统一设置UITabBarController管理的所有VC的tabBarItem图标文字的颜色大小等属性

1.  统一设置UITabBarController管理的所有VC的tabBarItem图标文字的颜色大小等属性

//设置字体大小及颜色(可变字典才可用[]方法设值)
    NSMutableDictionary *attr = [NSMutableDictionary dictionary];
    attr[NSFontAttributeName] = [UIFont systemFontOfSize:19];
    attr[NSForegroundColorAttributeName] = [UIColor grayColor];

    NSMutableDictionary *selectAttr = [NSMutableDictionary dictionary];
    selectAttr[NSFontAttributeName] = [UIFont systemFontOfSize:5];//NSFontAttributeName是个const变量，一旦设置了19，后面就不能再改了，这里改为5无效
    selectAttr[NSBackgroundColorAttributeName] = [UIColor darkGrayColor];

    UITabBarItem *item = [UITabBarItem appearance];
    [item setTitleTextAttributes:attr forState:UIControlStateNormal];
    [item setTitleTextAttributes:selectAttr forState:UIControlStateHighlighted];

2. 就不用像下面这样单独给每一个VC.tabBarItem设置字体大小和颜色了
[vc01.tabBarItem setTitleTextAttributes:attr forState:UIControlStateNormal];
    [vc01.tabBarItem setTitleTextAttributes:selectAttr forState:UIControlStateHighlighted];

[vc02.tabBarItem setTitleTextAttributes:attr forState:UIControlStateNormal];
    [vc02.tabBarItem setTitleTextAttributes:selectAttr forState:UIControlStateHighlighted];

[vc03.tabBarItem setTitleTextAttributes:attr forState:UIControlStateNormal];
    [vc03.tabBarItem setTitleTextAttributes:selectAttr forState:UIControlStateHighlighted];

原理是：

setTitleTextAttributes:forState: 函数后面有备注UI_APPEARANCE_SELECTOR，

可以通过 [Class appearance] setTitleTextAttributes: forState: ]; 来同一设置字体颜色。

//设置UITabBarItem:UIBarItem的颜色属性

NS_CLASS_AVAILABLE_IOS(2_0) @interface UIBarItem : NSObject <NSCoding, UIAppearance>

- (void)setTitleTextAttributes:(nullable NSDictionary<NSString *,id> *)attributes forState:(UIControlState)state NS_AVAILABLE_IOS(5_0) UI_APPEARANCE_SELECTOR;

- (nullable NSDictionary<NSString *,id> *)titleTextAttributesForState:(UIControlState)state NS_AVAILABLE_IOS(5_0) UI_APPEARANCE_SELECTOR