class Event extends Object
{
    public $name;               // 事件名
    public $sender;             // 事件发布者，通常是调用了 trigger() 的对象或类。
    public $handled = false;    // 是否终止事件的后续处理
    public $data;               // 事件相关数据

    private static $_events = [];

    public static function on($class, $name, $handler, $data = null,
        $append = true)
    {
        // ... ...
        // 用于绑定事件handler
    }

    public static function off($class, $name, $handler = null)
    {
        // ... ...
        // 用于取消事件handler绑定
    }

    public static function hasHandlers($class, $name)
    {
        // ... ...
        // 用于判断是否有相应的handler与事件对应
    }

    public static function trigger($class, $name, $event = null)
    {
        // ... ...
        // 用于触发事件
    }
}

public function trigger($name, Event $event = null)
{
    $this->ensureBehaviors();
    if (!empty($this->_events[$name])) {
        if ($event === null) {
            $event = new Event;
        }
        if ($event->sender === null) {
            $event->sender = $this;
        }
        $event->handled = false;
        $event->name = $name;

        // 遍历handler数组，并依次调用
        foreach ($this->_events[$name] as $handler) {
            $event->data = $handler[1];

            // 使用PHP的call_user_func调用handler
            call_user_func($handler[0], $event);

            // 如果在某一handler中，将$evnet->handled 设为true，
            // 就不再调用后续的handler
            if ($event->handled) {
                return;
            }
        }
    }
    Event::trigger($this, $name, $event);   // 触发类一级的事件
}

abstract class Application extends Module
{
    // 定义了两个事件
    const EVENT_BEFORE_REQUEST = ‘beforeRequest‘;
    const EVENT_AFTER_REQUEST = ‘afterRequest‘;

    public function run()
    {
        try {

            $this->state = self::STATE_BEFORE_REQUEST;

            // 先触发EVENT_BEFORE_REQUEST
            $this->trigger(self::EVENT_BEFORE_REQUEST);

            $this->state = self::STATE_HANDLING_REQUEST;

            // 处理Request
            $response = $this->handleRequest($this->getRequest());

            $this->state = self::STATE_AFTER_REQUEST;

            // 处理完毕后触发EVENT_AFTER_REQUEST
            $this->trigger(self::EVENT_AFTER_REQUEST);

            $this->state = self::STATE_SENDING_RESPONSE;
            $response->send();

            $this->state = self::STATE_END;

            return $response->exitStatus;

        } catch (ExitException $e) {

            $this->end($e->statusCode, isset($response) ? $response : null);
            return $e->statusCode;

        }
    }
}

// 定义事件的关联数据
class MsgEvent extend yii\base\Event
{
    public $dateTime;   // 微博发出的时间
    public $author;     // 微博的作者
    public $content;    // 微博的内容
}

// 在发布新的微博时，准备好要传递给handler的数据
$event = new MsgEvent;
$event->title = $title;
$event->author = $auhtor;

// 触发事件
$msg->trigger(Msg::EVENT_NEW_MESSAGE, $event);

$coal->on(Coal::EVENT_DISASTER, [$government, ‘onDisaster‘]);

class Government extend yii\base\Component
{
    ... ...

    public function onDisaster($event)
    {
        echo ‘DISASTER! from ‘ . $event->sender;
    }
}

// 第四个参数设为false，使得该handler在整个handler数组中处于第一个
$coal->on(Coal::EVENT_DISASTER, [$baddy, ‘onDisaster‘], null, false);

calss Baddy extend yii\base\Component
{
    ... ...

    public function onDisaster($event)
    {
        // 将事件标记为已经处理完毕，阻止后续事件handler介入。
        $event->handled = true;
    }
}

// 删除所有EVENT_DISASTER事件的handler
$coal->off(Coal::EVENT_DISASTER);

// 删除一个PHP全局函数的handler
$coal->off(Coal::EVENT_DISASTER, ‘global_onDisaster‘);

// 删除一个对象的成员函数的handler
$coal->off(Coal::EVENT_DISASTER, [$baddy, ‘onDisaster‘]);

// 删除一个类的静态成员函数的handler
$coal->off(Coal::EVENT_DISASTER, [‘path\to\Baddy‘, ‘static_onDisaster‘]);

// 删除一个匿名函数的handler
$coal->off(Coal::EVENT_DISASTER, $anonymousFunction);