以前基本是使用一个按钮的两种状态，Normal/Disabled或者Normal/Selected，使用UIButton的下面两个方法设置不同状态下的文字颜色和图片

- (void)setTitleColor:(nullable UIColor *)color forState:(UIControlState)state UI_APPEARANCE_SELECTOR;
- (void)setImage:(nullable UIImage *)image forState:(UIControlState)state;

今天有一个复选框需要用到两种状态的组合，Selected和Disabled，用上面的方式进行设置

[self setImage:[UIImage imageNamed:@"round_uncheck"] forState:UIControlStateNormal];
[self setImage:[UIImage imageNamed:@"round_check"] forState:UIControlStateSelected];
[self setImage:[UIImage imageNamed:@"round_check_disabled"] forState:UIControlStateSelected];

运行后发现状态跟图标一直对不上，于是分析了一下按钮的状态，其实现在按钮已经有四种状态：

1. 正常状态，enabled|unselected
2. 选中状态，enabled|selected
3. 禁用状态，disabled|unselected
4. 选中状态下禁用，disabled|selected

我们设置了按钮的三种状态下的样式，Normal,Disabled,Selected, 显然只能满足前面三种状态，在第四中状态disabled|selected 时，UIButton就不知道要使用哪张图片了。

回到代码看下UIControlState这个枚举

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
};

其实UIControlState是一个NS_OPTIONS，可以使用按位与操作符连接两个状态，所以修改代码如下：

[self setImage:[UIImage imageNamed:@"round_uncheck"] forState:UIControlStateNormal|UIControlStateDisabled];
[self setImage:[UIImage imageNamed:@"round_check"] forState:UIControlStateSelected];
[self setImage:[UIImage imageNamed:@"round_check_disabled"] forState:UIControlStateSelected|UIControlStateDisabled];

这样设置后按钮的图片就跟预期的一致了。