如何在SwiftUI中汇总过滤后的核心数据？

Question

如何在SwiftUI应用程序中汇总过滤后的核心数据值（使用谓词？

我有1个名为NPTransaction的实体。它有5个属性，我想对名为value（Integer 64）的属性的值求和。基本上，它将汇总所选月份的收入值（现在在示例代码中将其设置为当前Date（），但稍后将其设置为整个当月）。

我发现了有关Swift的类似问题，但无法使此代码在我的应用中正常工作。链接到相关问题：How to sum the numbers(Int16) of stored core data - Swift 3

基于链接中的代码，我现在有以下代码：

import SwiftUI

struct DashboardView: View {
    @Environment(\.managedObjectContext) var managedObjectContext
    @State var selectedDate = Date()

   // This code in private var below produces many errors like:
   // Value of type 'AppDelegate' has no member 'managedObjectContext'
   // Use of undeclared type 'NSEntityDescription'
   // Use of undeclared type 'NSFetchRequest'
   // - so I assume that this linked question may be outdated or I should take different approach using SwiftUI
    private var income: Int64 {
        let context = (UIApplication.shared.delegate as! AppDelegate).managedObjectContext
        let entityDesc: NSEntityDescription = NSEntityDescription.entity(forEntityName: "NPTransaction", in: context)!
        let request: NSFetchRequest<NSFetchRequestResult> = NSFetchRequest()
        request.entity = entityDesc

        request.predicate = NSPredicate(format: "date == %@", selectedDate)
        let records = try! context.fetch(request)

        try! context.fetch(request) as! [NSManagedObject]
        let monthlyIncome = result.reduce(0) { $0 + ($1.value(forKey: "value") as? Int64 ?? 0) }
        return monthlyIncome
    }

    var body: some View {
        NavigationView {
            VStack {                    
               Text("Monthly income:")
               Text("\(income) USD")
                }

// more code...

我只是在学习Swift和SwiftUI，所以也许有更好和完全不同的方式来解决此问题？

Answer 1

这应该解决您当前的代码，您正尝试减少称为result的不存在的变量，我相信您是从链接的代码中获得的。您还将进行两次不必要的提取。希望这会有所帮助。

request.predicate = NSPredicate(format: "date == %@", selectedDate)
let records = try! context.fetch(request) as! [NSManagedObject]
let monthlyIncome = records.reduce(0) { $0 + ($1.value(forKey: "value") as? Int64 ?? 0) }
return monthlyIncome

此外，您可以通过NSFetchRequest和NSExpression对CoreData中的值求和。这是我在本文中找到的示例：Group by, Count and Sum in CoreData

let keypathExp1 = NSExpression(forKeyPath: "col")
let expression = NSExpression(forFunction: "sum:", arguments: [keypathExp1])
let sumDesc = NSExpressionDescription()
sumDesc.expression = expression
sumDesc.name = "sum"
sumDesc.expressionResultType = .integer64AttributeType

let request = NSFetchRequest(entityName: "Record")
request.returnsObjectsAsFaults = false
request.propertiesToFetch = [sumDesc]
request.resultType = .dictionaryResultType