根據之前一篇文章，我們知道 Yii2 的事件分兩類，一是類級別的事件，二是實例級別的事件。類級別的事件是基於 yii\base\Event 實現，實例級別的事件是基於 yii\base\Component 實現。
今天先來看下類級別事件的實現，代碼是 yii\base\Event 類。
<?php
namespace yii\base;
/**
* Event is the base class for all event classes.
*/
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
*/
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.
* 觸發事件的對象
*/
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.
* 記錄事件是否已被處理，當 handled 被設置為 true 時，執行到這個 event 的時候，會停止，並忽略剩下的 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.
*/
public $data;
/**
* 存儲所有的 event，因為是 static 的屬性，所有的 event 對象/類都共享這一份數據
*/
private static $_events = [];
/**
* Attaches an event handler to a class-level event.
*
* When a class-level event is triggered, event handlers attached
* to that class and all parent classes will be invoked.
*
* For example, the following code attaches an event handler to `ActiveRecord`'s
* `afterInsert` event:
*
* ~~~
* Event::on(ActiveRecord::className(), ActiveRecord::EVENT_AFTER_INSERT, function ($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()]].
*
* 為一個類添加事件
*
* @param string $class the fully qualified class name to which the event handler needs to attach.
* @param string $name the event name.
* @param callable $handler the event handler.
* @param mixed $data the data to be passed to the event handler when the event is triggered.
* 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 最左邊的斜槓
$class = ltrim($class, '\\');
// 如果 append 為true，就放到 $_events 中名字為 $name 的數組的最後，否則放到最前面
if ($append || empty(self::$_events[$name][$class])) {
self::$_events[$name][$class][] = [$handler, $data];
} else {
array_unshift(self::$_events[$name][$class], [$handler, $data]);
}
}
/**
* Detaches an event handler from a class-level event.
*
* This method is the opposite of [[on()]].
*
* 移除一個類的事件
*
* @param string $class the fully qualified class name from which the event handler needs to be detached.
* @param string $name the event name.
* @param callable $handler the event handler to be removed.
* If it is null, all handlers attached to the named event will be removed.
* @return boolean whether a handler is found and detached.
* @see on()
*/
public static function off($class, $name, $handler = null)
{
$class = ltrim($class, '\\');
if (empty(self::$_events[$name][$class])) {
// 不存在該事件
return false;
}
if ($handler === null) {
// 如果 handler 為空，直接將在該類下該事件移除，即移出所有的是這個名字的事件
unset(self::$_events[$name][$class]);
return true;
} else {
$removed = false;
// 如果 $handler 不為空，循環 $_events 找到相應的 handler，只移除這個 handler 和 data 組成的數組
foreach (self::$_events[$name][$class] as $i => $event) {
if ($event[0] === $handler) {
unset(self::$_events[$name][$class][$i]);
$removed = true;
}
}
if ($removed) {
// 移除之後，使數組重新變成一個自然數組
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.
* 檢測在某個類或者對象是否具有某個事件
* @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)) {
// 如果是一個 object，就獲取其類名
$class = get_class($class);
} else {
// 如果是一個類名，就去掉 class 最左邊的斜槓
$class = ltrim($class, '\\');
}
// 如果該類中找不到，就去父類中找，直到找到或者沒有父類了為止
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.
* 觸發某個類或者對象的某個事件
* @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，就創建一個 Event 對象
$event = new static;
}
// 設置event對象的屬性，默認是未被處理的
$event->handled = false;
$event->name = $name;
if (is_object($class)) {
if ($event->sender === null) {
// 如果 $class 是個對象，並且是 sender 為空，就將 $class 賦給 sender，即 $class 就是觸發事件的對象
$event->sender = $class;
}
$class = get_class($class);
} else {
$class = ltrim($class, '\\');
}
// 循環類的 $_event，直到遇到 $event->handled 為真或者沒有父類了為止
do {
if (!empty(self::$_events[$name][$class])) {
foreach (self::$_events[$name][$class] as $handler) {
// 將參數賦到 event 對象的 data 屬性上
$event->data = $handler[1];
// 調用 $handler 方法
// 在方法中，可以用 $this->data 取到相應的參數
// 也可以在其中設置 $this->handled 的值，中斷後續事件的觸發
call_user_func($handler[0], $event);
// 當某個 handled 被設置為 true 時，執行到這個事件的時候，會停止，並忽略剩下的事件
if ($event->handled) {
return;
}
}
}
} while (($class = get_parent_class($class)) !== false);
}
}
通過上面代碼可以看出，類級別的 Event，其本質就是在 Event 類中的 $_events 變量中存儲事件，觸發事件的時候，只需將其取出，執行即可。
$_events裡面的數據結構大概如下：
[
'add' => [
'Child' => [
[function ($event) { ... }, $data],
[[$object, 'handleAdd'], null],
[['ChildClass', 'handleAdd'], $data],
['handleAdd', $data]
],
'ChildClass' => [
...
]
],
'delete' => [
...
]
]
之後講到yii\base\Component類時，我們會再來說一下實例級別的事件。
對 Yii2 源碼有興趣的同學可以關注項目 yii2-2.0.3-annotated，現在在上面已經添加了不少關於 Yii2 源碼的注釋，之後還會繼續添加～
有興趣的同學也可以參與進來，提交 Yii2 源碼的注釋。
