your droidcon online session today was real

Question

< Brian Gardner> your droidcon online session today was really good and it has me excited

Leland Richardson [G] · Answer

Was it recorded I have seen a ton of positive feedback about it and would love to check it out

Colton Idle · Answer

< Leland Richardson G > yep Should be recorded and going onto droidcon videos soon I ll ping ya if I see it smile

Brian Gardner · Answer

Thanks I m glad you liked it I had a ton of fun with the session and I m looking forward to making more video content soon

Brian Gardner · Answer

The recording is now live <https www droidcon com media detail video=412304809>

Leland Richardson [G] · Answer

< Brian Gardner> this was great really nice intro for people I m a bit curious about the state issue you mentioned with AdapterList Is this a known issue reported somewhere cc < Ryan Mentley> Also i m curious if you intentionally were saying to use `mutableStateOf ` where you used it instead of `state ` The latter seems more correct based on the usage

Ryan Mentley · Answer

Which state issue Timecode

Brian Gardner · Answer

I reported the issue here <https issuetracker google com u 2 issues 151860426>

Leland Richardson [G] · Answer

in the video above he is creating a toggleable button in each item of an adapterlist but said that it wasn t updating when the state would get changed My hunch is that the problem was actually just that mutableStateOf was being used instead of `state` though

Brian Gardner · Answer

I likely wasn t intentional about the state part Why would the `mutableStateOf ` work differently than `state`

Leland Richardson [G] · Answer

`mutableStateOf` is like a constructor and will create a new instance each time with the value you pass in The `state` function is a composable function which will only execute the `init` lambda the first time and it will remember the instance each time when creating state ina composable function this is usually what you want it is basically shorthand for `remember mutableStateOf init `

Ryan Mentley · Answer

The `mutableStateOf` in the preview I think is the one that probably should lost be `state`

Brian Gardner · Answer

Ok so creating the mutable state won t trigger a recompose unless it s wrapped in a `remember ` block inside the composable function

Ryan Mentley · Answer

The code looks otherwise mostly correct but there s one suspicious bit < Leland Richardson G > is it valid to pass a `MutableState` directly like this

Leland Richardson [G] · Answer

< Brian Gardner> it does actually trigger an update but when it recomposes the function you just create a different mutable state instance initialized to the same original value as before so it doesn t look like it s doing anything

Ryan Mentley · Answer

or should the value be passed directly

Leland Richardson [G] · Answer

it s valid to do either

Brian Gardner · Answer

ah ok I didn t realize it was calling the composable function again to recreate the view I wasn t sure how the recompose step worked but that makes sense

Ryan Mentley · Answer

alright in that case that does look like it s probably an AdapterList bug because the rest of the mutable state code outside of the preview looks correct

Ryan Mentley · Answer

I d have to debug it to be sure though

Leland Richardson [G] · Answer

yeah it calls the function again similar to react flutter etc you want to write your composable functions as if they can be called again at any time so if the function itself is introducing the state you want to make sure and hold onto it properly hence use remember or state

Brian Gardner · Answer

Another question I have is where it s appropriate to create the state since the functions can be called again It seems like the state should be declared at the top level screen component and passed down to the children At least if the view state needs to change based on user interaction

Leland Richardson [G] · Answer

there s a whole can of worms that can be opened by that question more or less i try to think of it as create it in the place where the state is owned That can be hard to define but in general a good rule of thumb first candidate is going to be the closest ancestor composable where the value is used entirely below it and nowhere else If that scope changes just move the state up until it is satisfied again

Leland Richardson [G] · Answer

but there s a lot more that goes into it and i might give a different answer depending on the context

Leland Richardson [G] · Answer

for instance if there s some state variable like current logged in user that is going to be used at a much higher level than something more locally scoped or even just something less business logic specific and more ui logic specific

Brian Gardner · Answer

That makes sense The main scenario I think of is loading data from a database or web server I can setup the state when the data loads but when I need to modify the data based on the user interaction how do I update the screen with the new data

Brian Gardner · Answer

Assuming the ViewModel or presenter handles the data loading and updates would I just create a new composable with the updated data or is there a better way to swap it out

Leland Richardson [G] · Answer

in those cases i think the better thing is to use something other than state like LiveData or Flow etc and then join that data into the composable hierarchy using state but keep state as an implementation detail

Leland Richardson [G] · Answer

you can see how this is done with some of our interop libraries with Rx and LiveData

Brian Gardner · Answer

Yeah I saw that compose has the flow rxjava to state stuff now which is cool I ll have to look into that more

Leland Richardson [G] · Answer

but in this case it is important to leverage tools such as `onCommit` to do it properly

Leland Richardson [G] · Answer

also we will be releasing some new APIs around coroutines and such that will make a lot of this really really nice if your data layer uses suspend functions

Brian Gardner · Answer

Sweet We re definitely moving towards suspend functions for our data layers so that will be nice

Leland Richardson [G] · Answer

nice

Brian Gardner · Answer

I haven t messed with `onCommit` or `onActive` too much so I ll look into those next

Brian Gardner · Answer

Thanks for talking through this stuff with me

Leland Richardson [G] · Answer

no problem

Leland Richardson [G] · Answer

thanks for creating such good content it was a very well done live coding example

Leland Richardson [G] · Answer

also if you can can you try updating your example to use `state` and see if that fixes the adapterlist bug you filed

Brian Gardner · Answer

I m glad you liked it I can take some time tomorrow morning to rework my sample to see if using `state` works

Leland Richardson [G] · Answer

awesome thanks