public function on(string $event, \Closure $closure): void
    {
        $this->events[$event] = $closure;
    }
public function on(string $event, \Closure $closure): void
    {
        $this->events[$event] = $closure;
    }
public function on ( string $ event , \ Closure $ closure ) : void { $ this -> events [ $ event ] = $ closure ; }
add event handler
@param string   $event
@param \Closure $closure
add event handler