上篇介绍了Swift宏的定义与生声明，本篇主要看看是Swift宏的具体实现。结合Swift中Codable协议，封装一个工具让类或者结构体自动实现Codable协议，并且添加一些协议中没有的功能。

关于Codable协议

    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、缺字段的情况下不使用Optional会抛异常而不是使用缺省值等等。

基于以上情况，之前也写了一些Codable协议的补充，比如之前使用属性包装器增加了协议的默认值的提供具体地址https://github.com/duzhaoquan/DQTool.git

Swift Macro 的参考链接

1. 【WWDC23】一文看懂 Swift Macro
2. swift-macro-examples
3. Swift AST Explorer
4. CodableWrapper

实现目标：

Swift5.9之后新出了宏，通过宏可以更加优雅的封装Codable协议，增加新功能

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

具体的实现

定义几个宏

• @Codable
• @CodableSubclass
• @CodableKey(..)
• @CodableNestedKey(..)
• @CodableTransformer(..)

先简单的声明与实现

声明Codable和CodableKey宏。

// CodableWrapperMacros/CodableWrapper.swift@attached(member, names: named(init(from:)), named(encode(to:)))
@attached(conformance)
public macro Codable() = #externalMacro(module: "CodableWrapperMacros", type: "Codable")@attached(member)
public macro CodableKey(_ key: String ...) = #externalMacro(module: "CodableWrapperMacros", type: "CodableKey")

实现Codable和CodableKey宏。

// CodableWrapperMacros/Codable.swift
import SwiftSyntax
import SwiftSyntaxMacrospublic struct Codable: MemberMacro {public static func expansion(of _: AttributeSyntax,providingConformancesOf declaration: some DeclGroupSyntax,in _: some MacroExpansionContext) throws -> [(TypeSyntax, GenericWhereClauseSyntax?)]{return []}public static func expansion(of node: SwiftSyntax.AttributeSyntax,providingMembersOf declaration: some SwiftSyntax.DeclGroupSyntax,in context: some SwiftSyntaxMacros.MacroExpansionContext) throws -> [SwiftSyntax.DeclSyntax]{return []}
}

// CodableWrapperMacros/CodableKey.swift
import SwiftSyntax
import SwiftSyntaxMacrospublic struct CodableKey: ConformanceMacro, MemberMacro {public static func expansion(of node: SwiftSyntax.AttributeSyntax,providingMembersOf declaration: some SwiftSyntax.DeclGroupSyntax,in context: some SwiftSyntaxMacros.MacroExpansionContext) throws -> [SwiftSyntax.DeclSyntax]{return []}
}

添加宏定义

​​// CodableWrapperMacros/Plugin.swift
import SwiftCompilerPlugin
import SwiftSyntaxMacros@main
struct CodableWrapperPlugin: CompilerPlugin {let providingMacros: [Macro.Type] = [Codable.self,CodableKey.self,]
}

 在这里，@Codable实现了两种宏，一种是一致性宏（Conformance Macro），另一种是成员宏（Member Macro）。

一些关于这些宏的说明：

• @Codable和Codable协议的宏名不会冲突，这样的命名一致性可以降低认知负担。
• Conformance Macro用于自动让数据模型遵循Codable协议（如果尚未遵循）。
• Member Macro用于添加init(from decoder: Decoder)和func encode(to encoder: Encoder)这两个方法。在@attached(member, named(init(from:)), named(encode(to:)))中，必须声明新增方法的名称才是合法的。

实现自动遵循Codable协议

// CodableWrapperMacros/Codable.swiftpublic struct Codable: ConformanceMacro, MemberMacro {public static func expansion(of node: AttributeSyntax,providingConformancesOf declaration: some DeclGroupSyntax,in context: some MacroExpansionContext) throws -> [(TypeSyntax, GenericWhereClauseSyntax?)] {return [("Codable", nil)]}public static func expansion(of node: SwiftSyntax.AttributeSyntax,providingMembersOf declaration: some SwiftSyntax.DeclGroupSyntax,in context: some SwiftSyntaxMacros.MacroExpansionContext) throws -> [SwiftSyntax.DeclSyntax]{return []}
}

