Swift 3 核心数据

Posted 2023-03-05

【中文标题】Swift 3 核心数据

【英文标题】：Swift 3 Core Data

【发布时间】：2016-09-15 17:41:15

【问题描述】：

尝试在 Swift 3 中执行 NSFetchRequest 时出现以下错误

Generic parameter 'ResultType' could not be inferred

我检查了很多链接，但我无法弄清楚如何解决它。

这就是我正在做的事情

ViewController.swift

func loadData()
    let request = NSFetchRequest(entityName: "Grocery") //Error occurs here(Generic parameter 'ResultType' could not be inferred)
    do
        let results = try manageObjectContext.execute(request)
        groceries = results as! [NSManagedObject]
        tableView.reloadData()
    catch
        fatalError("Error is retriving Gorcery items")
    

【问题讨论】：

Swift 4 中 CoreData 的快速介绍：medium.com/swift2go/…

【参考方案1】：

试试这个：

let request = NSFetchRequest<NSFetchRequestResult>(entityName: "Grocery")

它应该工作:)

因此您的代码应如下所示：

func loadData()
 let request = NSFetchRequest<NSFetchRequestResult>(entityName: "Grocery")
do
    let results = try manageObjectContext.execute(request)
    groceries = results as! [NSManagedObject]
    tableView.reloadData()
catch
    fatalError("Error is retriving Gorcery items")
 

【讨论】：

谢谢，它有效，但是我将其更改为使用 NSManagedObject 子类

【参考方案2】：

致谢：Nitesh Patil https://medium.com/@imnitpa/swift-3-core-data-7b00b50f5782

import UIKit
import CoreData

class ViewController: UIViewController 

    override func viewDidLoad() 
        super.viewDidLoad()

        view.backgroundColor = .white

        let appDelegate = UIApplication.shared.delegate as!AppDelegate
        let context = appDelegate.persistentContainer.viewContext

        //adding new user and saving in database...
        let newUser = NSEntityDescription.insertNewObject(forEntityName: "Users", into: context)
        newUser.setValue("Batman", forKey: "username")
        newUser.setValue("Robin", forKey: "password")

        do 
            try context.save()
            print("saved")
         catch 
            print("Error occured...")
        

        // restoring data back from database
        let request = NSFetchRequest < NSFetchRequestResult > (entityName: "Users")
        request.returnsObjectsAsFaults = false

        do 
            let results =
                try context.fetch(request)

            if results.count > 0 

                for result in results as![NSManagedObject] 

                    if let username = result.value(forKey: "username") as ? String 

                        print(username)
                    
                
            

            else 
                print("No results")
            
         catch 
            print("Couldn't fetch results")
        
    

【参考方案3】：

你也可以用这个：

let request : NSFetchRequest<Grocery> = Grocery.fetchRequest()

当你执行请求时，你会得到 [Grocery] 类型的结果：

let results : [Grocery] = try manageObjectContext.execute(request)

或

let results = try manageObjectContext.execute(request)