UIButton在正常（UIControlStateNormal

UIButton在不同状态下的设置：

UIControlStateNormal 正常状态

UIControlStateHighlighted 高亮状态，按钮按下时状态，在正常状态时按下

UIControlStateSelected 选中状态

UIControlStateSelected|UIControlStateHighlighted 高亮状态，按钮按下时状态，在选中状态时按下

例如：

[btn setImage:  forState:UIControlStateNormal];

[btn setImage:  forState:UIControlStateHighlighted];

[btn setImage:  forState:UIControlStateSelected];

[btn setImage:  forState:UIControlStateSelected|UIControlStateHighlighted];