src/Bot.php
public function on(\Closure $checker, \Closure $handler)
    {
        $this->managers[] = new Manager($checker, $handler);

        return $this;
    }
public function on(\Closure $checker, \Closure $handler)
    {
        $this->managers[] = new Manager($checker, $handler);

        return $this;
    }
public function on ( \ Closure $ checker , \ Closure $ handler ) { $ this -> managers [ ] = new Manager ( $ checker , $ handler ) ; return $ this ; }
Register event handler callback

@param \Closure $checker checker function
@param \Closure $handler handler function

@return \Viber\Bot
Register event handler callback