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在实体 Shift 中，我有两个属性 startTime 和 endTime，我想取差值，然后对所有获取的值求和。

let fetchRequest = NSFetchRequest<NSDictionary>(entityName: "Shift")
        fetchRequest.predicate = shiftPredicate
        fetchRequest.resultType = .dictionaryResultType

        let durationExpression = NSExpressionDescription()
        durationExpression.name = "duration"
        durationExpression.expression = NSExpression(format: "endTime - startTime")
        durationExpression.expressionResultType = .integer16AttributeType

// INSERT A

 do { let result = try
            managedObjectContext.fetch(fetchRequest)
            print("\(result)")
            let resultDict = result.first
            print("\(String(describing: resultDict?["sumDuration"]))")

        } catch let error as NSError {
            print ("Could not fetch \(error), \(error.userInfo)")
        }

现在这个 fetch 返回一个包含 [{duration, 3}, {duration, 6}] 的字典数组，但我想总结所有的持续时间值。

我在 INSERT A 尝试了以下操作

       let sumDurationExpression = NSExpressionDescription()
        sumDurationExpression.name = "sumDuration"
        sumDurationExpression.expression = NSExpression(forFunction: "sum:", arguments: ["duration"])
        sumDurationExpression.expressionResultType = .integer16AttributeType
        fetchRequest.propertiesToFetch!.append(sumDurationExpression)

但这会因发送无法识别的选择器而崩溃，我也尝试过

        durationExpression.expression = NSExpression(format: "sum: (endTime - startTime")
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