hi everybody I ve got a problem with a LazyColumn of element

Question

hi everybody I ve got a problem with a LazyColumn of elements that have a favourite button basically when I tap the favourite button the item that is being favourited a document in my case is changed in the underlying data structure in the VM but the view isn t updated so I never see any change in the button state thread

Chris Fillmore · Answer

Can you please put your code blocks in the thread That s why people are reacting with thread

Manuel Lorenzo · Answer

of course sorry

Manuel Lorenzo · Answer

```class MainViewModel private val ioDispatcher CoroutineDispatcher = <http Dispatchers IO|Dispatchers IO> ViewModel var documentList = emptyList lt PDFDocument gt toMutableStateList fun toggleFavoriteDocument pdfDocument PDFDocument documentList find it == pdfDocument let it favorite = it favorite ``` The composables are ``` Composable fun DocumentRow document PDFDocument onDocumentClicked String Boolean gt Unit onFavoriteValueChange Uri gt Unit HeartIcon isFavorite = document favorite onValueChanged = onFavoriteValueChange document uri Composable fun HeartIcon isFavorite Boolean color Color = Color 0xffE91E63 onValueChanged Boolean gt Unit IconToggleButton checked = isFavorite onCheckedChange = onValueChanged Icon tint = color imageVector = if isFavorite Icons Filled Favorite else Icons Default FavoriteBorder contentDescription = null ``` am I doing something wrong because when I call the `toggleFavouriteDocument` in the `ViewModel` I see it s marked or unmarked as favourite but there is no recomposition at all anywhere edited

Chris Fillmore · Answer

It s no problem +1 Sorry I can t offer any contribution to your specific problem good luck getting it resolved

Oleksandr Balan · Answer

Does your `favorite` property in `PDFDocument` baked by `State`

Manuel Lorenzo · Answer

what do you mean although I guess if I don t know what you mean that means that it is NOT backed by `State`

Oleksandr Balan · Answer

Could you show us your `PDFDocument` class

Manuel Lorenzo · Answer

``` Parcelize data class PDFDocument var uri String var favorite Boolean Parcelable ```

Oleksandr Balan · Answer

Having a mutable properties var in this case inside the mutable container MutableList is generally a bad idea Compose simply could not track this changes It tracks only changes in the `State` properties So I guess something like this should work ``` Parcelize data class PDFDocument val uri String val initialFavorite Boolean Parcelable var favorite by mutableStateOf initialFavorite ``` However I would recommend to make your models fully immutable ``` Parcelize data class PDFDocument val uri String val favorite Boolean Parcelable``` And create a new ones for example with a ` copy ` when the state changes ```fun toggleFavoriteDocument pdfDocument PDFDocument documentList replaceAll if it == pdfDocument it copy favorite = it favorite else it ```

Albert Chang · Answer

Use regular class for a mutable class and data class for a immutable class Do not use data class for a mutable class

Manuel Lorenzo · Answer

thanks unfortunately it s not working yet

Manuel Lorenzo · Answer

I have this ```fun toggleFavoriteDocument pdfDocument PDFDocument documentList find it == pdfDocument let it copy fav = it fav also documentListLiveData postValue documentList ``` and when I set a breakpoint in the last `postValue` to see the content of the `documentList` the documents are never `favorite = true` nor `fav = true`

Manuel Lorenzo · Answer

doing this works but the icon is never changed ```fun toggleFavoriteDocument pdfDocument PDFDocument documentList forEachIndexed index document gt pdfDocument takeIf document == pdfDocument let documentList index = document copy fav = document favorite also documentListLiveData postValue documentList ```

Manuel Lorenzo · Answer

perhaps there s something wrong in the way I hoisted the state in `DocumentRow` and `HeartIcon`

Oleksandr Balan · Answer

Oh you are using `LiveData` in this case something like this should work ```fun toggleFavoriteDocument pdfDocument PDFDocument val newDocuments = documentListLiveData value map if it == pdfDocument it copy favorite = it favorite else it documentListLiveData postValue newDocuments ``` Why do you have a `documentList` property then

Manuel Lorenzo · Answer

Yes a mutable list

Manuel Lorenzo · Answer

I have tried however with both LiveData and mutable state list

Manuel Lorenzo · Answer

I ll make it work thanks a lot everybody