Strongly Typed UserDefaults

CachedValue.swift

import Foundation

/// `CachedValue` provides a strongly typed interface to UserDefaults.
/// It can cache any ``Codable`` type.
///
/// Usage:
/// 1. Make a struct conforming to `CachedValue`
///     * Add a `typealias ValueType = YourType`
///     * Optionally add a `static var defaultValue: YourType = YourInstance`.  (Your `value` accessor will then be non-optional.)
///     * Optionally add a `static var cacheKey: String = "a-very-specific-string-key"`.  (By default your key will be your CachedValue conforming type's name.)
///     * Optionally add a `static var store: UserDefaults = MyNonStandardUserDefaults()`.  (By default CachedValue will use `UserDefaults.standard`.)
/// 2. Instantiate your type.
/// 3. Assign to or read from derived `.value` property.
/// 4. You can call `clear()` on your instance to remove the underlying data from UserDefaults.
///
/// Example:
/// ```
///   struct AValue: CachedValue {
///      typealias ValueType = String
///      // Note: No default value
///      // Derived accessor type:
///      // var value: String?
///   }
///
///   struct BValue: CachedValue {
///       typealias ValueType = String
///       static var defaultValue: String = "Some default value"
///       // Derived accessor type:
///       // var value: String
///   }
///
///   // Session 1
///   var a = AValue()
///   var b = BValue()
///   print(a.value) // nil
///   print(b.value) // Some default value
///   a.value = "Value A"
///   b.value = "Value B"
///   print(a.value) // Optional("Value A")
///   print(b.value) // Value B
///
///   // Session 2
///   var a = AValue()
///   var b = BValue()
///   print(a.value) // Optional("Value A")
///   print(b.value) // Value B
///   a.clear()
///   b.clear()
///   print(a.value) // nil
///   print(b.value) // Some default value
/// ```
public protocol CachedValue {
    associatedtype ValueType: Codable
    associatedtype DefaultValueType
    associatedtype StoreType: UserDefaults
    static var cacheKey: String { get }
    static var defaultValue: DefaultValueType { get }
    static var store: StoreType { get }
}

public extension CachedValue {
    static var cacheKey: String { String(describing: Self.self) }
    static var defaultValue: ValueType? { nil }
    static var store: UserDefaults { UserDefaults.standard }
}

public extension CachedValue {
    func clear() {
        Self.store.set(nil, forKey: Self.cacheKey)
    }
}

public extension CachedValue where DefaultValueType == ValueType {
    var value: ValueType {
        get {
            let decoder = JSONDecoder()
            if let data = Self.store.object(forKey: Self.cacheKey) as? Data,
               let value = try? decoder.decode(ValueType.self, from: data) {
                return value
            } else {
                return Self.defaultValue
            }
        }
        set {
            let encoder = JSONEncoder()
            if let data = try? encoder.encode(newValue) {
                Self.store.set(data, forKey: Self.cacheKey)
            }
        }
    }
}

public extension CachedValue where DefaultValueType == Optional<ValueType> {
    var value: ValueType? {
        get {
            let decoder = JSONDecoder()
            if let data = Self.store.object(forKey: Self.cacheKey) as? Data,
               let value = try? decoder.decode(ValueType.self, from: data) {
                return value
            } else {
                return nil
            }
        }
        set {
            let encoder = JSONEncoder()
            if let data = try? encoder.encode(newValue) {
                Self.store.set(data, forKey: Self.cacheKey)
            }
        }
    }
}