Should I store the state of a variable that depends upon another variable?

Is it okay to save the state of a dependent variable?

I am learning Android development through the official Android course. In one of the projects, I came across a line of code that saves the state of a variable that depends on another variable whose state has already been saved. The code is as follows:

> In this code, we have already stored the state of the variable currentDessertIndex. Since the value of currentDessertPrice and currentDessertImageId depends upon the currentDessertIndex whenever the recomposition happens, the two variables will be assigned the correct value of even if we do not store the state of those.

@Composable
private fun DessertClickerApp(
    desserts: List<Dessert>
) {

    var revenue by rememberSaveable { mutableStateOf(0) }
    var dessertsSold by rememberSaveable { mutableStateOf(0) }

    val currentDessertIndex by rememberSaveable { mutableStateOf(0) }

    var currentDessertPrice by rememberSaveable {
        mutableStateOf(desserts[currentDessertIndex].price)
    }
    var currentDessertImageId by rememberSaveable {
        mutableStateOf(desserts[currentDessertIndex].imageId)
    }
    ...
}

> This is what I think should be the right way to do it.

@Composable
private fun DessertClickerApp(
    desserts: List<Dessert>
) {

    var revenue by rememberSaveable { mutableStateOf(0) }
    var dessertsSold by rememberSaveable { mutableStateOf(0) }

    val currentDessertIndex by rememberSaveable { mutableStateOf(0) }

    var currentDessertPrice = desserts[currentDessertIndex].price
    
    var currentDessertImageId = desserts[currentDessertIndex].imageId
    
    ...
}

So, what do you suggest? Am I correct or does this invites any isssue?

Link to the Line of code

I changed the way the the state was stored. Now I expect to tell me if I am correct or does this invite any issue.

Ideally, you want a dependent state to change only when the state it depends on changes.

var currentDessertPrice = desserts[currentDessertIndex].price
var currentDessertImageId = desserts[currentDessertIndex].imageId

However, in the above lines, the dependant states will be calculated every time the composable recomposes which is unnecessary. So, you have to use remember to remember the values between recomposition.

Also, I think that this can be a good use case of derivedStateOf.

Yes it's generally better to only save the state of independent variables.

If the dependent variables are complex (not in your case), you can save their states too but you'll have to be careful to update them when the independent variable changes. Otherwise you will encounter hard to find bugs where some parts of your composable doesn't update.