Squashed commit of the following:

Author: winddpan

.gitignore

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 xcuserdata/
-*.xcuserdata
+*.xcuserdata
+.DS_Store

.swiftpm/xcode/package.xcworkspace/contents.xcworkspacedata

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+# 关于CodableWrapper
+
+* Codable很好，但是有一些缺陷：比如严格要求数据源，定义为String给了Int就抛异常、支持自定义CodingKey但是写法十分麻烦、缺字段的情况下不适用Optional会抛异常而不是使用缺省值等等。
+* 之前发布过[PropertyWrapper版](https://github.com/winddpan/CodableWrapper/tree/0.3.3)，主要使用PropertyWrapper标记属性来提高了Codable的使用体验，使用了几个比较tricky的黑科技，所以API也比市面上的同类库要简单。
+* 现在Swift5.9支持宏了，决定写一个没有任何tricky的宏版本的CodableWrapper。目前已开发完毕[CodableWrapper/swift5.9-macro](https://github.com/winddpan/CodableWrapper/tree/swift5.9-macro)
+
+## 首先确定目标
+1. 支持缺省值，JSON缺少字段容错
+2. 支持 `String` `Bool` `Number` 等基本类型互转
+3. 驼峰大小写自动互转
+4. 自定义解析key
+5. 自定义解析规则 (Transformer)
+6. 方便的 `Codable Class` 子类
+
+## 设计API
+* 定义宏：
+  * `@Codable` 
+  * `@CodableSubclass` 
+  * `@CodingKey(..)`
+  * `@CodingNestedKey(..)` 
+  * `@CodingTransformer(..)`
+
+* 例子：
+    ```
+    @Codable
+    struct BasicModel {
+        var defaultVal: String = "hello world"
+        var defaultVal2: String = Bool.random() ? "hello world" : ""
+        let strict: String
+        let noStrict: String?
+        let autoConvert: Int?
+
+        @CodingKey("hello")
+        var hi: String = "there"
+
+        @CodingNestedKey("nested.hi")
+        @CodingTransformer(StringPrefixTransform("HELLO -> "))
+        var codingKeySupport: String
+
+        @CodingNestedKey("nested.b")
+        var nestedB: String
+
+        var testGetter: String {
+            nestedB
+        }
+    }
+    ```
+
+## 手动实现这些目标特性
+想要宏自动生成代码，需要先弄清楚怎么手动实现这些目标特性。先想象一下编译器怎么生成Codable的实现的：
+
+```
+struct BasicModel: Codable {
+    var defaultVal: String = "hello world"
+}
+```
+
+```
+// 编译器生成的
+struct BasicModel: Codable {
+    var defaultVal: String = "hello world"
+
+    enum CodingKeys: String, CodingKey {
+        case defaultVal
+    }
+
+    init(from decoder: Decoder) throws {
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+        self.defaultVal = try container.decode(String.self, forKey: .defaultVal)
+    }
+
+    func encode(to encoder: Encoder) throws {
+        var container = encoder.container(keyedBy: CodingKeys.self)
+        try container.encode(self.defaultVal, forKey: .defaultVal)
+    }
+}
+```
+
+### 手动实现目标1：支持缺省值
+
+```
+init(from decoder: Decoder) throws {
+    let container = try decoder.container(keyedBy: CodingKeys.self)
+    self.defaultVal = (try? container.decode(String.self, forKey: .defaultVal)) ?? "hello world"
+}
+```
+
+### 手动实现目标2：支持 `String` `Bool` `Number` 等基本类型互转
+
+```
+init(from decoder: Decoder) throws {
+    let container = try decoder.container(keyedBy: CodingKeys.self)
+
+    if let value = try? container.decode(Int.self, forKey: .defaultVal) {
+        self.defaultVal = String(value)
+    } else if let value = try? container.decode(String.self, forKey: .defaultVal) {
+        self.defaultVal = String(value)
+    } else if let value = try? container.decode(Float.self, forKey: .defaultVal) {
+        self.defaultVal = String(value)
+    } 
+    // 各种基本类型尝试转换 else if ...
+    else {
+        self.defaultVal = "hello world"
+    }
+}
+```
+
+这么解析肯定不是个办法，这里结合了两个开源库的实现，第一步使用[AnyDecodable](https://github.com/Flight-School/AnyCodable/blob/master/Sources/AnyCodable/AnyDecodable.swift)解析出JSON内的数据，解析出来可能是任意基本类型。第二步使用[BuiltInBridgeType](https://github.com/alibaba/HandyJSON/blob/master/Source/BuiltInBridgeType.swift)和[BuiltInBridgeType](https://github.com/alibaba/HandyJSON/blob/master/Source/BuiltInBridgeType.swift)将解析出来的基本类型尝试转换成目标类型。完整实现如下：
+
+```swift
+extension Decodable {
+    static func decode<K>(from container: KeyedDecodingContainer<K>, forKey key: KeyedDecodingContainer<K>.Key) throws -> Self {
+        return try container.decode(Self.self, forKey: key)
+    }
+}
+
+struct BasicModel: Codable {
+    var defaultVal: String = "hello world"
+
+    enum CodingKeys: CodingKey {
+        case defaultVal
+    }
+
+    init(from decoder: Decoder) throws {
