ios 判断字符串是否为空

#define NULLString(string) ((![string isKindOfClass:[NSString class]])||[string isEqualToString:@""] || (string == nil) || [string isEqualToString:@""] || [string isKindOfClass:[NSNull class]]||[[string stringByTrimmingCharactersInSet:[NSCharacterSet whitespaceCharacterSet]] length]==0)