编译一下。右键@Codable -> Expand Macro查看扩写的代码，看起来还可以。

但如果BasicModel本身就遵循了Codable，编译就报错了。所以希望先检查数据模型是否遵循Codable协议，如果没有的话再遵循它，怎么办呢？ 打开Swift AST Explorer 编写一个简单Struct和Class，可以看到整个AST，declaration: some DeclGroupSyntax对象根据模型是struct还是class分别对应了StructDecl和ClassDecl。补充上检查代码之后如下，增加了检查时否时class或者struct，否则抛出错误。代码如下

public static func expansion(of node: AttributeSyntax,providingConformancesOf declaration: some DeclGroupSyntax,in context: some MacroExpansionContext) throws -> [(TypeSyntax, GenericWhereClauseSyntax?)] {var inheritedTypes: InheritedTypeListSyntax?if let declaration = declaration.as(StructDeclSyntax.self) {inheritedTypes = declaration.inheritanceClause?.inheritedTypeCollection} else if let declaration = declaration.as(ClassDeclSyntax.self) {inheritedTypes = declaration.inheritanceClause?.inheritedTypeCollection} else {throw ASTError("use @Codable in `struct` or `class`")}if let inheritedTypes = inheritedTypes,inheritedTypes.contains(where: { inherited in inherited.typeName.trimmedDescription == "Codable" }){return []}return [("Codable" as TypeSyntax, nil)]
}

实现 @Codable 功能

先定义个 ModelMemberPropertyContainer，init(from decoder: Decoder) 和 func encode(to encoder: Encoder) 的扩展都在里面实现。

public static func expansion(of node: SwiftSyntax.AttributeSyntax,providingMembersOf declaration: some SwiftSyntax.DeclGroupSyntax,in context: some SwiftSyntaxMacros.MacroExpansionContext) throws -> [SwiftSyntax.DeclSyntax]
{let propertyContainer = try ModelMemberPropertyContainer(decl: declaration, context: context)let decoder = try propertyContainer.genDecoderInitializer(config: .init(isOverride: false))let encoder = try propertyContainer.genEncodeFunction(config: .init(isOverride: false))return [decoder, encoder]
}

// CodableWrapperMacros/ModelMemberPropertyContainer.swiftimport SwiftSyntax
import SwiftSyntaxMacrosstruct GenConfig {let isOverride: Bool
}struct ModelMemberPropertyContainer {let context: MacroExpansionContextfileprivate let decl: DeclGroupSyntaxinit(decl: DeclGroupSyntax, context: some MacroExpansionContext) throws {self.decl = declself.context = context}func genDecoderInitializer(config: GenConfig) throws -> DeclSyntax {return """init(from decoder: Decoder) throws {fatalError()}""" as DeclSyntax}func genEncodeFunction(config: GenConfig) throws -> DeclSyntax {return """func encode(to encoder: Encoder) throws {fatalError()}""" as DeclSyntax}
}

填充init(from decoder: Decoder) 

 需要得知属性名、@CodableKey的参数、@CodableNestedKey的参数、@CodableTransformer的参数、初始化表达式。获取memberProperties列表：