+        let container = try decoder.container(keyedBy: CodingKeys.self)
+        defaultVal = container.tryNormalKeyDecode(type: String.self, key: .defaultVal) ?? "hello world"
+    }
+
+    func encode(to encoder: Encoder) throws {
+        var container = encoder.container(keyedBy: CodingKeys.self)
+        try container.encode(defaultVal, forKey: .defaultVal)
+    }
+}
+
+extension KeyedDecodingContainer {
+    func tryNormalKeyDecode<Value>(type _: Value.Type, key: K) -> Value? {
+        let value = try? decodeIfPresent(AnyDecodable.self, forKey: key)?.value
+        if let value = value {
+            if let converted = value as? Value {
+                return converted
+            }
+            if let _bridged = (Value.self as? _BuiltInBridgeType.Type)?._transform(from: value), let __bridged = _bridged as? Value {
+                return __bridged
+            }
+            // 如果是非基本类型，那继续尝试Decode
+            if let valueType = Value.self as? Decodable.Type {
+                if let value = try? valueType.decode(from: self, forKey: key) as? Value {
+                    return value
+                }
+            }
+        }
+        return nil
+    }
+}
+```
+
+### 手动实现目标3：支持驼峰大小写自动互转
+```swift
+// 为了简化CodingKey，使用AnyCodingKey实现CodingKey
+public struct AnyCodingKey: CodingKey {
+    public var stringValue: String
+    public var intValue: Int?
+
+    public init?(stringValue: String) {
+        self.stringValue = stringValue
+        intValue = nil
+    }
+
+    public init?(intValue: Int) {
+        stringValue = "\(intValue)"
+        self.intValue = intValue
+    }
+
+    public init(index: Int) {
+        stringValue = "\(index)"
+        intValue = index
+    }
+}
+
+struct BasicModel: Codable {
+    var defaultVal: String = "hello world"
+
+    init(from decoder: Decoder) throws {
+        // CodingKeys 改为 AnyCodingKey
+        let container = try decoder.container(keyedBy: AnyCodingKey.self)
+        defaultVal = container.tryNormalKeyDecode(type: String.self, key: "defaultVal") ?? "hello world"
+    }
+}
+
+extension KeyedDecodingContainer where K == AnyCodingKey {
+    func tryNormalKeyDecode<Value>(type: Value.Type, key: String) -> Value? {
+        func _decode(key: String) -> Value? {
+            let value = try? decodeIfPresent(AnyDecodable.self, forKey: key)?.value
+            if let value = value {
+                if let converted = value as? Value {
+                    return converted
+                }
+                if let _bridged = (Value.self as? _BuiltInBridgeType.Type)?._transform(from: value), let __bridged = _bridged as? Value {
+                    return __bridged
+                }
+                if let valueType = Value.self as? Decodable.Type {
+                    if let value = try? valueType.decode(from: self, forKey: key) as? Value {
+                        return value
+                    }
+                }
+            }
+            return nil
+        }
+
+        for newKey in [key, key.snakeCamelConvert()].compactMap({ $0 }) {
+            if let value = _decode(key: newKey) {
+                return value
+            }
+        }
+        return nil
+    }
+}
+
+extension String {
+    func snakeCamelConvert() -> String? {
+        // 驼峰大小写互转
+        ...
+    }
+}
+```
+
+### 手动实现目标4：自定义解析Key和NestedKey
+[container.decode](https://github.com/winddpan/CodableWrapper/blob/swift5.9-macro/Sources/CodableWrapper/Decoder.swift)封装  [container.encode](https://github.com/winddpan/CodableWrapper/blob/swift5.9-macro/Sources/CodableWrapper/Encoder.swift)封装
+
+```swift
+struct TestModel: Codable {
+    // @CodingKey("u1", "u2", "u9")
+    let userName: String = ""
+
+    // @CodingNestedKey("data.u1", "data.u2", "data.u9")
+    let userName2: String = ""
+
+    init(from decoder: Decoder) throws {
+        let container = try decoder.container(keyedBy: AnyCodingKey.self)
+        userName = container.decode(type: String.self, keys: ["u1", "u2", "u9"], nestedKeys: []) ?? ""
+        userName2 = container.decode(type: String.self, keys: [], nestedKeys: ["data.u1", "data.u2", "data.u9"]) ?? ""
