核心数据 – 如何从实体属性获取最大值(Swift)

发布时间：2020-12-14 02:27:10 所属栏目：百科来源：网络整理

导读：食谱 recipeID：Int recipeName：String 我有一个带有属性recipeID的实体配方. 如何在Swift中将max(recipeID)作为Int值？我很快,请帮助我. 提前致谢. func fetchMaxID() { let context = (UIApplication.sharedApplication().delegate as! AppDelegate).man

食谱

> recipeID：Int
> recipeName：String

我有一个带有属性recipeID的实体配方.
如何在Swift中将max(recipeID)作为Int值？

我很快,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 _ {

    }
}

Apple推荐并且速度最快的方式是使用NSExpressions. moc是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
}

（编辑：李大同）

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