( $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;
	}

	/**
	 * Executes the callback functions hooked on a specific action hook.
	 *
	 * @since 4.7.0
	 *
	 * @param mixed $args Arguments to pass to the hook callbacks.
	 */
	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;
		}
	}

	/**
	 * Processes the functions hooked into the 'all' hook.
	 *
	 * @since 4.7.0
	 *
	 * @param array $args Arguments to pass to the hook callbacks. Passed by reference.
	 */
	public function do_all_hook( &$args ) {
		$nesting_level = $this->nesting_level++;
		$this->iterations[ $nesting_level ] = array_keys( $this->callbacks );

		do {
			$priority = current( $this->iterations[ $nesting_level ] );
			foreach ( $this->callbacks[ $priority ] as $the_ ) {
				call_user_func_array( $the_['function'], $args );
			}
		} while ( false !== next( $this->iterations[ $nesting_level ] ) );

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

	/**
	 * Return the current priority level of the currently running iteration of the hook.
	 *
	 * @since 4.7.0
	 *
	 * @return int|false If the hook is running, return the current priority level. If it isn't running, return false.
	 */
	public function current_priority() {
		if ( false === current( $this->iterations ) ) {
			return false;
		}

		return current( current( $this->iterations ) );
	}

	/**
	 * Normalizes filters set up before WordPress has initialized to WP_Hook objects.
	 *
	 * @since 4.7.0
	 * @static
	 *
	 * @param array $filters Filters to normalize.
	 * @return WP_Hook[] Array of normalized filters.
	 */
	public static function build_preinitialized_hooks( $filters ) {
		/** @var WP_Hook[] $normalized */
		$normalized = array();

		foreach ( $filters as $tag => $callback_groups ) {
			if ( is_object( $callback_groups ) && $callback_groups instanceof WP_Hook ) {
				$normalized[ $tag ] = $callback_groups;
				continue;
			}
			$hook = new WP_Hook();

			// Loop through callback groups.
			foreach ( $callback_groups as $priority => $callbacks ) {

				// Loop through callbacks.
				foreach ( $callbacks as $cb ) {
					$hook->add_filter( $tag, $cb['function'], $priority, $cb['accepted_args'] );
				}
			}
			$normalized[ $tag ] = $hook;
		}
		return $normalized;
	}

	/**
	 * Determines whether an offset value exists.
	 *
	 * @since 4.7.0
	 *
	 * @link https://secure.php.net/manual/en/arrayaccess.offsetexists.php
	 *
	 * @param mixed $offset An offset to check for.
	 * @return bool True if the offset exists, false otherwise.
	 */
	public function offsetExists( $offset ) {
		return isset( $this->callbacks[ $offset ] );
	}

	/**
	 * Retrieves a value at a specified offset.
	 *
	 * @since 4.7.0
	 *
	 * @link https://secure.php.net/manual/en/arrayaccess.offsetget.php
	 *
	 * @param mixed $offset The offset to retrieve.
	 * @return mixed If set, the value at the specified offset, null otherwise.
	 */
	public function offsetGet( $offset ) {
		return isset( $this->callbacks[ $offset ] ) ? $this->callbacks[ $offset ] : null;
	}

	/**
	 * Sets a value at a specified offset.
	 *
	 * @since 4.7.0
	 *
	 * @link https://secure.php.net/manual/en/arrayaccess.offsetset.php
	 *
	 * @param mixed $offset The offset to assign the value to.
	 * @param mixed $value The value to set.
	 */
	public function offsetSet( $offset, $value ) {
		if ( is_null( $offset ) ) {
			$this->callbacks[] = $value;
		} else {
			$this->callbacks[ $offset ] = $value;
		}
	}

	/**
	 * Unsets a specified offset.
	 *
	 * @since 4.7.0
	 *
	 * @link https://secure.php.net/manual/en/arrayaccess.offsetunset.php
	 *
	 * @param mixed $offset The offset to unset.
	 */
	public function offsetUnset( $offset ) {
		unset( $this->callbacks[ $offset ] );
	}

	/**
	 * Returns the current element.
	 *
	 * @since 4.7.0
	 *
	 * @link https://secure.php.net/manual/en/iterator.current.php
	 *
	 * @return array Of callbacks at current priority.
	 */
	public function current() {
		return current( $this->callbacks );
	}

	/**
	 * Moves forward to the next element.
	 *
	 * @since 4.7.0
	 *
	 * @link https://secure.php.net/manual/en/iterator.next.php
	 *
	 * @return array Of callbacks at next priority.
	 */
	public function next() {
		return next( $this->callbacks );
	}

	/**
	 * Returns the key of the current element.
	 *
	 * @since 4.7.0
	 *
	 * @link https://secure.php.net/manual/en/iterator.key.php
	 *
	 * @return mixed Returns current priority on success, or NULL on failure
	 */
	public function key() {
		return key( $this->callbacks );
	}

	/**
	 * Checks if current position is valid.
	 *
	 * @since 4.7.0
	 *
	 * @link https://secure.php.net/manual/en/iterator.valid.php
	 *
	 * @return boolean
	 */
	public function valid() {
		return key( $this->callbacks ) !== null;
	}

	/**
	 * Rewinds the Iterator to the first element.
	 *
	 * @since 4.7.0
	 *
	 * @link https://secure.php.net/manual/en/iterator.rewind.php
	 */
	public function rewind() {
		reset( $this->callbacks );
	}

}