developing, you'll most likely find code snippets that use add_action().

Reference: developer.wordpress.org/reference/func…

So, you are registering a function that will be called when an event (action) occurs.

You provide the priority of it and how many arguments from that even you expect.

Why is priority useful?

If you depend on a plugin, and they use the same action but set the priority to 9 and save data.

Your function that is set to a priority of 10 can get that data from the database.

How such actions (events) are triggered?
They are done with do_action().

Reference: developer.wordpress.org/reference/func…

Let's consider a real action 'user_register'. This action is triggered when a user has been created.

Reference: developer.wordpress.org/reference/hook…

Let's now see how we can use that. We can hook into that action to perform something else.

If we just need the user ID, we don't need to define arguments (default: 1). If we also need other data, we would set 2.

Let's now talk about Filters. Those are used in general to change data before it will be used for something.

When starting out, you've probably seen code snippets with add_filter().

Reference: developer.wordpress.org/reference/func…

There is a big difference between filters and actions:

Functions registered to actions don't have to return anything.

Functions registered to filters are required to return a value.

As actions have do_action() to trigger hooked functions.

Filters use apply_filters().

Reference: developer.wordpress.org/reference/func…

Let's cover a simple filter 'the_title' from get_the_title():
developer.wordpress.org/reference/func…

We expect from get_the_title to return a title. If we don't return a value with our function, there will be an error. That's the difference we explained above.

When using add_filter, we can decide to accept the post ID or just the title. Let's say that we want to get the date from it and put it in the title.

And we need to make if there is nothing retrieved from ID, just return the provided title.

Now that we covered how to hook our functions to filters and action, what about removing them?

Sometimes, customizing a site can involve removing already registered actions and filters.

When calling functions to remove them, you need to specify the exact priority as well.

Those functions are:
- remove_action: developer.wordpress.org/reference/func…
- remove_filter: developer.wordpress.org/reference/func…

You specify which hook, function, and priority to remove.

Have this in mind:
- function won't be called if an action/filter was already triggered

Example:
You are hooking into 'woocommerce_init'.
But you are doing that much later and WooCommerce was initialized already.
It won't work.

- if functions are registered through a class, they won't be removed by just putting the function name.

add_action( 'init', [ $object, 'the_function' ] );
remove_action( 'init', 'the_function'); ❌
remove_action( 'init', [ $object, 'the_function'] ); ✅

Another thing:
When hooking into a filter/action that potentially calls the same, remove your before or you'll end up in an infinity loop.

Fix:
1. Inside our function, remove the action/filter
2. Process code
3. Re-hook our function

I hope you've learned something from this thread.

And if you have someone who is starting out with WordPress, retweet them the first tweet :D

https://twitter.com/igorbenic/status/1493283928257482752