NSString* orderby = [object valueForKey:@"orderby"]; 

        NSLog(@"orderby::%d",orderby.length==0); //可以

        NSLog(@"orderby::%d",[orderby isEqualToString:@""]); //不可以

        NSLog(@"orderby::%d",[orderby isEqual:Nil]); //不可以

        NSLog(@"orderby::%d",!orderby); //可以