Properties

Implementation status: Partial GA (Phase 138 Class Properties v6 MVP)

A property is a stored value associated with a type or an instance.

Stored Properties

class Person {
    var name: String
    let id: Int
    init(name: String, id: Int) {
        self.name = name
        self.id = id
    }
}

Keyword	Member
var	Mutable
let	Immutable

Global and Static Properties

let globalVersion = 1

class Config {
    static var debug = false
    static let version: Int = 1
}

Partial GA — since Phase 134, static var / static let MVP inside a class (type-level storage slots). Phase 138 GA — willSet / didSet MVP on a class static var (lang-props-v6:static-observers-mvp; with oldValue / newValue bindings; not the full set of struct / global observers).

class Counter {
    static var value: Int = 0 {
        willSet { print(newValue) }
        didSet { print(oldValue) }
    }
}
// Counter.value = 7 → willSet(7), didSet(0)

Computed Properties

Partial GA — Swift-style computed MVP inside classes and extensions:

class Box {
    var base: Int
    init(base: Int) { self.base = base }
    var doubled: Int {
        get { return self.base * 2 }
    }
    var value: Int {
        get { return self.base }
        set { self.base = newValue }
    }
}

extension Box {
    var labeled: String {
        get { return "x" }
        set { self.base = 0 }
    }
}

Capability	Status
class var x: T { get { … } } get-only	Phase 134 GA
class var x: T { get set } (newValue)	Phase 135 GA
Subclass override var computed (polymorphic read/write)	Phase 135 GA
extension var x: T { get } / { get set }	Phase 136 GA (lang-props-v4:extension-computed-mvp)
@Published on a class stored var	Phase 137 GA (lang-props-v5:published-mvp; a narrow willSet notification print("published:<name>") before assignment; not full Combine)
class static var + willSet / didSet	Phase 138 GA (lang-props-v6:static-observers-mvp)
lazy thread safety / struct @Published / the full set of global observers	139+

Fingerprint: lang-props-v6:docs-honesty — the complete Swift property model → 139+.

Property Observers

willSet / didSet: Partial GA (since Phase 65 direct assignment to stored properties can trigger them; deepened in Phase 134):

Capability	Status
Stored property willSet / didSet	Partial GA
oldValue / newValue bindings	GA
Inheritance: a subclass instance writing an inherited field triggers the superclass observer	Phase 134 GA
Suppressed inside init: self.field = … does not trigger observers	Phase 134 GA (aligned with Swift)
class static var willSet / didSet	Phase 138 GA
Observers on computed properties	Rejected (S056)
@Published, global observer runtime	Partial GA (@Published → Phase 137; the full set of global observers → 139+)

class Animal {
    var name: String
        willSet { print("will") }
        didSet { print("did") }
    init(name: String) { self.name = name }  // Assignment inside init does not print
}
class Dog: Animal {
    init(name: String) { super.init(name: name) }
}
// Writing name through a Dog instance triggers Animal's willSet/didSet

Property Override

A subclass may override var to replace a superclass computed property; reading and writing through a base reference goes through the subclass getter/setter (vtable polymorphism):

class Base {
    var label: String { get { return "base" } }
    init() {}
}
class Derived: Base {
    override var label: String { get { return "derived" } }
    init() {}
}
// let b: Base = Derived(); b.label → "derived"

Lazy Properties

lazy var: Phase 134 GA (class only; initialization triggers on first read; not thread-safe MVP → full semantics in 139+).

class Box {
    lazy var label: String = "ready"
    init() {}
}

Differences from Swift

Swift	SYX
Computed properties + the full set of observers	Mainly stored; computed get/set + override + extension computed + static observers MVP GA
lazy var thread safety	MVP is not thread-safe
@Published and friends	Phase 137 MVP (class stored; full Combine → 139+)
static observers	Phase 138 MVP (class static var only; full Swift static/lazy/global → 139+)
Unified struct property syntax / the full wrapper+observer ordering set	139+

---

Next chapter → Methods

See also → Language Reference: Declarations