if (!value || value == (id)kCFNull) return nil;

任意对象转换为数字的算法 摘自 YYModel
static force_inline NSNumber *YYNSNumberCreateFromID(__unsafe_unretained id value) {
static NSCharacterSet *dot;
static NSDictionary *dic;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
dot = [NSCharacterSet characterSetWithRange:NSMakeRange('.', 1)];
dic = @{@"TRUE" : @(YES),
@"True" : @(YES),
@"true" : @(YES),
@"FALSE" : @(NO),
@"False" : @(NO),
@"false" : @(NO),
@"YES" : @(YES),
@"Yes" : @(YES),
@"yes" : @(YES),
@"NO" : @(NO),
@"No" : @(NO),
@"no" : @(NO),
@"NIL" : (id)kCFNull,
@"Nil" : (id)kCFNull,
@"nil" : (id)kCFNull,
@"NULL" : (id)kCFNull,
@"Null" : (id)kCFNull,
@"null" : (id)kCFNull,
@"(NULL)" : (id)kCFNull,
@"(Null)" : (id)kCFNull,
@"(null)" : (id)kCFNull,
@"<NULL>" : (id)kCFNull,
@"<Null>" : (id)kCFNull,
@"<null>" : (id)kCFNull};
});

if (!value || value == (id)kCFNull) return nil;
if ([value isKindOfClass:[NSNumber class]]) return value;
if ([value isKindOfClass:[NSString class]]) {
    NSNumber *num = dic[value];
    if (num) {
        if (num == (id)kCFNull) return nil;
        return num;
    }
    if ([(NSString *)value rangeOfCharacterFromSet:dot].location != NSNotFound) {
        const char *cstring = ((NSString *)value).UTF8String;
        if (!cstring) return nil;
        double num = atof(cstring);
        if (isnan(num) || isinf(num)) return nil;
        return @(num);
    } else {
        const char *cstring = ((NSString *)value).UTF8String;
        if (!cstring) return nil;
        return @(atoll(cstring));
    }
}
return nil;

}