alschmut / StructBuilderMacro

An attached macro for structs, classes and enums, which produces a peer struct implementing the builder pattern. Initialise your object with minimal effort using default values.

@Buildable Swift Macro

@Buildable is an attached macro for structs, classes and enums, which produces a peer struct implementing the builder pattern. Initialise your object with minimal effort using default values.

import Buildable

@Buildable
struct Person {
    let name: String
    let age: Int
}

let person = PersonBuilder(age: 42).build()

Important!
- This macro is intended to be used for simple structs, enums and classes (see below limitations)
- Please report any issues you might encounter
- Please star this repository, if your project makes use of it :)

Table of Contents

• Detailed Example with generated builder
• Installation
• Motivation
• Limitations
• Builder default values
• Roadmap

Detailed Example with generated builder

import Buildable

@Buildable
struct Person {
    let name: String
    let age: Int
    let address: Address
    let hobby: String?
    let favouriteSeason: Season
    
    var likesReading: Bool {
        hobby == "Reading" 
    }
    
    static let minimumAge = 21
}

@Buildable
enum Season {
    case .winter
    case .spring
    case .summer
    case .autumn
}

@Buildable
class AppState {
    let persons: [Person]

    init(
        persons: [Person]
    ) {
        self.person = person
    }
}

let anyPerson = PersonBuilder().build()
let max = PersonBuilder(name: "Max", favouriteSeason: .summer).build()
let appState = AppStateBuilder(persons: [max]).build()

Expanded macro

struct PersonBuilder {
    var name: String = ""
    var age: Int = 0
    var address: Address = AddressBuilder().build()
    var hobby: String?
    var favouriteSeason: SeasonBuilder = SeasonBuilder().build()

    func build() -> Person {
        return Person(
            name: name,
            age: age,
            address: address,
            hobby: hobby,
            favouriteSeason: favouriteSeason
        )
    }
}

struct SeasonBuilder {
    var value: Season = .spring
    
    func build() -> Season {
        return value
    }
}

struct AppStateBuilder {
    var persons: [Person] = []

    func build() -> AppState {
        return AppState(
            persons: persons
        )
    }
}

Installation

The library can be installed using Swift Package Manager.

Motivation

I use builders in the below two scenarios. Having them generated reduces a lot of the boilerplate code:

• When creating mock data for unit tests
• When creating fixtures for SwiftUI Previews

Limitations

• The macro only works on struct, enum and class definitions
• The list of default values is limited to the values specified in the below table. All other types will require another builder to be defined
• If a builder for a specific declaration can not be generated, you can always choose to create it yourself by following the below builder naming pattern:

  struct <MyType>Builder {
  
      func build() -> <MyType> {
          return ...
      }
  }

• If a class or a struct has one or more initialisers, the macro will use the first/top one
• For structs without an initialiser, the macro makes a best guess to decide how the implicit memberwise initializer could look like. This best guess might fail for declarations that have not been considered during implementation of the macro
• As of Swift 5.9 (13.02.2024) it is not possible to use the generated builders inside the SwiftUI #Preview closure

Builder default values

The list of default values is limited to the values specified in the below table. If a type e.x. UnknownType is not part of the list, the macro will set the default value to UnknownTypeBuilder().build(), assuming that the UnknownTypeBuilder was created somewhere else.

Type	Default Value
UnknownType	UnknownTypeBuilder().build()
String	""
Int	0
Bool	false
Double	0
Float	0
Date	Date()
UUID	UUID()
[AnyType]	[]
[AnyType:AnyType]	[:]
AnyType?	(implicitly nil)
AnyType!	(implicitly nil)
Int8	0
Int16	0
Int32	0
Int46	0
UInt	0
UInt8	0
UInt16	0
UInt32	0
UInt46	0
Data	Data()
URL	URL(string: "https://www.google.com")!
CGFloat	0
CGPoint	CGPoint()
CGRect	CGRect()
CGSize	CGSize()
CGVector	CGVector()

Roadmap

The @Buildable macro was created out of personal interest to reduce repetitive code in my own projects. I might continue developing the macro depending use cases I stumble across, though, I do not guarantee to keep the project up to date myself. Please create GitHub issues for any feature or bugfix you would like to see within the macro. Contributions or fixes from the Community are most welcome.