willSet causing copy-on-write types to copy on mutations?

[tclementdev]

Is it correct that having a willSet on a property will cause copy-on-write types to always copy their internals when the property is mutated? I guess it would make sense because both the current value and the new value are available in the willSet. Using the @Published property wrapper seems to be causing this as well, I guess it uses a willSet internally. Is there some approach to avoid the copy?

[xwu (Xiaodi Wu)]

SE-0268 (Review #2) — Review didSet Semantics

There are two ways to give willSet semantics for a non-instantaneous modification (e.g. calling a mutating method on the property): You can do what Swift does today, which is to force the modification to happen to a temporary copy and then call willSet immediately before writing back. You can call willSet before the modification begins. The second makes more sense to me abstractly, but it's totally incompatible with willSet getting passed the new value, and it's a radically different point in…

[tclementdev]

Thank you! Seems quite unfortunate that SwiftUI relies on willSet.