Attached Member Macros for Boilerplate Reduction

Member Macro Overview

@AddID
struct User { var name: String }
// After expansion:
// struct User { var name: String; var id = UUID() }

Conforming to MemberMacro

import SwiftSyntaxMacros

struct AddIDMacro: MemberMacro {
  static func expansion(
    of node: AttributeSyntax,
    providingMembersOf decl: some DeclGroupSyntax,
    in context: some MacroExpansionContext
  ) throws -> [DeclSyntax] {
    return ["var id = UUID()"]
  }
}

Generating init

// Macro expansion returns:
[
  "init(name: String) { self.name = name; self.id = UUID() }"
]

@Observable Macro Internals

@Observable
class Counter {
  var count = 0
  // macro adds: _$observationRegistrar, access(), withMutation()
}

Reading the Decorated Type

if let structDecl = decl.as(StructDeclSyntax.self) {
  let members = structDecl.memberBlock.members
  // iterate to inspect stored properties
}

Extracting Stored Properties

let storedProps = members.compactMap { member -> VariableDeclSyntax? in
  guard let varDecl = member.decl.as(VariableDeclSyntax.self),
        varDecl.bindingSpecifier.tokenKind == .keyword(.var)
  else { return nil }
  return varDecl
}

Generating Equatable from Members

// Generated code:
static func == (lhs: User, rhs: User) -> Bool {
  lhs.name == rhs.name && lhs.id == rhs.id
}

Combining Member + Conformance Macros

struct AutoEquatableMacro: MemberMacro, ConformanceMacro { ... }

Attribute Arguments

@Clamped(range: 0...100)
var score: Int = 50
// macro reads range from attribute node and generates clamping accessors

Macro Error Reporting

guard decl.is(StructDeclSyntax.self) else {
  throw MacroError.message("@AddID can only be applied to structs")
}

Build Performance

// All generated members are emitted as source code
// and compiled normally — no reflection at runtime

Quick Check

Lesson Recap