Hi, how to use `@Stable` annotation properly? From...

# compose

[Erlan Amanatov]

Hi, how to use @Stable annotation properly?
From documentation

The result of equals will always return the same result for the same two instances.

So, should I override equals and hashcode methods for the following class?

@Stable
class State (
    text: String,
    enabled: Boolean
) {
    var text by mutableStateOf(text)
    var enabled by mutablesStateOf(enabled)
}

I am not sure because I see a lot of Stable annotated classes where those methods are not overridden

[ephemient]

if you implement structural equals, that will not be a valid type for `@Stable`:

val state1 = State("1", false)
val state2 = State("1", false)
state1 == state2
state2.text = "2"
state1 != state2

[Erlan Amanatov]

yes, I was talking about structural equals. Why is that not valid for Stable?

[Erlan Amanatov]

so the result for instances state1 and state2 is not the same 🤔

[ephemient]

for comparison, a standard @Stable type:

val state1 = rememberScrollState(initial = 0)
val state2 = rememberScrollState(initial = 0)
state1 != state2 // regardless of whether state1.value == state2.value

[Erlan Amanatov]

got it 👍
so the conclusion is that we never should override equals for Stable annotated classes.
What about @Immutable annotation? As far as I understand, in that case we have to override those methods (considering that all other requirements are met)

[ephemient]

I can't think of a practical application but "never" seems too strong. in principle this should be fine

@Stable class Keyed(val key: String, private val map: SnapshotStateMap<String, String>) {
    var value: String by map
    override fun equals(other: Any?): Boolean = other is Keyed && key == other.key && map == other.map
    override fun hashCode(): Int = key.hashCode() * 31 + map.hashCode()
}
val map = remember { mutableMapOf("foo" to "hello", "bar" to "world") }
val state1 = Keyed("foo", map)
val state2 = Keyed("foo", map)
val state3 = Keyed("bar", map)
state1 == state2; state1 != state3; state2 != state3 // regardless of anything value is set to

[ephemient]

also in principle an immutable type without structural equals (only referential equals) should be legal… it'll just be not useful for skipping

[Colton Idle]

FWIW. I've shipped 4 apps to prod with compose without any real issues.

I've never used @Stable 🙈