Hi! Two questions: 1. Does `@Stable` imply `@Immutable` or vice versa? 1. If I have a `data class...

# compose

[dimsuz]

Hi! Two questions:

1. Does @Stable imply @Immutable or vice versa?
2. If I have a data class which contains only vals (which are either other data classes or privitives or non-mutable collection types), does it mean that Compose compiler plugin will be able to infer `@Stable`/`@Immutable` automatically? I found an earlier thread that seemed to suggest that.
3. It would be great if both of these points were somehow covered in documentation (or did I miss that?)

[Adam Powell]

Immutable implies Stable. Stable means that for object instances a and b, the result of a.equals(b) will always return the same result regardless of any changes made to either a or b. In practice this means any change to a is reflected in b and vice versa; they share some sort of internal structure.

[Adam Powell]

Immutable implies Stable because immutable can't change at all, trivially satisfying this contract.

[Adam Powell]

The compose plugin will infer stability if your data class has fields of only other stable types, yes. Primitives and strings are considered stable. Read-only collections such as List<T> are not.

[eygraber]

What would be an example of something that was stable but not immutable?

[Adam Powell]

A type with several by mutableStateOf properties. It can change, but it's always equal to itself and notifies compose of those changes when they happen. Similarly, a type that points at the same stable objects and compares as equal if the objects it points to are reference equal

[dimsuz]

I see, thanks!

So if my data class has List<T>, Map<T> or other effectively immutable data structure I should explicitly mark this class as Immutable, otherwise I can rely on compose smartness 🙂

[Adam Powell]

yes, @Stable and @Immutable make a promise to compose that it can't verify without knowing the semantic things you know about your code. You can declare a type @Immutable that contains a List<T>, but if you assign a mutableListOf to that property and start changing it you're going to have a bad time

[dimsuz]

Got it! 👍