如何根据查找具有特定值的实体在我的核心数据之间进行迭代

Posted 2023-03-12

【中文标题】如何根据查找具有特定值的实体在我的核心数据之间进行迭代

【英文标题】：How do iterate between my core data depending to find entities with a particular value

【发布时间】：2020-06-30 15:51:28

【问题描述】：

我是 swift 的初学者。我有一个名为 Task 的 coredata 实体，其属性为 Date。我想在该实体中的所有 nsmanaged 对象之间进行迭代，并提取具有特定日期的对象并将它们放入一个数组中。

func loadTasks()
    let df = DateFormatter()
    df.dateFormat = "dd-MM-yyyy" // assigning the date format
    let now = df.string(from: Date()) // extracting the date with the given format
    let appDel : AppDelegate = UIApplication.shared.delegate as! AppDelegate
    let context: NSManagedObjectContext = appDel.persistentContainer.viewContext// handler to access the core date database by using the context from the app delegate        loadTasks()
    let request = NSFetchRequest<NSFetchRequestResult>(entityName: "Tasks")
    
    do 
      coreTasks = try context.fetch(request) as! [NSManagedObject]
        for item in coreTasks 
            for date in item.value(forKey: "date")
                if (date == now) 
                    todaysTasks.append(date)
            
        
     catch let error as NSError 
      print("Could not fetch. \(error), \(error.userInfo)")
    

我试过了，但我到处都遇到语法错误。

【问题讨论】：

请在您的问题中粘贴错误。

【参考方案1】：

有几个问题

替换

for item in request 
   for date in item.value(forKey: "date") 
       if (date == now) 

与

for item in coreTasks 
   if item.value(forKey: "date") as! String == now 

---

更好的语法是filter记录

func loadTasks()
    let df = DateFormatter()
    df.locale = Locale(identifier: "en_US_POSIX")
    df.dateFormat = "dd-MM-yyyy" // assigning the date format
    let now = df.string(from: Date()) // extracting the date with the given format
    let appDel = UIApplication.shared.delegate as! AppDelegate
    let context = appDel.persistentContainer.viewContext// handler to access the core date database by using the context from the app delegate        loadTasks()
    let request = NSFetchRequest<Tasks>(entityName: "Tasks")
    
    do 
      coreTasks = try context.fetch(request)
      let todayItems = coreTasks.filter$0.date == now
      todaysTasks.append(contentsOf: todayItems)

     catch let error as NSError 
      print("Could not fetch. \(error), \(error.userInfo)")
    

最好是应用谓词

func loadTasks()
    let df = DateFormatter()
    df.locale = Locale(identifier: "en_US_POSIX")
    df.dateFormat = "dd-MM-yyyy" // assigning the date format
    let now = df.string(from: Date()) // extracting the date with the given format
    let appDel = UIApplication.shared.delegate as! AppDelegate
    let context = appDel.persistentContainer.viewContext// handler to access the core date database by using the context from the app delegate        loadTasks()
    let request = NSFetchRequest<Tasks>(entityName: "Tasks")
    request.predicate = NSPredicate(format: "date == %@", now)

    do 
      todaysTasks = try context.fetch(request)
     catch let error as NSError 
      print("Could not fetch. \(error), \(error.userInfo)")
    

并且始终以单数形式命名 Core Data 实体 (Task)。

【讨论】：

谢谢你我做到了。在 item.value(forKey: "date") 中获取日期。这条线现在给我一个错误。 “For-in 循环需要‘任何？’符合'Sequence'；你的意思是解开可选的吗？"

你必须投类型，请仔细阅读我的回答。并且不要使用value(forKey anyway。使用更好的语法。