nsopenglview子类中键盘事件的恼人的碰撞声

我已经重写了nsopenglview来处理键盘事件。事件检测工作正常,但每次我按一个键,就会听到烦人的撞击声。我怎样才能告诉我的观点冷静下来呢?

下面是我的keyup:方法如下:

-(void) keyUp:(NSEvent *)theEvent
{
    NSString *characters = [theEvent charactersIgnoringModifiers];

    if ( [characters length] != 1 )
        return;

    unichar keyChar = [characters characterAtIndex:0];

    if ( keyChar == NSLeftArrowFunctionKey ) 
    {
        //do something
        return;
    }

    if ( keyChar == NSRightArrowFunctionKey ) 
    {
        //do something
        return;
    }

    if ( keyChar == NSUpArrowFunctionKey ) 
    {
        //do something
        return;
    }

    if ( keyChar == NSDownArrowFunctionKey ) 
    {
        //do something
        return;
    }
}