It is advised not to pass viewmodel objects down to child co

Question

It is advised not to pass viewmodel objects down to child composables is this due to increased coupling and worse testability or are there some other reasons too Context Some of my screens mostly consist of sub screens that definitely don t need to be reusable would it be bad practice to just directly pass the viewmodel down a few layers of parent layout composables

Csaba Szugyiczki · Answer

you also loose the ability to create preview s for your subscreens

Csaba Szugyiczki · Answer

your subscreens will share a huge state with things the particular subscreen should not even know about eg fields used in Subscreen1 will be available for Subscreen2

vide · Answer

Thank you for the response I ll create separate state holders for the sub screens

vide · Answer

Just as a curiosity though what would be the practical consequences of having the subscreens share some state

Csaba Szugyiczki · Answer

There is no direct consequence I guess you could get away with it just fine But you have to be really careful how you implement things You introduce much more complexity than needed You give room for mistakes that otherwise could not even exist

Csaba Szugyiczki · Answer

Handling and triggering Recompositions might also be trickier harder to reason about You mentioned worse testability I mentioned not being able to have previews and the only upside is it is easier to implement

Csaba Szugyiczki · Answer

So it is hard to tell if you will regret it or it will work great If you are sure a few layers wont end up being every layer then it might work but I would not risk it personally just to cut a few corners

Robert Williams · Answer

You haven t really explained why you want to do this If your goal is to have subscreens observe some subset of the state then you re probably reimplementing some of what Compose already does which is likely to be a bad idea

Robert Williams · Answer

If you just want to avoid some boilerplate of passing lots of callbacks so the subscreens can send events back to the VM I d say that s probably fine

Robert Williams · Answer

as already explained though this will make your subscreens harder to test and really doesn t save much boilerplate

vide · Answer

I was actually kind of doing both before the subscreens only need a subset of the state and callbacks What do you mean by Compose already does this

vide · Answer

Would the better pattern here be separate state holders for the subscreens like I was planning to refactor

Robert Williams · Answer

Only the top level should be observing for changes in state and then passing subsets of the required state to its children

Robert Williams · Answer

children don t need the VM for this because it s driven by the parent

Colton Idle · Answer

From a few days ago <https kotlinlang slack com archives CJLTWPH7S p1658639141600549>

Colton Idle · Answer

I basically do what the last answer in the above ^ thread says BUT i have made exceptions in some cases where I have a subscreen inside of a screen A good example is when I have a complex bottom sheet in my Screen then I will just pass the VM but after that point I don t pass it down any further