iOS/Objective C/Core数据：从查询中获得属性的所有值

Question

我想在从核心数据中获取某个属性后，将所有属性的值相加。是否有一些捷径可以快速返回这个结果？

这是我的标准代码。寻找的东西，将总量可达orderprice的值来获得所有订单的总和

- (id) getSalesTotal{ 
    NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"Orders"]; 
    fetchRequest.resultType = NSDictionaryResultType; 

    NSError *error  = nil; 
    self.managedObjectContext = [IDModel sharedInstance].managedObjectContext; 

    NSArray *results = [self.managedObjectContext executeFetchRequest:fetchRequest 
                    error:&error]; 
    NSLog(@"error is %@",error); 
    // NSLog(@"context%@",results); 
    NSLog(@"beforevalueforkey%@",results); 
    NSMutableArray * ordertotal = [[results valueForKey:@"orderprice"] mutableCopy]; 
    return ordertotal;//Is there something like sum(ordertotal)????? 
} 

Answer 1

您可以使用键 - 值编码：

NSNumber *orderTotal = [arrayWhoseSumIsToBeCalculated valueForKeyPath:@"@sum.self"];