liuzhida33 · September 8, 2022 08:47 · Sep 8, 2022
diff --git a/CodableDefault.swift b/CodableDefault.swift
@@ -0,0 +1,88 @@
+
+protocol CodableDefaultSource {
+    associatedtype Value: Codable
+    static var defaultValue: Value { get }
+}
+
+enum CodableDefault {}
+
+extension CodableDefault {
+    @propertyWrapper
+    struct Wrapper<Source: CodableDefaultSource> {
+        typealias Value = Source.Value
+        var wrappedValue = Source.defaultValue
+    }
+}
+
+extension CodableDefault.Wrapper: Decodable {
+    init(from decoder: Decoder) throws {
+        let container = try decoder.singleValueContainer()
+        wrappedValue = try container.decode(Value.self)
+    }
+}
+
+extension CodableDefault.Wrapper: Encodable {
+    func encode(to encoder: Encoder) throws {
+        var container = encoder.singleValueContainer()
+        try container.encode(wrappedValue)
+    }
+}
+
+extension KeyedDecodingContainer {
+    func decode<T>(_ type: CodableDefault.Wrapper<T>.Type,
+                   forKey key: Key) throws -> CodableDefault.Wrapper<T> {
+        try decodeIfPresent(type, forKey: key) ?? .init()
+    }
+}
+
+extension KeyedEncodingContainer {
+    mutating func encode<T>(_ value: CodableDefault.Wrapper<T>, forKey key: Key) throws {
+        try encode(value.wrappedValue, forKey: key)
+    }
+}
+
+extension CodableDefault {
+    typealias Source = CodableDefaultSource
+    typealias List = Codable & ExpressibleByArrayLiteral
+    typealias Map = Codable & ExpressibleByDictionaryLiteral
+
+    enum Sources {
+        enum True: Source {
+            static var defaultValue: Bool { true }
+        }
+
+        enum False: Source {
+            static var defaultValue: Bool { false }
+        }
+
+        enum EmptyString: Source {
+            static var defaultValue: String { "" }
+        }
+
+        enum EmptyList<T: List>: Source {
+            static var defaultValue: T { [] }
+        }
+
+        enum EmptyMap<T: Map>: Source {
+            static var defaultValue: T { [:] }
+        }
+
+        enum Zero: Source {
+            static var defaultValue: Int { 0 }
+        }
+
+        enum One: Source {
+            static var defaultValue: Int { 1 }
+        }
+    }
+}
+
+extension CodableDefault {
+    typealias True = Wrapper<Sources.True>
+    typealias False = Wrapper<Sources.False>
+    typealias EmptyString = Wrapper<Sources.EmptyString>
+    typealias EmptyList<T: List> = Wrapper<Sources.EmptyList<T>>
+    typealias EmptyMap<T: Map> = Wrapper<Sources.EmptyMap<T>>
+    typealias Zero = Wrapper<Sources.Zero>
+    typealias One = Wrapper<Sources.One>
+}