swift - 核心数据 - 如何从实体属性中获取最大值(Swift)

Question

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食谱

• 配方ID:整数
• 食谱名称:字符串

我有一个带有属性 recipeID 的实体 Recipe。如何在 Swift 中将 max(recipeID) 作为 Int 值获取？

我是 swift 的新手，请帮助我。提前致谢。

func fetchMaxID() {
    let context = (UIApplication.sharedApplication().delegate as! AppDelegate).managedObjectContext
    let fetchRequest = NSFetchRequest(entityName: "Recipe")

    fetchRequest.fetchLimit = 1
    let sortDescriptor = NSSortDescriptor(key: "recipeID", ascending: false)
    fetchRequest.sortDescriptors = [sortDescriptor]
    do {
        let maxID = try [managedObjectContext?.executeFetchRequest(fetchRequest)].first
        print(maxID)
    } catch _ {

    }
}

Answer 1

Apple 推荐并且最快的方式是使用 NSExpressions。 moc 是一个 NSManagedObjectContext。

private func getLastContactSyncTimestamp() -> Int64? {

    let request: NSFetchRequest<NSFetchRequestResult> = NSFetchRequest()
    request.entity = NSEntityDescription.entity(forEntityName: "Contact", in: self.moc)
    request.resultType = NSFetchRequestResultType.dictionaryResultType

    let keypathExpression = NSExpression(forKeyPath: "timestamp")
    let maxExpression = NSExpression(forFunction: "max:", arguments: [keypathExpression])

    let key = "maxTimestamp"

    let expressionDescription = NSExpressionDescription()
    expressionDescription.name = key
    expressionDescription.expression = maxExpression
    expressionDescription.expressionResultType = .integer64AttributeType

    request.propertiesToFetch = [expressionDescription]

    var maxTimestamp: Int64? = nil

    do {

        if let result = try self.moc.fetch(request) as? [[String: Int64]], let dict = result.first {
           maxTimestamp = dict[key]
        }

    } catch {
        assertionFailure("Failed to fetch max timestamp with error = \(error)")
        return nil
    }

    return maxTimestamp
}