UIFont如何获取系统字体样式

familyName为系统字体样式集合
在UIFont中同时定义了属性和类方法，所以两种方法都可以获取到

#if UIKIT_DEFINE_AS_PROPERTIES@property(class, nonatomic, readonly) NSArray<NSString *> *familyNames;#else+ (NSArray<NSString *> *)familyNames;#endif//每种系统字体样式中，又可以细分为几种for(NSString *fontfamilyname in [UIFont familyNames])    {        NSLog(@"family:'%@'",fontfamilyname);        for(NSString *fontName in         [UIFont fontNamesForFamilyName:fontfamilyname])        {            NSLog(@"\tfont:'%@'",fontName);        }        NSLog(@"-------------%d",i++);    }