フック　フィルターフック　アクションフック｜WordPress

<?php
//plugin.php

// Initialize the filter globals.
require( dirname( __FILE__ ) . '/class-wp-hook.php' );

/** @var WP_Hook[] $wp_filter */
global $wp_filter, $wp_actions, $wp_current_filter;

//初期の状態では$wp_filterは空の状態。
//なので、wp_filter = arrya();で配列が作成される。
if ( $wp_filter ) {
	$wp_filter = WP_Hook::build_preinitialized_hooks( $wp_filter );
} else {
	$wp_filter = array();
}

if ( ! isset( $wp_actions ) ) {
	$wp_actions = array();
}

if ( ! isset( $wp_current_filter ) ) {
	$wp_current_filter = array();
}

<?php
//plugin.phpのapply_filter()    
    
/**
 * Call the functions added to a filter hook.
 *
 * The callback functions attached to filter hook $tag are invoked by calling
 * this function. This function can be used to create a new filter hook by
 * simply calling this function with the name of the new hook specified using
 * the $tag parameter.
 *
 * The function allows for additional arguments to be added and passed to hooks.
 *
 *     // Our filter callback function
 *     function example_callback( $string, $arg1, $arg2 ) {
 *         // (maybe) modify $string
 *         return $string;
 *     }
 *     add_filter( 'example_filter', 'example_callback', 10, 3 );
 *
 *     /*
 *      * Apply the filters by calling the 'example_callback' function we
 *      * "hooked" to 'example_filter' using the add_filter() function above.
 *      * - 'example_filter' is the filter hook $tag
 *      * - 'filter me' is the value being filtered
 *      * - $arg1 and $arg2 are the additional arguments passed to the callback.
 *     $value = apply_filters( 'example_filter', 'filter me', $arg1, $arg2 );
 *
 * @since 0.71
 *
 * @global array $wp_filter         Stores all of the filters.
 * @global array $wp_current_filter Stores the list of current filters with the current one last.
 *
 * @param string $tag     The name of the filter hook.
 * @param mixed  $value   The value on which the filters hooked to `$tag` are applied on.
 * @param mixed  $var,... Additional variables passed to the functions hooked to `$tag`.
 * @return mixed The filtered value after all hooked functions are applied to it.
 */
function apply_filters( $tag, $value ) {
	global $wp_filter, $wp_current_filter;

	$args = array();

	// Do 'all' actions first.
	if ( isset( $wp_filter['all'] ) ) {
		$wp_current_filter[] = $tag;
		$args                = func_get_args();
		_wp_call_all_hook( $args );
	}

	if ( ! isset( $wp_filter[ $tag ] ) ) {
		if ( isset( $wp_filter['all'] ) ) {
			array_pop( $wp_current_filter );
		}
        return $value;
        
    //第２引数の$valueを返している。
	}

	if ( ! isset( $wp_filter['all'] ) ) {
		$wp_current_filter[] = $tag;
	}

	if ( empty( $args ) ) {
		$args = func_get_args();
	}

	// don't pass the tag name to WP_Hook
	array_shift( $args );

	$filtered = $wp_filter[ $tag ]->apply_filters( $value, $args );

	array_pop( $wp_current_filter );

	return $filtered;
}

//class-wp-hook.phpのapply_filters()
/**
	 * Calls the callback functions added to a filter hook.
	 *
	 * @since 4.7.0
	 *
	 * @param mixed $value The value to filter.
	 * @param array $args  Arguments to pass to callbacks.
	 * @return mixed The filtered value after all hooked functions are applied to it.
	 */
	public function apply_filters( $value, $args ) {
		if ( ! $this->callbacks ) {
			return $value;
		}

		$nesting_level = $this->nesting_level++;

		$this->iterations[ $nesting_level ] = array_keys( $this->callbacks );
		$num_args                           = count( $args );

		do {
			$this->current_priority[ $nesting_level ] = $priority = current( $this->iterations[ $nesting_level ] );

			foreach ( $this->callbacks[ $priority ] as $the_ ) {
				if ( ! $this->doing_action ) {
					$args[0] = $value;
				}

