初步判断string是不是json格式

+ (BOOL)isJsonString:(NSString *)jsonString{
    if (jsonString.length < 2) return NO;
    if (!([jsonString hasPrefix:@"{"] || [jsonString hasPrefix:@"["])) return NO;
    // {:123  }:125  [: 91  ]:93
    return [jsonString characterAtIndex:jsonString.length-1]-[jsonString characterAtIndex:0] == 2;
}

jsonToObj

+ (id)jsonToObj:(NSString *)jsonString{
    if (![self isJsonString:jsonString]) return nil;
    NSError *err;
    NSData *jsonData = [jsonString dataUsingEncoding:NSUTF8StringEncoding];
    id obj;
    if (jsonData) {
        obj = [NSJSONSerialization JSONObjectWithData:jsonData options:NSJSONReadingMutableContainers error:&err];
    }
    if (err) {
        return nil;
    }
    return obj;
}