核心数据 – 如何从实体属性获取最大值(Swift)

概述食谱 > 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! AppDel 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}

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