vendor/yiisoft/yii2/base/Event.php

      $event) {                if ($event[0] === $handler) {                    unset(self::$_events[$name][$class][$i]);                    $removed = true;                }            }            if ($removed) {                // 移除之后，使数组重新变成一个自然数组                self::$_events[$name][$class] = array_values(self::$_events[$name][$class]);            }            return $removed;        }    }    /**     * Returns a value indicating whether there is any handler attached to the specified class-level event.     * Note that this method will also check all parent classes to see if there is any handler attached     * to the named event.     * 检测在某个类或者对象是否具有某个事件     * @param string|object $class the object or the fully qualified class name specifying the class-level event.     * @param string $name the event name.     * @return boolean whether there is any handler attached to the event.     */    public static function hasHandlers($class, $name)    {        if (empty(self::$_events[$name])) {            // 不存在，直接返回            return false;        }        if (is_object($class)) {            // 如果是一个 object，就获取其类名            $class = get_class($class);        } else {            // 如果是一个类名，就去掉 class 最左边的斜杠            $class = ltrim($class, '\\');        }        // 如果该类中找不到，就去父类中找，直到找到或者没有父类了为止        do {            if (!empty(self::$_events[$name][$class])) {                return true;            }        } while (($class = get_parent_class($class)) !== false);        return false;    }    /**     * Triggers a class-level event.     * This method will cause invocation of event handlers that are attached to the named event     * for the specified class and all its parent classes.     * 触发某个类或者对象的某个事件     * @param string|object $class the object or the fully qualified class name specifying the class-level event.     * @param string $name the event name.     * @param Event $event the event parameter. If not set, a default [[Event]] object will be created.     */    public static function trigger($class, $name, $event = null)    {        if (empty(self::$_events[$name])) {            return;        }        if ($event === null) {            // 事件不存在，就创建一个 Event 对象            $event = new static;        }        // 设置event对象的属性，默认是未被处理的        $event->handled = false;        $event->name = $name;        if (is_object($class)) {            if ($event->sender === null) {                // 如果 $class 是个对象，并且是 sender 为空，就将 $class 赋给 sender，即 $class 就是触发事件的对象                $event->sender = $class;            }            $class = get_class($class);        } else {            $class = ltrim($class, '\\');        }        // 循环类的 $_event，直到遇到 $event->handled 为真或者没有父类了为止        do {            if (!empty(self::$_events[$name][$class])) {                foreach (self::$_events[$name][$class] as $handler) {                    // 将参数赋到 event 对象的 data 属性上                    $event->data = $handler[1];                    // 调用 $handler 方法                    // 在方法中，可以用 $this->data 取到相应的参数                    // 也可以在其中设置 $this->handled 的值，中断后续事件的触发                    call_user_func($handler[0], $event);                    // 当某个 handled 被设置为 true 时，执行到这个事件的时候，会停止，并忽略剩下的事件                    if ($event->handled) {                        return;                    }                }            }        } while (($class = get_parent_class($class)) !== false);    }}