Persisting.swift

//
//  Persisting.swift
//
//  Created by James Sedlacek on 1/30/25.
//

import SwiftUI

/// A property wrapper that manages the persistence of Codable types using UserDefaults.
///
/// The `Persisting` property wrapper provides a convenient way to persist Codable types
/// in UserDefaults by automatically handling the encoding and decoding process.
///
/// # Usage Example
/// ```swift
/// enum TimeFormat: String, Codable, Hashable {
///     case twelveHour = "12h"
///     case twentyFourHour = "24h"
/// }
///
/// struct SettingsView: View {
///     @Persisting("timeFormat") private var format = TimeFormat.twelveHour
///
///     var body: some View {
///         Picker("Time Format", selection: $format) {
///             Text("12 Hour").tag(TimeFormat.twelveHour)
///             Text("24 Hour").tag(TimeFormat.twentyFourHour)
///         }
///     }
/// }
/// ```
@MainActor
@propertyWrapper
public struct Persisting<T: Codable & Hashable>: DynamicProperty {
    @AppStorage private var storage: Data
    private let defaultValue: T

    private let encoder: JSONEncoder = .init()
    private let decoder: JSONDecoder = .init()

    /// The wrapped value of the Codable type.
    public var wrappedValue: T {
        get {
            guard let decoded = try? decoder.decode(T.self, from: storage) else {
                return defaultValue
            }
            return decoded
        }
        nonmutating set {
            guard let encoded = try? encoder.encode(newValue) else { return }
            storage = encoded
        }
    }

    /// A binding to the Codable value stored in `UserDefaults`.
    public var projectedValue: Binding<T> {
        Binding(
            get: { wrappedValue },
            set: { wrappedValue = $0 }
        )
    }

    /// Initializes the property wrapper.
    ///
    /// - Parameters:
    ///   - wrappedValue: The default value if no value is found in UserDefaults.
    ///   - key: The key used to store the value in UserDefaults.
    ///   - store: The UserDefaults store to use. Defaults to `.standard`.
    public init(
        wrappedValue: T,
        _ key: String,
        store: UserDefaults? = nil
    ) {
        defaultValue = wrappedValue
        let initialData = (try? encoder.encode(wrappedValue)) ?? Data()
        _storage = .init(wrappedValue: initialData, key, store: store)
    }
}