+    }
+
+    func encode(to encoder: Encoder) throws {
+        var container = encoder.container(keyedBy: CodingKeys.self)
+        try container.encode(value: self.userName, keys: ["u1", "u2", "u9"], nestedKeys: [])
+        try container.encode(value: self.userName2, keys: [], nestedKeys: ["data.u1", "data.u2", "data.u9"])
+    }
+}
+```
+
+### 手动实现目标5：自定义解析规则Transformer 
+```swift
+// 定义TransformType协议
+public protocol TransformType {
+    associatedtype Object
+    associatedtype JSON: Codable
+
+    func transformFromJSON(_ json: JSON?) -> Object
+    func transformToJSON(_ object: Object) -> JSON?
+}
+```
+
+```swift
+// 定义一个不遵循Codable的结构体
+struct DateWrapper {
+    let timestamp: TimeInterval
+
+    var date: Date {
+        Date(timeIntervalSince1970: timestamp)
+    }
+
+    init(timestamp: TimeInterval) {
+        self.timestamp = timestamp
+    }
+
+    static var transformer = TransformOf<DateWrapper, TimeInterval>(fromJSON: { DateWrapper(timestamp: $0 ?? 0) }, toJSON: { $0.timestamp })
+}
+
+struct DateModel: Codable {
+    // @CodingTransformer(DateWrapper.transformer)
+    var time: DateWrapper? = DateWrapper(timestamp: 0)
+
+    // @CodingTransformer(DateWrapper.transformer)
+    var time1: DateWrapper = .init(timestamp: 0)
+
+    // @CodingTransformer(DateWrapper.transformer)
+    var time2: DateWrapper?
+
+    // @CodingTransformer(DateWrapper.transformer)
+    var time3: DateWrapper
+
+    init(from decoder: Decoder) throws {
+        let container = try decoder.container(keyedBy: AnyCodingKey.self)
+        let time = try? container.decode(type: TimeInterval.self, keys: ["time"], nestedKeys: [])
+        self.time = DateWrapper.transformer.transformFromJSON(time) ?? DateWrapper(timestamp: 0)
+
+        let time1 = try? container.decode(type: TimeInterval.self, keys: ["time1"], nestedKeys: [])
+        self.time1 = DateWrapper.transformer.transformFromJSON(time1) ?? .init(timestamp: 0)
+
+        let time2 = try? container.decode(type: TimeInterval.self, keys: ["time2"], nestedKeys: [])
+        self.time2 = DateWrapper.transformer.transformFromJSON(time2)
+
+        let time3 = try? container.decode(type: TimeInterval.self, keys: ["time3"], nestedKeys: [])
+        self.time3 = DateWrapper.transformer.transformFromJSON(time3)
+    }
+
+    func encode(to encoder: Encoder) throws {
+        let container = encoder.container(keyedBy: AnyCodingKey.self)
+        if let time = self.time, let value = DateWrapper.transformer.transformToJSON(self.time) {
+            try container.encode(value: value, keys: ["time"], nestedKeys: [])
+        }
+        if let value = DateWrapper.transformer.transformToJSON(self.time1) {
+            try container.encode(value: value, keys: ["time1"], nestedKeys: [])
+        }
+        if let time2 = self.time2, let value = DateWrapper.transformer.transformToJSON(time2) {
+            try container.encode(value: value, keys: ["time2"], nestedKeys: [])
+        }
+        if let value = DateWrapper.transformer.transformToJSON(time3) {
+            try container.encode(value: value, keys: ["time3"], nestedKeys: [])
+        }
+    }
+}
+```
+
+### 手动实现目标6：“不方便”的 `Codable Class` 子类
+```
+class ClassModel1: Codable {
+    var val: String?
+}
+
+class ClassSubmodel1: ClassModel1 {
+    var subVal: String = "1_1"
+
+    required init(from decoder: Decoder) throws {
+        let container = try decoder.container(keyedBy: AnyCodingKey.self)
+        self.subVal = (try? container.decode(type: type(of: self.subVal), keys: ["subVal"], nestedKeys: [])) ?? ("1_1")
+        try super.init(from: decoder)
+    }
+
+    override func encode(to encoder: Encoder) throws {
+        try super.encode(to: encoder)
+        let container = encoder.container(keyedBy: AnyCodingKey.self)
+        try container.encode(value: self.subVal, keys: ["subVal"], nestedKeys: [])
+    }
+}
+```
+
+下一篇文章将介绍Codable宏的开发和实现。
+
+-----
+**文章目录**
+* [一、设计目标和手动实现这些目标特性](https://juejin.cn/post/7251501945272270908)
+* [二、Codable宏的开发和实现](https://juejin.cn/post/7252170693676499004)