objective-c 中判断字符串 是否为空

NSManagedObject* object=[self.listDataobjectAtIndex:i];

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