Custom Hook Listeners

Monolog-based logging package for WordPress.

Custom Hook Listeners

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Custom Hook Listeners

The suggested way to log custom code with Wonolog is to use custom hook listeners.

Hook listeners can be seen, in fact, as Wonolog extensions, as they are reusable pieces of code that integrate with Wonolog and extend its functionality to permit the logging of third-party code.

Hook listeners allow to completely decouple the code to be logged from Wonolog, because it does not need to contain any do_action( 'wonolog.log' ) call.

The only thing that is required is that the code to be logged fires actions when “meaningful things” happen during execution, allowing Wonolog to listen to these actions and create log records.

Let’s clarify with an example.

The Fictional “MyFiles” Plugin

Let’s assume there’s a plugin named “MyFiles” that handles upload and download of files for logged-in WordPress users.

This plugin will fire some actions:

// Somewhere in the plugin code:
do_action( 'myfiles_file_uploaded', $file_info, $uploader_user_id );

// Somewhere in the plugin code:
do_action( 'myfiles_file_upload_failed', $file_info, $reason, $uploader_user_id );

// Somewhere in the plugin code:
do_action( 'myfiles_file_downloaded', $file_name, $downloader_user_id );

We will create a custom hook listener that can be used to log actions fired by the plugin, and we will se how to integrate the custom hook listener with Wonolog.

Custom Listener Example

For the website where this plugin is installed, we can write a Wonolog hook listener that will look more or less like this (please note that the following code is PHP 5.6+):

namespace MyWebiste;

use Inpsyde\Wonolog\Data;
use Inpsyde\Wonolog\HookListener\ActionListenerInterface;

class MyFilesListener implements ActionListenerInterface {

    const TARGET_CHANNEL_NAME = 'MyFilesPlugin';

    public function id() {

        return 'MyFiles Listener';

    public function listen_to() {

        return [

    public function update( array $args ) {

        $method = [ $this, current_filter() ];
        if ( is_callable( $method ) ) {
            return $method( ...$args );

    private function myfiles_file_uploaded( $file_info, $user_id ) {

        return new Data\Debug(
            'A file has been uploaded.',      // Message.
            self::TARGET_CHANNEL_NAME,        // Channel.
            compact( 'file_info', 'user_id' ) // Context.

    private function myfiles_file_upload_failed( $file_info, $reason, $user_id ) {

        return new Data\Error(
            "A file download failed: {$reason}.",
            compact( 'file_info', 'user_id' )

    private function myfiles_file_downloaded( $file_name, $user_id ) {

        return new Data\Debug(
            'A file has been downloaded.',
            compact( 'file_name', 'user_id' )

Things to note in the above code:

  • The listen_to() method returns the list of all actions the listener targets.
  • The update() method is called when each of those listened hooks is fired, and all the arguments passed to the hook are passed as array to the method. Based on the actual hook, a different private method is called, passing received hook arguments in order (thanks to PHP 5.6 variadic arguments).
  • Each of those private methods returns a Wonolog log object that will be handled by Wonolog according to its configuration (i.e., handlers, processors, channels etc.).
  • All log objects returned have a custom channel, ‘MyFilesPlugin’, so Wonolog will have to know about it to be able to handle the log records.

Integrating a Custom Listener

With the listener class from above available, we still have to tell Wonolog to use it.

The MU plugin to do that could be something like this:

 * Plugin name: Wonolog Configuration
use Inpsyde\Wonolog;
use MyWebsite\MyFilesListener;

if ( ! defined( 'Inpsyde\Wonolog\LOG' ) ) {

// Add the custom channel to Wonolog.
add_filter( 'wonolog.channels', function( array $channels ) {  

    $channels[] = MyFilesListener::TARGET_CHANNEL_NAME;

    return $channels;
} );

Wonolog\bootstrap()->use_hook_listener( new MyFilesListener() );

What about Filters?

When possible, it is preferable to use actions to trigger log records. Filters need to return a value, and we most probably do not want that logging changes the return value.

However, especially if the code we want to log is not under our control or we don’t want to edit it for any reason, it could be necessary to use filters in hook listeners.

This is why Wonolog ships with a FilterListenerInterface that can be used instead of, or alongside, ActionListenerInterface.

Considering that callbacks attached to filters must return something in WordPress, and considering that the update() method returns null or log data objects, FilterListenerInterface has an additional method filter() that receives all the arguments passed to filter as array (just like update()) and has to return the filter return value.

If you do not want the hook listener to affect the filtering, something like this will do already:

public function filter( array $args ) {

    return $args[0];

Custom Hook Priority

Neither ActionListenerInterface, nor FilterListenerInterface provide a way to specify the priority that Wonolog uses to listen to hooks.

By default, Wonolog uses a very late priority, which is fine in most cases.

But we know that there are always edge cases.

For this reason, there’s an additional interface: HookPriorityInterface.

This interface has a priority() method that has to return the priority to use (the returned value will be used when Wonolog calls add_action() and add_filter(), respectively).

The value returned by priority() will be used for all hooks listened to. To control the priority on a per-hook basis, without creating a different hook listener, Wonolog provides a filter, 'wonolog.listened-hook-priority', that can be used for the scope.

This filter will provide callbacks with the both the current priority as first argument, and the hook as second, allowing to change priority on a per-hook basis.

The priority passed to filter callbacks is initially set to the default priority used by Wonolog, or to the priority returned by priority() for listeners implementing HookPriorityInterface.

This means that priority can be customized even without implementing HookPriorityInterface but only hooking 'wonolog.listened-hook-priority'.

For example, the MyFilesListener class from above could do something like this:

class MyFilesListener implements ActionListenerInterface {
    // ...

    public function listen_to() {

        $target_hooks = [
            // hook_name                 => priority
            'myfiles_file_uploaded'      => 0,
            'myfiles_file_upload_failed' => 20,
            'myfiles_file_downloaded'    => 999,

        add_filter( 'wonolog.listened-hook-priority', function( $priority, $hook  ) use ( $target_hooks ) {

            return isset( $target_hooks[ $hook ] ) ? $target_hooks[ $hook ] : $priority;
        }, 10, 2 );

        return array_keys( $target_hooks );

    // ...

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