UIButton 点击切换不同图片

_Bool isopen;
if (!isopen) {
    [btn setBackgroundImage:[UIImage imageNamed:@"selebtndown"] forState:UIControlStateNormal];
} else {
       [btn setBackgroundImage:[UIImage imageNamed:@"selebtnup"] forState:UIControlStateNormal];
   }
   isopen = !isopen;