如何从coredata swift中随机选择一个元素

Posted 2023-03-12

【中文标题】如何从coredata swift中随机选择一个元素

【英文标题】：How to randomly choose an element from coredata swift

【发布时间】：2016-01-21 06:51:44

【问题描述】：

import UIKit
import CoreData

class Period1Controller: UIViewController, UITextFieldDelegate 


@IBOutlet weak var enterName: UITextField!
@IBOutlet weak var presentName: UITextView!
@IBOutlet weak var independceStatue: UITextField!
@IBOutlet weak var driverSelection: UITextField!


var entitys = [NSManagedObject]()
var nameList : [String] = []
var period1 = ""
var period1NameList = ""

override func viewDidLoad() 
    super.viewDidLoad()




func setValues() 
    nameList = [enterName.text!]


//this button is for saving the element into the core data
@IBAction func setName(sender: UIButton) 
    setValues()
    for item in nameList
        period1 += (item + "  ")
        period1NameList += item
    
    let context = (UIApplication.sharedApplication().delegate as! AppDelegate).managedObjectContext
    let entity = NSEntityDescription.entityForName("Entity", inManagedObjectContext: context)
    let otherEntity = NSManagedObject(entity: entity!, insertIntoManagedObjectContext: context)
    otherEntity.setValue(period1NameList, forKey: "period1Core")
    do 
        try context.save()
        print("Item Saved")
     catch 
        print("Saved Failed")
    
    presentName.text = period1
    enterName.text = ""


// this button is for taking out element from core data and randomly pick a value from the element I took out from core data
@IBAction func start(sender: UIButton) 
    setValues()
    let context = (UIApplication.sharedApplication().delegate as! AppDelegate).managedObjectContext
    let request = NSFetchRequest(entityName: "Entity")
    do 
        let results = try context.executeFetchRequest(request)
        entitys = results as! [NSManagedObject]
     catch let error as NSError 
        print("Could not fetch \(error), \(error.userInfo)")
    
    let otherEntity = entitys.last
    let randomIndex = Int(arc4random_uniform(UInt32(otherEntity.count)))
    driverSelection.text! = nameList[randomIndex]

我要做的是从我的核心数据中随机选择一个元素并将其设置为等于 driverSelection 文本字段。 在我的代码中，我将 coredata = 中的元素设置为otherEntity。这使otherEntity 成为NSmanagedObject，但由于otherEntity 是NSmanagedObject，我无法使用.count 方法。有没有办法可以让我从NSmanagedObject中随机选择一个元素？？？

【参考方案1】：

斯威夫特 5

我一直在寻找这个答案，我发现你可以用 fetchOffset 得到一行，用 moc.count 得到数组计数

像这样……

func randomIconAK(_ moc: NSManagedObjectContext) -> Icon? 
  //
  //  RETURNS ONE ITEM AT RANDOM
  //
  let fetchRequest: NSFetchRequest<NSFetchRequestResult> = NSFetchRequest(entityName: "Icon")

  if let fetchRequestCount = try? moc.count(for: fetchRequest) 
    fetchRequest.fetchOffset = Int.random(in: 0...fetchRequestCount)
  

  fetchRequest.fetchLimit = 1

  var fetchResults: [Icon]?
  moc.performAndWait 
    fetchResults = try? fetchRequest.execute() as? [Icon]
  

  if let wrapFetchResults = fetchResults 
    if wrapFetchResults.count > 0 
      return wrapFetchResults.first
     else 
      return nil
    
   else 
    return nil
  
//randomIconAK

【参考方案2】：

如果您尝试从数据库中的当前对象数中提取随机索引，则应使用results.count 而不是otherEntity.count，如下所示：

    @IBAction func start(sender: UIButton) 
        setValues()
        let context = (UIApplication.sharedApplication().delegate as! AppDelegate).managedObjectContext
        let request = NSFetchRequest(entityName: "Entity")
        var results = [AnyObject]()

        do 
            results = try context.executeFetchRequest(request)

         catch let error as NSError 
            print("Could not fetch \(error), \(error.userInfo)")
        
        let randomIndex = Int(arc4random_uniform(UInt32(results.count)))
        driverSelection.text! = nameList[randomIndex]

    

【讨论】：

它不起作用。它返回一个错误，提示“无法将类型“[AnyObject]”的值分配给类型“[NSManagedObject]”

对，抱歉，结果需要是 [AnyObject] 而不是 [NSManagedObject] 类型，我已修复。

如果回答了您的问题，请接受答案。

再问一个问题。当我按下“开始”按钮时，它返回我索引超出范围。是不是因为它没有从我的核心数据中检索元素？

检查你的nameList是否为空。即使 DB 为空，randomIndex 也会为零，这是一个有效的索引，但如果 nameList 为空，那么您将超出边界异常。