UIButton选择状态下长按时会变回原始状态

问题大概就像这样

（请无视那红字）
一般而言这是高亮状态的设置有所缺乏。完善代码如下：

[_pupilBtn setImage:[UIImage imageNamed:@"a1"]
             forState:UIControlStateNormal];
[_pupilBtn setImage:[UIImage imageNamed:@"a1"]
             forState:UIControlStateHighlighted];
[_pupilBtn setImage:[UIImage imageNamed:@"a2"]
             forState:UIControlStateSelected];
[_pupilBtn setImage:[UIImage imageNamed:@"a2"]
             forState:UIControlStateSelected | UIControlStateHighlighted];