如何在 Swift 中聚合 [关闭]

Question

我在 swift 中使用聚合函数时遇到问题，我将一个数字保存到 coredata 中。但是我一直在尝试获取它并按某个实体组织它。例如，加利福尼亚有 150 只狗，加利福尼亚有 60 只猫，俄勒冈州有 200 只狗，俄勒冈州有 60 只猫。我把它放到一个 VC 中，让人们输入每个州有多少只狗或猫。但在下一个中，我希望能够按州排序并合计每个州的动物数量。

Answer 1

您可以使用 NSExpression 类从 CoreData 获取聚合信息。这是我专门为您的案例创建的示例，因为我以前从未这样做过并且发现这很有趣；）

我的 CoreData 模型如下所示：

对于 Animal 的类型我使用枚举：

enum AnimalType: Int {
    case Cat, Dog
}

视图控制器的代码：

class AggregationResult {
    let locationName: String
    let animalType: AnimalType
    let count: Int

    init(locationName: String, animalType: AnimalType, count: Int) {
        self.locationName = locationName
        self.animalType = animalType
        self.count = count
    }
}

class ViewController: UIViewController {

    @IBOutlet weak var tableView: UITableView!

    private var dataSource: [AggregationResult] = []

    override func viewDidLoad() {
        super.viewDidLoad()

        load()
    }

    private func load() {

        let countExpressionDesc = NSExpressionDescription()
        countExpressionDesc.name = "countAnimals"
        countExpressionDesc.expression = NSExpression(forFunction: "count:", arguments: [NSExpression(forKeyPath: "type")])
        countExpressionDesc.expressionResultType = .Integer32AttributeType

        let request = NSFetchRequest(entityName: "Animal")
        request.propertiesToFetch = ["location.name", "type", countExpressionDesc]
        request.propertiesToGroupBy = ["location.name", "type"] // <-- all these properties should be put in 'propertiesToFetch' otherwise we will have crash
        //request.predicate = NSPredicate(format: "location.name == %@", argumentArray: ["Oregon"]) // <-- uncomment to find animals from Oregon only
        request.resultType = .DictionaryResultType

        let appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
        let results = try! appDelegate.managedObjectContext.executeFetchRequest(request)

        if let results = results as? [NSDictionary] {

            print("results: \(results)")

            for dict in results {
                if let
                    name = dict["location.name"] as? String,
                    type = dict["type"] as? Int,
                    atype = AnimalType(rawValue: type),
                    count = dict["countAnimals"] as? Int {

                    let ar = AggregationResult(locationName: name, animalType: atype, count: count)
                    dataSource.append(ar)
                }
            }

            tableView.reloadData()
        }
    }
}

extension ViewController: UITableViewDataSource {
    func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return dataSource.count
    }

    func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCellWithIdentifier("Cell", forIndexPath: indexPath)

        let item = dataSource[indexPath.row]
        cell.textLabel!.text = "\(item.locationName)"
        cell.detailTextLabel!.text = "\(item.count) \(item.animalType == .Cat ? "cats" : "dogs")"

        return cell
    }
}

你感兴趣的获取放在load函数中。 最后我得到的结果是：

results: [{
    countAnimals = 60;
    "location.name" = California;
    type = 0;
}, {
    countAnimals = 150;
    "location.name" = California;
    type = 1;
}, {
    countAnimals = 100;
    "location.name" = Oregon;
    type = 0;
}, {
    countAnimals = 200;
    "location.name" = Oregon;
    type = 1;
}]

表格看起来像：

希望这会有所帮助:)