Using Codable with AppStorage, the easy way!

CodableAppStorage.swift

import Foundation
import SwiftUI

// MARK: Demo

struct Example: RawRepresentable, Codable {
    let id: UUID
}

// This is functionally the same as above, but we have a neater type without creating a new protocol.
typealias RawCodable = RawRepresentable & Codable
struct Example2: RawCodable {
    let id: UUID
}

struct ContentView: View {
    @AppStorage("1")
    var first = [1]
    
    @AppStorage("2")
    var second = ["test": 123]
    
    @AppStorage("3")
    var third = Example(id: .init())
    
    var body: some View {
        Text(first.description)
        Text(second.description)
        Text(third.id.description)
    }
}

// MARK: General Codables

extension RawRepresentable where Self: Codable {
    public init?(rawValue: String) {
        guard let value: Self = Coders.encode(rawValue) else { return nil }
        self = value
    }
    public var rawValue: String {
        Coders.decode(self, default: "{}")
    }
}

// MARK: Collections
// Slightly more complex than structs.

extension Collection where Self: RawRepresentable, Self: Codable, Element: Codable {
    public var rawValue: String {
        Coders.decode(self, default: "[]")
    }
}

extension Array: RawRepresentable where Element: Codable {
    public init?(rawValue: String) {
        guard let value: Self = Coders.encode(rawValue) else { return nil }
        self = value
    }
}

extension Set: RawRepresentable where Element: Codable {
    public init?(rawValue: String) {
        guard let value: Self = Coders.encode(rawValue) else { return nil }
        self = value
    }
}

// MARK: Dictionary

extension Dictionary: RawRepresentable where Key: Codable, Value: Codable {
    public init?(rawValue: String) {
        guard let value: Self = Coders.encode(rawValue) else { return nil }
        self = value
    }
    public var rawValue: String {
        Coders.decode(self, default: "{}")
    }
}

// MARK: Private Helpers

fileprivate struct Coders {
    private static var _decoder: JSONDecoder { JSONDecoder() }
    private static var _encoder: JSONEncoder { JSONEncoder() }
    fileprivate static func encode<Value: Codable>(_ value: String) -> Value? {
        guard let data = value.data(using: .utf8), let decoded = try? _decoder.decode(Value.self, from: data) else {
            return nil
        }
        return decoded
    }
    fileprivate static func decode<Value: Codable>(_ value: Value, default: String) -> String {
        guard let data = try? _encoder.encode(value), let value = String(data: data, encoding: .utf8) else {
            return `default`
        }
        return value
    } 
}

Bonus: skip all the raw representable stuff & just work with Codable types directly:

@propertyWrapper
public struct CodableAppStorage<Value: Codable>: DynamicProperty {
    @AppStorage
    private var value: Data
    
    private let decoder = JSONDecoder()
    private let encoder = JSONEncoder()
    
    public init(wrappedValue: Value, _ key: String, store: UserDefaults? = nil) {
        _value = .init(wrappedValue: try! encoder.encode(wrappedValue), key, store: store)
    }
    
    public var wrappedValue: Value {
        get { try! decoder.decode(Value.self, from: value) }
        nonmutating set { value = try! encoder.encode(newValue) }
    }
    
    public var projectedValue: Binding<Value> {
        Binding(
            get: { wrappedValue }, 
            set: { wrappedValue = $0 }
        )
    }
}

This code is obviously unsafe with all the force unwrapping, but it can be adjusted as needed!

Just saw your CodableAppStorage propertyWrapper solution. It looks elegant and practical to me:
No extensions of builtin types and no copy/paste of code. Why should one use the CodableAppStorage.swift solution then?
Thanks, Lourens

Back in January 2024 I wrote almost identical code using the propertyWrapper as above. Wish I had done a better search then. Now I'm trying to get the key to be the variable name (plus maybe a namespace identifier) but haven't figured out Swift macros enough yet.