				// Avoid the array_slice if possible.
				if ( $the_['accepted_args'] == 0 ) {
					$value = call_user_func_array( $the_['function'], array() );
				} elseif ( $the_['accepted_args'] >= $num_args ) {
					$value = call_user_func_array( $the_['function'], $args );
				} else {
					$value = call_user_func_array( $the_['function'], array_slice( $args, 0, (int) $the_['accepted_args'] ) );
				}
			}
		} while ( false !== next( $this->iterations[ $nesting_level ] ) );

		unset( $this->iterations[ $nesting_level ] );
		unset( $this->current_priority[ $nesting_level ] );

		$this->nesting_level--;

		return $value;
	}

?>

<?php
//plugin.php内のadd_filter()
//$wp_filter[$tag]がセットされているか確かめる。
//初回はセットされていない。
//new WP_Hook()で各フィルターフックのインスタンスを作成。
//WP_Hookクラスに用意されているadd_filter()を呼び出す。
function add_filter( $tag, $function_to_add, $priority = 10, $accepted_args = 1 ) {
	global $wp_filter;

	if ( ! isset( $wp_filter[ $tag ] ) ) {
		$wp_filter[ $tag ] = new WP_Hook();
	}
	$wp_filter[ $tag ]->add_filter( $tag, $function_to_add, $priority, $accepted_args );
	return true;
}

//class-wp-hook.php内のadd_filter

/**
	 * Hooks a function or method to a specific filter action.
	 *
	 * @since 4.7.0
	 *
	 * @param string   $tag             The name of the filter to hook the $function_to_add callback to.
	 * @param callable $function_to_add The callback to be run when the filter is applied.
	 * @param int      $priority        The order in which the functions associated with a
	 *                                  particular action are executed. Lower numbers correspond with
	 *                                  earlier execution, and functions with the same priority are executed
	 *                                  in the order in which they were added to the action.
	 * @param int      $accepted_args   The number of arguments the function accepts.
	 */
	public function add_filter( $tag, $function_to_add, $priority, $accepted_args ) {
		$idx              = _wp_filter_build_unique_id( $tag, $function_to_add, $priority );
		$priority_existed = isset( $this->callbacks[ $priority ] );

		$this->callbacks[ $priority ][ $idx ] = array(
			'function'      => $function_to_add,
			'accepted_args' => $accepted_args,
		);

		// if we're adding a new priority to the list, put them back in sorted order
		if ( ! $priority_existed && count( $this->callbacks ) > 1 ) {
			ksort( $this->callbacks, SORT_NUMERIC );
		}

		if ( $this->nesting_level > 0 ) {
			$this->resort_active_iterations( $priority, $priority_existed );
		}
    }

<?php
//plugin.phpのdo_action()
/**
 * Execute functions hooked on a specific action hook.
 *
 * This function invokes all functions attached to action hook `$tag`. It is
 * possible to create new action hooks by simply calling this function,
 * specifying the name of the new hook using the `$tag` parameter.
 *
 * You can pass extra arguments to the hooks, much like you can with `apply_filters()`.
 *
 * Example usage:
 *
 *     // The action callback function.
 *     function example_callback( $arg1, $arg2 ) {
 *         // (maybe) do something with the args.
 *     }
 *     add_action( 'example_action', 'example_callback', 10, 2 );
 *
 *     /*
 *      * Trigger the actions by calling the 'example_callback()' function
 *      * that's hooked onto `example_action` above.
 *      *
 *      * - 'example_action' is the action hook.
 *      * - $arg1 and $arg2 are the additional arguments passed to the callback.
 *     $value = do_action( 'example_action', $arg1, $arg2 );
 *
 * @since 1.2.0
 * @since 5.3.0 Formalized the existing and already documented `...$arg` parameter
 *              by adding it to the function signature.
 *
 * @global array $wp_filter         Stores all of the filters and actions.
 * @global array $wp_actions        Increments the amount of times action was triggered.
 * @global array $wp_current_filter Stores the list of current filters with the current one last.
 *
 * @param string $tag    The name of the action to be executed.
 * @param mixed  ...$arg Optional. Additional arguments which are passed on to the
 *                       functions hooked to the action. Default empty.
 */
function do_action( $tag, ...$arg ) {
	global $wp_filter, $wp_actions, $wp_current_filter;

	if ( ! isset( $wp_actions[ $tag ] ) ) {
		$wp_actions[ $tag ] = 1;
	} else {
		++$wp_actions[ $tag ];
	}

