如何使用Swift宏向 struct 体及其init函数添加新成员

如果您只需要一个额外的初始值设定项，MemberMacro可以生成:

enum CountedMacro: MemberMacro {
    static func expansion(of node: AttributeSyntax, providingMembersOf declaration: some DeclGroupSyntax, in context: some MacroExpansionContext) throws -> [DeclSyntax] {
        let initialisers = declaration.memberBlock.members.compactMap { member in
            member.decl.as(InitializerDeclSyntax.self)
        }
        guard initialisers.count > 0 else {
            context.diagnose(...)
            return []
        }
        let newInitialisers = initialisers.map(generateNewInitialiser).map(DeclSyntax.init)
        return ["var count = 0"] + newInitialisers
    }
    
    static func generateNewInitialiser(from initialiser: InitializerDeclSyntax) -> InitializerDeclSyntax {
        var newInitialiser = initialiser
        // add parameter
        let newParameterList = FunctionParameterListSyntax {
            newInitialiser.signature.parameterClause.parameters
            "count: Int"
        }
        newInitialiser.signature.parameterClause.parameters = newParameterList
        
        // add statement initialising count
        newInitialiser.body?.statements.append("self.count = count")
        
        return newInitialiser
    }
}

如果要在现有的初始化器中添加一个新的count参数，宏目前不能这样做.一种解决方法是要求宏的用户将初始化器标记为私有.然后，宏可以 for each 专用初始化器生成新的public个初始化器.这实际上"隐藏"了现有的初始值设定项.

// in generateNewInitialiser...
let modifiersToBeRemoved: [TokenSyntax] = ["public", "private", "internal", "fileprivate", "required"]
newInitialiser.modifiers = newInitialiser.modifiers.filter { m in modifiersToBeRemoved.contains { m.name == $0 } }
newInitialiser.modifiers.insert(DeclModifierSyntax(name: "public"), at: newInitialiser.modifiers.startIndex)

另一种更灵活的设计是将其拆分为three个宏:

• @Counted是一个成员宏，它添加var count = 0以及附加的member attribute macro that adds @CountedInitialiserto all initialisers without a@CountedIgnored`宏.
• @CountedInitialiser是使用generateNewInitialiser生成公共初始值设定项的对等宏.
• @CountedIgnored是不执行任何操作的对等宏.它仅用于告诉@Counted忽略初始值设定项

这允许宏的用户准确地 Select 他们想要增加的初始值设定项.例如:

@Counted
struct Foo {
    let name: String

    @CountedIgnored
    init() { name = "Default" }

    // @CountedInitialiser will be added to this when @Counted expands
    private init(name: String) { self.name = name }
}

根据SWIFT数据，@Observable个(@ObservationTracked/@ObservationIgnored)和@Model个(@_PersistedProperty/@Transient)可以看到类似的技术.