php是怎么实现事件监听的

class Event
{
    protected static $listens = array();
     
    public static function listen($event, $callback, $once=false){
        if(!is_callable($callback)) return false;
        self::$listens[$event][] = array('callback'=>$callback, 'once'=>$once);
        return true;
    }
     
    public static function one($event, $callback){
        return self::listen($event, $callback, true);
    }
     
    public static function remove($event, $index=null){
        if(is_null($index))
            unset(self::$listens[$event]);
        else
            unset(self::$listens[$event][$index]);
    }
     
    public static function trigger(){
        if(!func_num_args()) return;
        $args = func_get_args();
        $event = array_shift($args);
        if(!isset(self::$listens[$event])) return false;
        foreach((array) self::$listens[$event] as $index=>$listen){
            $callback = $listen['callback'];
            $listen['once'] && self::remove($event, $index);
            call_user_func_array($callback, $args);
        }
    }
}

// 增加监听walk事件
Event::listen('walk', function(){
    echo "I am walking...n";
});
// 增加监听walk一次性事件
Event::listen('walk', function(){
    echo "I am listening...n";
}, true);
// 触发walk事件
Event::trigger('walk');
/*
I am walking...
I am listening...
*/
Event::trigger('walk');
/*
I am walking...
*/
 
Event::one('say', function($name=''){
    echo "I am {$name}n";
});
 
Event::trigger('say', 'deeka'); // 输出 I am deeka
Event::trigger('say', 'deeka'); // not run
 
class Foo
{
    public function bar(){
        echo "Foo::bar() is calledn";
    }
     
    public function test(){
        echo "Foo::foo() is called, agrs:".json_encode(func_get_args())."n";
    }
}
 
$foo    = new Foo;
 
Event::listen('bar', array($foo, 'bar'));
Event::trigger('bar');
 
Event::listen('test', array($foo, 'test'));
Event::trigger('test', 1, 2, 3);
 
class Bar
{
    public static function foo(){
        echo "Bar::foo() is calledn";
    }
}
 
Event::listen('bar1', array('Bar', 'foo'));
Event::trigger('bar1');
 
Event::listen('bar2', 'Bar::foo');
Event::trigger('bar2');
 
function bar(){
    echo "bar() is calledn";
}
 
Event::listen('bar3', 'bar');
Event::trigger('bar3');