如何在 SwiftUI 中汇总过滤的核心数据？

Question

如何在 SwiftUI 应用中对过滤后的 Core Data 值求和（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（现在它在示例代码中设置为 current Date()，但稍后将设置为整个当前月份）。

我发现了关于 Swift 的类似问题，但我无法让这段代码在我的应用程序中运行。 相关问题链接：How to sum the numbers(Int16) of stored core data - Swift 3

根据链接中的代码，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   // - so I assume that this linked question may be outdated or I should take different approach using SwiftUI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，所以也许有更好的完全不同的方法来解决这个问题？

Answer 1

我在这个问题中发现了类似的问题： Sum of (double) fetch request field 我稍微修改了代码并提出了这个解决方案：

import SwiftUI
import CoreData

struct DashboardView: View {
    @Environment(\.managedObjectContext) var managedObjectContext
    // FetchRequest with predicate set to "after now" 
    @FetchRequest(entity: NPTransaction.entity(), sortDescriptors: [NSSortDescriptor(keyPath: \NPTransaction.value, ascending: false)], predicate: NSPredicate(format: "date > %@", Date() as NSDate)) var fetchRequest: FetchedResults<NPTransaction>

    // sum results using reduce
    var sum: Int64 {
        fetchRequest.reduce(0) { $0 + $1.value }
    }

    var body: some View {
        NavigationView {
            VStack(alignment: .leading) {

                HStack {
                    VStack(alignment: .leading) {
                        Text("sum")
                        Text("\(sum)")
                            .font(.largeTitle)

                    }
                    Spacer()
                }
                .padding()

                Spacer()
            }.navigationBarTitle("Title")
        }
    }

}

struct DashboardView_Previews: PreviewProvider {
    static var previews: some View {
        let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
        return DashboardView().environment(\.managedObjectContext, context)
    }
}

Answer 2

这应该修复您当前的代码，您正在尝试减少名为 result 的不存在变量，我相信您是从链接的代码中获得的。您还进行了两次不必要的提取。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一个示例：Group by, Count and Sum in CoreData。

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

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

let monthlyIncome = try! context.fetch(request) as? Int64 

if let monthlyIncome = monthlyIncome {
    print("monthly income total: \(monthlyIncome)")
}

编辑：我用您的实体和列名修改了示例代码。