I have `mutableStateListOf()` object, which I named days, that holds `data class Cell(val text: Stri...

# compose

[Justin Yue]

I have mutableStateListOf() object, which I named days, that holds data class Cell(val text: String, var isSelected: Boolean). When I want to change the isSelected status, I have to reassign one element to a new Cell with the updated boolean. I would prefer to just reassign one element's isSelected boolean, but that doesn't seem to work. Does anybody know why?

[Justin Yue]

In one of my view models, I have this code:

var days = mutableStateListOf(
        Cell("Sun", false),
        Cell("Mon", false),
        Cell("Tue", false),
        Cell("Wed", false),
        Cell("Thu", false),
        Cell("Fri", false),
        Cell("Sat", false)
    )

And I have this function to change the boolean in a Cell:

fun changeDaySelection(index: Int) {
//        days[index].isSelected = !days[index].isSelected <- this line does cause recomposition 
        days[index] = Cell(days[index].text, !days[index].isSelected) // <- this line does cause recomposition
    }

Not entirely sure why the commented line wouldn't work

[CLOVIS]

Boolean is not a State.

[CLOVIS]

var isSelected: State<Boolean> should work, but I don't know if it's good practice.

[CLOVIS]

Another solution:

data class Cell(..., val isSelected: Boolean)
// note the val

var days by remember { mutableStateListOf(...) }
...
days = days[index].copy(isSelected = !days[index].isSelected)

[CLOVIS]

Eg: Compose knows only when you use = on the State class.

[Justin Yue]

Makes sense. I agree that making isSelected: State<Boolean> might not be the best approach, and I'll try out the 2nd solution. Thank you for your help.

[Zach Klippenstein (he/him) [MOD]]

You can do something like this to avoid having the state type in your api:

class Cell {
  var isSelected: Boolean by mutableStateOf(false)
}