Hi, is there a way to observe a MutableState value ? Like executing a code when the value is changed

# compose-desktop

[Ayfri]

Hi, is there a way to observe a MutableState value ? Like executing a code when the value is changed

[SrSouza]

You mean outside of a composition?

[Adam Powell]

yes, there are ways to do it, that's how composition and compose-ui do it. 🙂 the best way to do it will depend on what the use case is. What are you trying to do?

[Ayfri]

I have 2 values, the first is a string and the second is a value from an Enum

The second is changed through a DropDown menu, and I want the first to be equal to the name of the second

But the first will later be used for multiple enums with other values like the second

(I have a main DropDown menu which presents you other DropDowns that will all set my first value)

[Ayfri]

So I want to observe my second value to set the first value

[Adam Powell]

how is the first value currently represented?

[Adam Powell]

It sounds like the first value is always a function of the second, e.g.

val first: String get() = second.name

?

[hfhbd]

Or use derivedState. But a regular extension method sounds simpler.

[Adam Powell]

yeah derivedStateOf is a good choice if the calculation is expensive and the result should be cached; if not, just make it a function that calculates the result based on the input states

[Adam Powell]

this is a kind of key element of declarative thinking with snapshots and compose; instead of thinking in terms of, "observe changes in value A and update value B accordingly," think in terms of, "B is defined as `f(A)`"

[Adam Powell]

by thinking this way you leave no room for error, no complexity of correct subscription management

[Adam Powell]

everything is defined in terms of single sources of truth for each input instead of as data processing pipelines with discrete imperative stages and update events

[Ayfri]

I can't use your solution Adam because I will maybe have other values that sets the first one

My application is for creating JSON from templates, but there are types of JSONs and type of these types, like Recipe type and Texture type, and for recipes there is CraftingShapedRecipeTemplate and CraftingShapelessRecipeTemplate() and SmeltingRecipeTemplate() etc, same for Texture, there are multiple types

But I have 1 box that contains the fields the user have to complete for the template, like name , id etc, there are a lot of different fields through the templates

So to know what template I have, I have multiple Enums, like RecipeType and TextureType, and for each enum I have a function toTemplate() which transform itself into the template

So for now when my second value changes, I set the first one to the Enum name and in the box I get back the template by searching through all the Enum if in any I can find a Enum value matching, using MyEnum.values().find { it.name == firstValue } ?: MyOtherEnum.Values().find { it.name == firstValue } etc and then I execute .toTemplate() or I return if null

I don't know if it is a good architecture but I thought of this

[Adam Powell]

yeah I would probably find another way to do that. 🙂 Even using an Any and a when (value) { is EnumTypeOne -> ... would be better than going through strings

[Adam Powell]

but the general declarative principle will still guide you to a simpler design that will have fewer opportunities for bugs. "`first` is equal to..." and then whatever logic is involved in determining its value, absent any element of time or events.

[Ayfri]

Okay I'm gonna use the when (...) { is ... so