How to update a struct value?

Question

I have some monthly data that I need to calculate a specific value for before it gets displayed, using:

.onAppear{
     ForEach(monthlyTilt) {item in
          item.degrees = NewValue…        
     }
 }

With a Struct/var combination of:

struct MonthlyTilt: Identifiable {
    var id = UUID().uuidString
    var month: String
    var day: Int
    var degrees: Double
}

var monthlyTilt = [
    MonthlyTilt(month: "Jan",  day: 15,  degrees: 0.0),
    MonthlyTilt(month: "Feb",  day: 46,  degrees: 0.0),
    MonthlyTilt(month: "Mar",  day: 74,  degrees: 0.0),
    MonthlyTilt(month: "Apr",  day: 105, degrees: 0.0),
    MonthlyTilt(month: "May",  day: 135, degrees: 0.0),
    MonthlyTilt(month: "Jun",  day: 166, degrees: 0.0),
    MonthlyTilt(month: "Jul",  day: 196, degrees: 0.0),
    MonthlyTilt(month: "Aug",  day: 227, degrees: 0.0),
    MonthlyTilt(month: "Sept", day: 258, degrees: 0.0),
    MonthlyTilt(month: "Oct",  day: 288, degrees: 0.0),
    MonthlyTilt(month: "Nov",  day: 319, degrees: 0.0),
    MonthlyTilt(month: "Dec",  day: 349, degrees: 0.0)
]

The error message reads: Cannot assign to property: 'item' is a 'let' constant. monthlyTilt is set as a var.

How can I update the degree value?

Answer 1

You're trying to update a struct which is, by default, immutable. There are a few different approaches to this. One way is to populate the monthlyTilt var only when you get the data you need to build the structs. You'll probably want to update your datasource to include the month and day as well.

import SwiftUI

public struct FBKChatBotView: View {
    @State private var tilts: [MonthlyTilt] = []
    
    public var body: some View {
      VStack {
        ForEach(tilts) { tilt in
          Text("Tilt is \(tilt.degrees) degrees")
        }
      }
        .onAppear {
           someDataSource() { items in
             items.forEach { item in
               let tilt = MonthlyTilt(month: item.month, day: item.day, degrees: item.degrees)
               tilts.append(tilt)
             }
           }
        }
    }
}

An alternative would be to set up an ObservableObject to handle building that list of an @Published array of tilts and your view would update when that changed. Then you could eliminate the onAppear.

Or, you could create a mutating function in your struct that allows it to set/overwrite its own variables.

struct MonthlyTilt: Identifiable {
    var id = UUID().uuidString
    var month: String
    var day: Int
    var degrees: Double

    mutating func setDegrees(_ to: Double) {
       degrees = to
    }
}

I don't know why, but I never really like that last (mutating) option because it opens the door to other things also mutating the struct and then you lose the strictness of an immutable struct. I don't know if that's best practice or personal preference, but I almost always try not to make structs mutable.

Sample code is off the top of my head so excuse any typos