UIButton的状态优先级

发现问题

最近写项目中发现一个有趣儿的问题，UIButton 设置了 btn.enabled = NO; 但是 btn 显示的图片并非 UIControlStateDisabled 状态设置的图片。
问题出在哪里呢？请看下文分析。

UIButton的状态

typedef NS_OPTIONS(NSUInteger, UIControlState) {
    UIControlStateNormal       = 0,
    UIControlStateHighlighted  = 1 << 0,                  // used when UIControl isHighlighted is set
    UIControlStateDisabled     = 1 << 1,
    UIControlStateSelected     = 1 << 2,                  // flag usable by app (see below)
    UIControlStateFocused NS_ENUM_AVAILABLE_IOS(9_0) = 1 << 3, // Applicable only when the screen supports focus
    UIControlStateApplication  = 0x00FF0000,              // additional flags available for application use
    UIControlStateReserved     = 0xFF000000               // flags reserved for internal framework use
};

最常用的是前四种（UIControlStateNormal、UIControlStateHighlighted、UIControlStateDisabled、UIControlStateSelected）。

思考

No bb, Show code

[btn setImage:[UIImage imageNamed:@"img0"] forState:UIControlStateNormal];
[btn setImage:[UIImage imageNamed:@"img1"] forState:UIControlStateHighlighted]; 
[btn setImage:[UIImage imageNamed:@"img2"] forState:UIControlStateDisabled];
[btn setImage:[UIImage imageNamed:@"img2"] forState:UIControlStateSelected];

问题：如果如下设置，btn 会显示那张图片。

1.  
btn.highlighted = YES;
btn.enabled = YES;
btn.selected = NO;// 显示 Highlighted

2. 
btn.highlighted = YES;
btn.enabled = NO;
btn.selected = YES;// 显示 Normal

3.
btn.highlighted = NO;
btn.enabled = YES;
btn.selected = YES;// 显示 Selected

4. 
btn.highlighted = NO;
btn.enabled = NO;
btn.selected = YES;// 显示 Normal

总结

当 btn 的两种状态展示设置有冲突（不分设置的先后顺序）时，系统会默认显示 UIControlStateNormal 状态的图片。无冲突时，正常显示。