类不符合 Encodable

Question

However, getting an error message that says that my class 'Expenses' does not conform to protocol 'Decodable' & Type 'Expenses' does not conform to protocol 'Encodable'

import Foundation

class Expenses : ObservableObject, Codable  {
    
    @Published var items : [ExpenseItem] {
        // Step 1 creat did set on publsihed var.
        didSet {
            let encoder = JSONEncoder()
            if let encoded = try? encoder.encode(items) {
                UserDefaults.standard.set(encoded, forKey: "Items")
            }
        }
    }

    init() {
        if let items = UserDefaults.standard.data(forKey: "Items") {
            let decoder = JSONDecoder(
            if let decoded = try?
                decoder.decode([ExpenseItem].self, from: items) {
                self.items = decoded
                return
            }
        }
        self.items = []
    }     
}

my expense item is flagged as

struct ExpenseItem : Identifiable, Codable   {
    let id = UUID()
    let name : String
    let type : String
    let amount : Int       
}

Answer 1

Conformance to Encodable / Decodable is auto-synthesized when all stored properties conform to Encodable / Decodable , but using a property wrapper on a property means that now the property wrapper type needs to conform to Encodable / Decodable .

@Published property wrapper doesn't conform. It would have been nice to just implement conformance on the Published type itself, but unfortunately it doesn't expose the wrapped value, so without using reflection (I've seen suggestions online), I don't think it's possible.

You'd need to implement the conformance manually:

class Expenses : ObservableObject {
   @Published var items : [ExpenseItem]

   // ... rest of your code
}

extension Expense: Codable {
   enum CodingKeys: CodingKey {
      case items
   }
    
   func encode(to encoder: Encoder) throws {
      var container = encoder.container(keyedBy: CodingKeys.self)
      try container.encode(self.items, forKey: .items)
   }

   required init(from decoder: Decoder) throws {
      var container = try decoder.container(keyedBy: CodingKeys.self)
      self.items = try container.decode([ExpenseItem].self, forKey: .items)
   }      
}