	// Do 'all' actions first.
	if ( isset( $wp_filter['all'] ) ) {
		$wp_current_filter[] = $tag;
		$all_args            = func_get_args();
		_wp_call_all_hook( $all_args );
	}

	if ( ! isset( $wp_filter[ $tag ] ) ) {
		if ( isset( $wp_filter['all'] ) ) {
			array_pop( $wp_current_filter );
		}
		return;
	}

	if ( ! isset( $wp_filter['all'] ) ) {
		$wp_current_filter[] = $tag;
	}

	if ( empty( $arg ) ) {
		$arg[] = '';
	} elseif ( is_array( $arg[0] ) && 1 === count( $arg[0] ) && isset( $arg[0][0] ) && is_object( $arg[0][0] ) ) {
		// Backward compatibility for PHP4-style passing of `array( &$this )` as action `$arg`.
		$arg[0] = $arg[0][0];
	}

	$wp_filter[ $tag ]->do_action( $arg );

	array_pop( $wp_current_filter );
}
?>

<?php
//class-wp-hook.phpのdo_action()メソッド
/**
	 * Calls the callback functions that have been added to an action hook.
	 *
	 * @since 4.7.0
	 *
	 * @param array $args Parameters to pass to the callback functions.
	 */
	public function do_action( $args ) {
		$this->doing_action = true;
		$this->apply_filters( '', $args );

		// If there are recursive calls to the current action, we haven't finished it until we get to the last one.
		if ( ! $this->nesting_level ) {
			$this->doing_action = false;
		}
	}
?>

<?php
//plugin.phpのadd_action()

/**
 * Hooks a function on to a specific action.
 *
 * Actions are the hooks that the WordPress core launches at specific points
 * during execution, or when specific events occur. Plugins can specify that
 * one or more of its PHP functions are executed at these points, using the
 * Action API.
 *
 * @since 1.2.0
 *
 * @param string   $tag             The name of the action to which the $function_to_add is hooked.
 * @param callable $function_to_add The name of the function you wish to be called.
 * @param int      $priority        Optional. Used to specify the order in which the functions
 *                                  associated with a particular action are executed. Default 10.
 *                                  Lower numbers correspond with earlier execution,
 *                                  and functions with the same priority are executed
 *                                  in the order in which they were added to the action.
 * @param int      $accepted_args   Optional. The number of arguments the function accepts. Default 1.
 * @return true Will always return true.
 */
function add_action( $tag, $function_to_add, $priority = 10, $accepted_args = 1 ) {
	return add_filter( $tag, $function_to_add, $priority, $accepted_args );
}
?>

<?php
//plugin.php内のadd_filter()
//$wp_filter[$tag]がセットされているか確かめる。
//初回はセットされていない。
//new WP_Hook()で各フィルターフックのインスタンスを作成。
//WP_Hookクラスに用意されているadd_filter()を呼び出す。
function add_filter( $tag, $function_to_add, $priority = 10, $accepted_args = 1 ) {
	global $wp_filter;

	if ( ! isset( $wp_filter[ $tag ] ) ) {
		$wp_filter[ $tag ] = new WP_Hook();
	}
	$wp_filter[ $tag ]->add_filter( $tag, $function_to_add, $priority, $accepted_args );
	return true;
}

//class-wp-hook.php内のadd_filter

/**
	 * Hooks a function or method to a specific filter action.
	 *
	 * @since 4.7.0
	 *
	 * @param string   $tag             The name of the filter to hook the $function_to_add callback to.
	 * @param callable $function_to_add The callback to be run when the filter is applied.
	 * @param int      $priority        The order in which the functions associated with a
	 *                                  particular action are executed. Lower numbers correspond with
	 *                                  earlier execution, and functions with the same priority are executed
	 *                                  in the order in which they were added to the action.
	 * @param int      $accepted_args   The number of arguments the function accepts.
	 */
	public function add_filter( $tag, $function_to_add, $priority, $accepted_args ) {
		$idx              = _wp_filter_build_unique_id( $tag, $function_to_add, $priority );
		$priority_existed = isset( $this->callbacks[ $priority ] );

		$this->callbacks[ $priority ][ $idx ] = array(
			'function'      => $function_to_add,
			'accepted_args' => $accepted_args,
		);

		// if we're adding a new priority to the list, put them back in sorted order
		if ( ! $priority_existed && count( $this->callbacks ) > 1 ) {
			ksort( $this->callbacks, SORT_NUMERIC );
		}

		if ( $this->nesting_level > 0 ) {
			$this->resort_active_iterations( $priority, $priority_existed );
		}
    }