Sum of (double) fetch request field

问题

Is it possible to get the sum of a core data (through a fetch request) field (double) in Swift UI? Thanks in advance.

回答1:

Yes, you could use the reduce method.

var sum: Double {
    fetchRequest.reduce(0) { $0 + $1.number }
}

This starts with an initial value of 0, and for each item in the FetchRequest, it adds the number property to the accumulated total.

Assuming you have an entity called SomeEntity in your .xcdatamodeld file, with an attribute called number, the whole code might look like

struct ContentView: View {

    @Environment(\.managedObjectContext) var managedObjectContext
    @FetchRequest(entity: SomeEntity.entity(), sortDescriptors: [NSSortDescriptor(keyPath: \SomeEntity.number, ascending: false)]) var fetchRequest: FetchedResults<SomeEntity>

    var sum: Double {
        fetchRequest.reduce(0) { $0 + $1.number }
    }

    var body: some View {
        Form {
            Section {
                Button("Add number") {
                    let entity = SomeEntity(context: self.managedObjectContext)
                    entity = Double.random(in: 0...10)
                    try? self.managedObjectContext.save()
                }
            }

            Section(header: Text("Sum")) {
                Text("\(sum)")
            }

            Section {
                ForEach(fetchRequest, id: \.self) {
                    Text("\($0.number)")
                }
            }


        }
    }
} 

来源：https://stackoverflow.com/questions/58956207/sum-of-double-fetch-request-field