Hi everyone ! Im using the compose metrics and I get the following declare class: What is mean my c...

# compose

[Daniel Okanin]

Hi everyone ! Im using the compose metrics and I get the following declare class:
What is mean my class is stable but includes unstable fields? How is it effect? Is it effect if the function that uses this class be considers as skippable or not only? Is it ok to put “stable” annotation on class that includes unstable fields?

stable class ProViewModelItem {
  unstable val profileImage: ImageDescriptor?
  unstable val coverImage: ImageDescriptor?
  stable val numReviews: Int
  stable val reviewRating: Float
} 

This is how I declare this class:
@Stable
class ProViewModelItem(
    id: String?, title: String?,
    val profileImage: ImageDescriptor? = null,
    val coverImage: ImageDescriptor? = null,
    val numReviews: Int = 0,
    val reviewRating: Float = 0f,
) : HomeFeedViewModelItem(id, title)

[Francesc]

stable is a contract. It's ok if you uphold your part. How's your ImageDescriptor defined?

[Daniel Okanin]

@Francesc as java interface

[Daniel Okanin]

1. The result of equals will always return the same result for the same two instances.
2. When a public property of the type changes, composition will be notified.
3. All public property types are stable.
SO i dont uphold my pard?

[Francesc]

Stable means that it can change but any change will be notified. Does your class actually change? If it's immutable but the compiler can't infer it, you can mark it as Immutable.

[Daniel Okanin]

I see, so if I have unstable fields in my class and @Stable annotation, there is any negative effect?

[Stylianos Gakis]

If you say that something is @Stable but it is not you are making the compiler make a wrong assumption therefore you can not be guaranteed that things will work correctly. Things will most likely behave wrong without any warning. Aka never do this. I’d also recommend taking a look at this which was published recently https://medium.com/androiddevelopers/jetpack-compose-stability-explained-79c10db270c8

[Francesc]

Yes, that will cause issues. Usually you don't have to worry about recomposition, only if you see a performance issue would you need to delve into it. Adding the annotation when it does not apply will lead to subtle bugs

[Daniel Okanin]

@Stylianos Gakis @Francesc Thanks

[Daniel Okanin]

Hi @Francesc after mark @immutable annotation on class that includes object from other module that not support compose (so its automaticily marked as unstable)
I see the the inner field is still unstable but the the outer is stable, is it ok?

stable class PhotoViewModelItem {
  unstable val imageDescriptor: ImageDescriptor?
  stable val isSaved: MutableState<Boolean>
  stable val photoType: PhotoType?
  stable val tagsCount: Int
}

[Francesc]

if you control the module where the ImageDescriptor is defined, you could annotatte it there. If you don't, you could wrap it in a class and mark it as @Immutable - assuming the implementation is indeed immutable. This stuff was recently described here https://medium.com/androiddevelopers/jetpack-compose-stability-explained-79c10db270c8

[Daniel Okanin]

yes I read this blog post, see the example of the data class Snack - he puts @immutable annotation and still use Set as field (its known as unstable)

[Francesc]

right, you can annotate something with @Stable or @Immutable to help the compiler, but then you are responsible for honoring that contract

[Daniel Okanin]

and what is the contract with immutable? Just the object will never change after it created yes?

[Francesc]

the contract is basically that as long as equals and hashCode do not change, the object does not change. If it's immutable (doesn't change after construction) then those hold

[Francesc]

as far as I understand it, the compose compiler relies on equals to determine if it needs to recompose for immuable objects. If you use data classes you get that for free, otherwise you have to write the equals to ensure a new, different object , does not yield the same equals (you could break that if you omitted a field, for instance)

[Daniel Okanin]

Thanks a lot