前面阅读了Yii2的两个基本类Object和Component，了解了Yii的三个重要概念属性、事件、行为，下面开始阅读Event类，Event类是所有事件类的基类：
<?php
/**
* @link http://www.yiiframework.com/
* @copyright Copyright (c) 2008 Yii Software LLC
* @license http://www.yiiframework.com/license/
*/
namespace yii\base;
/**
* Event is the base class for all event classes.
* Event 是所有事件类的基类，是对Component的深化
*
* It encapsulates the parameters associated with an event.
* The [[sender]] property describes who raises the event.
* And the [[handled]] property indicates if the event is handled.
* If an event handler sets [[handled]] to be true, the rest of the
* uninvoked handlers will no longer be called to handle the event.
*
* Additionally, when attaching an event handler, extra data may be passed
* and be available via the [[data]] property when the event handler is invoked.
*
* @author Qiang Xue <qiang.xue@gmail.com>
* @since 2.0
*/
class Event extends Object
{
/**
* @var string the event name. This property is set by [[Component::trigger()]] and [[trigger()]].
* Event handlers may use this property to check what event it is handling.
* 事件的名字，通过[[Component::trigger()]] 和 [[trigger()]]方法设置，事件处理程序通过这个属性类检查处理的事件
*/
public $name;
/**
* @var object the sender of this event. If not set, this property will be
* set as the object whose "trigger()" method is called.
* This property may also be a `null` when this event is a
* class-level event which is triggered in a static context.
*
* 触发事件的对象，如果未设置，则设置为调用"trigger()"方法的对象，如果是在静态环境下触发的类级别的事件，怎为空
*/
public $sender;
/**
* @var boolean whether the event is handled. Defaults to false.
* When a handler sets this to be true, the event processing will stop and
* ignore the rest of the uninvoked event handlers.
*
* 记录事件是否已被处理，当 handled 被设置为 true 时，执行到这个 event 的时候，会停止，并忽略剩下的 event
*/
public $handled = false;
/**
* @var mixed the data that is passed to [[Component::on()]] when attaching an event handler.
* Note that this varies according to which event handler is currently executing.
* 事件加载时传入的数据，对应当前执行的事件处理程序
*/
public $data;
/**
* @var array contains all globally registered event handlers.
* 存储所有的 event，因为是 static 的属性，所有的 event 对象/类都共享这一份数据
*/
private static $_events = [];
/**
* Attaches an event handler to a class-level event.
* 为一个类添加事件
*
* When a class-level event is triggered, event handlers attached
* to that class and all parent classes will be invoked.
*
* For example, the following code attaches an event handler to `ActiveRecord`'s
* `afterInsert` event:
*
* ```php
* Event::on(ActiveRecord::className(), ActiveRecord::EVENT_AFTER_INSERT, function ($event) {
* Yii::trace(get_class($event->sender) . ' is inserted.');
* });
* ```
*
* The handler will be invoked for EVERY successful ActiveRecord insertion.
*
* For more details about how to declare an event handler, please refer to [[Component::on()]].
*
* @param string|object $class 定义的类级别的对象或者类名称.
* @param string $name 事件名
* @param callable $handler 事件处理程序.
* @param mixed $data 数据.
* When the event handler is invoked, this data can be accessed via [[Event::data]].
* @param boolean $append whether to append new event handler to the end of the existing
* handler list. If false, the new handler will be inserted at the beginning of the existing
* handler list.
* @see off()
*/
public static function on($class, $name, $handler, $data = null, $append = true)
{
// 去掉 class 最左边的斜线
$class = ltrim($class, '\\');
if ($append || empty(self::$_events[$name][$class])) {
// 如果 append 为true，就放到 $_events 中名字为 $name 的数组的最后
self::$_events[$name][$class][] = [$handler, $data];
} else {
//否则放到最前面
array_unshift(self::$_events[$name][$class], [$handler, $data]);
}
}
/**
* Detaches an event handler from a class-level event.
* [[on()]]方法的反方法，移除一个类的事件
* This method is the opposite of [[on()]].
*
* @param string|object $class 定义的类级别的对象或者类名称.
* @param string $name 事件名
* @param callable $handler 事件处理程序.
* If it is null, all handlers attached to the named event will be removed.
* @return boolean whether a handler is found and detached.
* @see on()
*/
public static function off($class, $name, $handler = null)
{
// 去掉 class 最左边的斜线
$class = ltrim($class, '\\');
if (empty(self::$_events[$name][$class])) {
// 如果不存在该事件，返回false
return false;
}
if ($handler === null) {
// 如果 handler 为空，也就是没有规定移除的事件处理程序，直接将该事件移除，即移出所有的是这个名字的事件
unset(self::$_events[$name][$class]);
return true;
} else {
$removed = false; //移除标记
// 如果 $handler 不为空，遍历 $_events 找到相应的 handler，只移除这个 handler 和 data 组成的数组
foreach (self::$_events[$name][$class] as $i => $event) {
if ($event[0] === $handler) {
unset(self::$_events[$name][$class][$i]);
$removed = true;
}
}
if ($removed) {
//如果删除成功，调用array_values方法重新赋值，初始化索引
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/会同时检测父类
*
* @param string|object $class 定义的类级别的对象或者类名称.
* @param string $name 事件名
* @return boolean whether there is any handler attached to the event.
*/
public static function hasHandlers($class, $name)
{
if (empty(self::$_events[$name])) {
//如果不存在，直接返回false
return false;
}
if (is_object($class)) {
// 如果是一个 object，就获取其类名 get_class 内置函数，用于获取对象的方法
$class = get_class($class);
} else {
// 如果是一个类名，就去掉 class 最左边的斜杠
$class = ltrim($class, '\\');
}
//将该类/该对象对应的类的父类以及该类/该对象对应的类实现的接口名称合并到一个数组中
$classes = array_merge(
[$class],
class_parents($class, true),//获取父类的类名
class_implements($class, true)//获取当前类和父类实现的所有接口类名
);
//判断是否存在事件处理程序，存在返回true
foreach ($classes as $class) {
if (!empty(self::$_events[$name][$class])) {
return true;
}
}
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/会同时作用于父类
*
* @param string|object $class 定义的类级别的对象或者类名称.
* @param string $name 事件名
* @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[$name])) {
//如果事件不存在，直接返回
return;
}
if ($event === null) {
// 构建Event对象，为传入到handler函数中做准备
$event = new static;
}
$event->handled = false;//事件是否被处理标志
$event->name = $name;//事件名
if (is_object($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);//取得类名称
} else {
$class = ltrim($class, '\\');//去掉类名称左边的斜线
}
//将该类/该对象对应的类的父类以及该类/该对象对应的类实现的接口名称合并到一个数组中
$classes = array_merge(
[$class],
class_parents($class, true),
class_implements($class, true)
);
foreach ($classes as $class) {//遍历所有的类
if (!empty(self::$_events[$name][$class])) {//找到符合条件的类
foreach (self::$_events[$name][$class] as $handler) {
$event->data = $handler[1];//设置data数据
call_user_func($handler[0], $event);//调用处理程序
if ($event->handled) {//事件是否被处理标志为真，结束执行
return;
}
}
}
}
}
}