Triggering Swift Property Observers From Initializers

Natalia Panferova:

In Swift, property observers such as willSet and didSet are not called when a property is set in an initializer. This is by design, as the initializer's purpose is to set up the initial state of an object, and during this phase, the object is not yet fully initialized. However, if we need to perform some actions similar to what we'd do in property observers during initialization, there are some workarounds.

Ole Begemann:

When a Swift property has a willSet clause, any mutation of that property will be made on a temporary copy.

⇒ At least two copies exist during the mutation.

⇒ This defeats the copy-on-write optimization for Array, String, etc. (which relies on uniquely referenced buffers).

⇒ A property of a COW type with a willSet will copy its contents on every mutation.

Relevant for SwiftUI code because @Published uses willSet: every mutation of a @Published var array: [T] will copy the array contents.

This happens even if the willSet doesn’t actually use the new value, e.g. if it was just being used to trigger a notification.

Previously:

• Macros in Swift 5.9
• SwiftUI Data Flow 2023
• Why Swift’s Copy-on-Write Is Safe
• Swift Copy-on-Write PSA: Mutating Dictionary Entries