How do you observe a mutable object that fires events when i

Question

How do you observe a mutable object that fires events when it changes Code that doesn t work ``` Composable fun Things model MyViewModel val mutableThing = model thing mutableThing addChangeListener recomposeThisStuff Text $ mutableThing size ``` So model thing may always be the same reference object But it changes internally

robnik · Answer

Or is Compose meant to be used mainly with immutable types I like Kotlin s immutable persistent collections but sometimes it s harder when dealing with more complex types to make everything immutable Not sure if I should be trying to do that or not

len · Answer

Convert `thing` to a `MutableState` in your ViewModel and just do `Text $ model thing size `

len · Answer

<https developer android com codelabs jetpack compose state 8> this codelab should help you

robnik · Answer

But the value `model thing` never changes As far as Compose is concerned that mutable state is not changing Am I supposed to observe the object and then call `thing value = thing value` to try and trigger a recompose

robnik · Answer

Note that in that linked article all the MutableStates contain immutable types like String

len · Answer

In your view model you have to change the value of your mutable state in order to trigger a recomposition

len · Answer

Try creating a function in your view model that updates the current value and see how the value changes in your UI

len · Answer

If your value is never updated you might be missing a remember call somewhere Make sure that your view model instance is always the same

robnik · Answer

Here s some code to illustrate my question <https pastebin com aTvs1i9w>

robnik · Answer

The Thing is a mutable object with its own way of notifying observers of changes It seems like this doesn t fit well with Compose s MutableState With a mutable object there is only one instance so you can t change the value of the MutableState

robnik · Answer

Ah I discovered the 2nd arg to `mutableStateOf ` So this one line change makes my example work `var thing1 Thing by mutableStateOf Thing neverEqualPolicy ` Seems odd though so I still wonder if there is a better way

Adam Powell · Answer

Given the rest of your setup and assuming you can t change `Thing` using `neverEqualPolicy` is fine there

robnik · Answer

What if I could change `Thing` Should I try to make it completely immutable or is there another way to make mutable objects that works better with Compose