class ResponseBuffer:
    def __init__(self):
        self._text = ""
        self._observers = []
    
    def add_observer(self, callback):
        self._observers.append(callback)
    
    def get_text(self):
        return self._text
    
    def append_text(self, text):
        self._text += text
        self._notify_observers()
    
    def set_text(self, text):
        self._text = text
        self._notify_observers()
    
    def clear(self):
        self._text = ""
        self._notify_observers()
    
    def _notify_observers(self):
        for observer in self._observers:
            observer(self._text)