struct ModelMemberPropertyContainer {let context: MacroExpansionContextfileprivate let decl: DeclGroupSyntaxfileprivate var memberProperties: [ModelMemberProperty] = []init(decl: DeclGroupSyntax, context: some MacroExpansionContext) throws {self.decl = declself.context = contextmemberProperties = try fetchModelMemberProperties()}func fetchModelMemberProperties() throws -> [ModelMemberProperty] {let memberList = decl.memberBlock.memberslet memberProperties = try memberList.compactMap { member -> ModelMemberProperty? inguard let variable = member.decl.as(VariableDeclSyntax.self),variable.isStoredPropertyelse {return nil}// nameguard let name = variable.bindings.map(\.pattern).first(where: { $0.is(IdentifierPatternSyntax.self) })?.as(IdentifierPatternSyntax.self)?.identifier.text else {return nil}guard let type = variable.inferType else {throw ASTError("please declare property type: \(name)")}var mp = ModelMemberProperty(name: name, type: type)let attributes = variable.attributes// isOptionalmp.isOptional = variable.isOptionalType// CodableKeyif let customKeyMacro = attributes?.first(where: { element inelement.as(AttributeSyntax.self)?.attributeName.as(SimpleTypeIdentifierSyntax.self)?.description == "CodableKey"}) {mp.normalKeys = customKeyMacro.as(AttributeSyntax.self)?.argument?.as(TupleExprElementListSyntax.self)?.compactMap { $0.expression.description } ?? []}// CodableNestedKeyif let customKeyMacro = attributes?.first(where: { element inelement.as(AttributeSyntax.self)?.attributeName.as(SimpleTypeIdentifierSyntax.self)?.description == "CodableNestedKey"}) {mp.nestedKeys = customKeyMacro.as(AttributeSyntax.self)?.argument?.as(TupleExprElementListSyntax.self)?.compactMap { $0.expression.description } ?? []}// CodableTransformif let customKeyMacro = attributes?.first(where: { element inelement.as(AttributeSyntax.self)?.attributeName.as(SimpleTypeIdentifierSyntax.self)?.description == "CodableTransformer"}) {mp.transformerExpr = customKeyMacro.as(AttributeSyntax.self)?.argument?.as(TupleExprElementListSyntax.self)?.first?.expression.description}// initializerExprif let initializer = variable.bindings.compactMap(\.initializer).first {mp.initializerExpr = initializer.value.description}return mp}return memberProperties}
}

 完善genDecoderInitializer：

    func genDecoderInitializer(config: GenConfig) throws -> DeclSyntax {// memberProperties: [ModelMemberProperty]let body = memberProperties.enumerated().map { idx, member inif let transformerExpr = member.transformerExpr {let transformerVar = context.makeUniqueName(String(idx))let tempJsonVar = member.namevar text = """let \(transformerVar) = \(transformerExpr)let \(tempJsonVar) = try? container.decode(type: type(of: \(transformerVar)).JSON.self, keys: [\(member.codingKeys.joined(separator: ", "))], nestedKeys: [\(member.nestedKeys.joined(separator: ", "))])"""if let initializerExpr = member.initializerExpr {text.append("""self.\(member.name) = \(transformerVar).transformFromJSON(\(tempJsonVar), fallback: \(initializerExpr))""")} else {text.append("""self.\(member.name) = \(transformerVar).transformFromJSON(\(tempJsonVar))""")}return text} else {let body = "container.decode(type: type(of: self.\(member.name)), keys: [\(member.codingKeys.joined(separator: ", "))], nestedKeys: [\(member.nestedKeys.joined(separator: ", "))])"if let initializerExpr = member.initializerExpr {return "self.\(member.name) = (try? \(body)) ?? (\(initializerExpr))"} else {return "self.\(member.name) = try \(body)"}}}.joined(separator: "\n")let decoder: DeclSyntax = """\(raw: attributesPrefix(option: [.public, .required]))init(from decoder: Decoder) throws {let container = try decoder.container(keyedBy: AnyCodingKey.self)\(raw: body)}"""return decoder}

• let transformerVar = context.makeUniqueName(String(idx)) 需要生成一个局部transformer变量，为了防止变量名冲突使用了makeUniqueName生成唯一变量名

