加载ttf字体

+ (UIFont *)flatFontOfSize:(CGFloat)size {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSURL * url = [[NSBundle mainBundle] URLForResource:@"Lato-Regular" withExtension:@"ttf"];
        CFErrorRef error;
        CTFontManagerRegisterFontsForURL((__bridge CFURLRef)url, kCTFontManagerScopeNone, &error);
        error = nil;
    });
    return [UIFont fontWithName:@"Lato-Regular" size:size];
}