[button setBackgroundImage:backImage forState:UIControlStateNormal];
[button setBackgroundImage:backImage forState:UIControlStateHighlighted];
[button setBackgroundImage:selecetedBackImage forState:UIControlStateSelected | UIControlStateHighlighted];
[button setBackgroundImage:selecetedBackImage forState:UIControlStateSelected];
[button setImage:iconImage forState:UIControlStateNormal];
[button setImage:iconImage forState:UIControlStateHighlighted];
[button setImage:iconImage forState:UIControlStateSelected|UIControlStateHighlighted];
[button setTitle:text forState:UIControlStateNormal];
[button.titleLabel setFont:[UIFont systemFontOfSize:12.0]];
自定义tabbar用button替代tabbaritem遇到了一个问题。就是在选中button的情况下，再次点击button，会变成默认的图片，看起来很不美观。
查了好久终于看到原来还是state的原因。
UIControlStateSelected | UIControlStateHighlighted
UIControlStateSelected
这俩代表的不同，UIControlStateSelected | UIControlStateHighlighted 是处于selected 的情况下，再次点击高亮的效果。
原文：http://www.cnblogs.com/LoveJiaQi/p/4683583.html