• attributesPrefix(option: [.public, .required]) 根据 struct/class 是 open/public 生成正确的修饰。所有情况展开如下：

  open class Model: Codable {public required init(from decoder: Decoder) throws {}
  }public class Model: Codable {public required init(from decoder: Decoder) throws {}
  }class Model: Codable {required init(from decoder: Decoder) throws {}
  }public struct Model: Codable {public init(from decoder: Decoder) throws {}
  }struct Model: Codable {init(from decoder: Decoder) throws {}
  }
  

  填充func encode(to encoder: Encoder)

  func genEncodeFunction(config: GenConfig) throws -> DeclSyntax {let body = memberProperties.enumerated().map { idx, member inif let transformerExpr = member.transformerExpr {let transformerVar = context.makeUniqueName(String(idx))if member.isOptional {return """let \(transformerVar) = \(transformerExpr)if let \(member.name) = self.\(member.name), let value = \(transformerVar).transformToJSON(\(member.name)) {try container.encode(value: value, keys: [\(member.codingKeys.joined(separator: ", "))], nestedKeys: [\(member.nestedKeys.joined(separator: ", "))])}"""} else {return """let \(transformerVar) = \(transformerExpr)if let value = \(transformerVar).transformToJSON(self.\(member.name)) {try container.encode(value: value, keys: [\(member.codingKeys.joined(separator: ", "))], nestedKeys: [\(member.nestedKeys.joined(separator: ", "))])}"""}} else {return "try container.encode(value: self.\(member.name), keys: [\(member.codingKeys.joined(separator: ", "))], nestedKeys: [\(member.nestedKeys.joined(separator: ", "))])"}}.joined(separator: "\n")let encoder: DeclSyntax = """\(raw: attributesPrefix(option: [.open, .public]))func encode(to encoder: Encoder) throws {let container = encoder.container(keyedBy: AnyCodingKey.self)\(raw: body)}"""return encoder
  }
  

  @CodableKey @CodableNestedKey @CodableTransformer增加Diagnostics

这些宏是用作占位标记的，不需要实际扩展。但为了增加一些严谨性，比如在以下情况下希望增加错误提示：

@CodableKey("a")
struct StructWraning1 {}

实现也很简单抛出异常即可

public struct CodableKey: MemberMacro {public static func expansion(of node: AttributeSyntax, providingMembersOf _: some DeclGroupSyntax, in context: some MacroExpansionContext) throws -> [DeclSyntax] {throw ASTError("`\(self.self)` only use for `Property`")}
}

 这里也就印证了 @CodableKey 为什么不用 @attached(memberAttribute)(Member Attribute Macro) 而使用 @attached(member)(Member Macro) 的原因。如果不声明使用@attached(member)，就不会执行MemberMacro协议的实现，在MemberMacro位置写上@CodableKey("a")也就不会报错。

实现@CodableSubclass，方便的Codable Class子类

先举例展示Codable Class子类的缺陷。编写一个简单的测试用例：是不是出乎意料，原因是编译器只给ClassModel添加了init(from decoder: Decoder)，ClassSubmodel则没有。要解决问题还需要手动实现子类的Codable协议，十分不便：

@CodableSubclass就是解决这个问题，实现也很简单，在适时的位置super call，方法标记成override就可以了。

func genDecoderInitializer(config: GenConfig) throws -> DeclSyntax {...let decoder: DeclSyntax = """\(raw: attributesPrefix(option: [.public, .required]))init(from decoder: Decoder) throws {let container = try decoder.container(keyedBy: AnyCodingKey.self)\(raw: body)\(raw: config.isOverride ? "\ntry super.init(from: decoder)" : "")}"""
}func genEncodeFunction(config: GenConfig) throws -> DeclSyntax {...let encoder: DeclSyntax = """\(raw: attributesPrefix(option: [.open, .public]))\(raw: config.isOverride ? "override " : "")func encode(to encoder: Encoder) throws {\(raw: config.isOverride ? "try super.encode(to: encoder)\n" : "")let container = encoder.container(keyedBy: AnyCodingKey.self)\(raw: body)}"""
}

具体代码实现地址：GitHub - duzhaoquan/CodableTool