I have fun with viewModelScope.launch written in viewModel for removing an item from cart, there's a...

# compose

[Nikhil Parab]

I have fun with viewModelScope.launch written in viewModel for removing an item from cart, there's a condition in compose item card which disables the remove icon when last part is remaining in the list, but when I click on the remove button on 2 of the last cards, both the items gets removed. I already tried mutex, checking with a mutableStateOf boolean value, adding @Synchronized on the fun, but nothing works ? how should I handle this race condition?

[Sergey Dmitriev]

Could you share your ViewModel’s and Compose code?

[Nikhil Parab]

there isn't much there, just simple fun in viewModel, and normal lazycolumn

LazyColumn(
   ...
) {
    items(
        items = selectedItems,
        key = { item -> item.id },
        contentType = { _ -> "SelectedItem" }
    ) { item ->
        val condition = !(item.inCertainCategory && item.part_desc?.contains("xyz", ignoreCase = true) == false) &&
                selectedItems.count {
                    it.inCertainCategory && it.part_desc?.contains("xyz", ignoreCase = true) == true
                } > 1
        SelectedPartItem(
            modifier = Modifier.animateItem(),
            item = item,
            isEditEnabled = condition,
            onAdd = { onAddMinusItem(true, item) },
            onMinus = { onAddMinusItem(false, item) },
            onRemove = { onRemoveItem(item) }
        )
    }
}

fun removeItem(...) {
    viewModelScope.launch {
        ...
    }
}

[Sergey Dmitriev]

which disables the remove icon when last part is remaining in the list

What “when last part is remaining in the list” means?

when I click on the remove button on 2 of the last cards, both the items gets removed.

That sounds correct, if remove button is available when you click on it the item gets removed 🤔

[Sergey Dmitriev]

Without more context I suggest
• Check how you update selectedItems
• Check the condition try to break it down and make more readable

[Nikhil Parab]

Yes, the button is disabled, when the second last item is removed, so the condition is working correct, just when I click those items simultenously this condition doesn't work

[Sergey Dmitriev]

But how can you click it if the button is disabled?

[Nikhil Parab]

just with 2 fingers, at the same time

[Sergey Dmitriev]

You are clicking the same item with 2 fingers?

[Nikhil Parab]

for e.g there are last 2 items remaining in the list, I click on both of them simultaneously, both of them gets removed

[Sergey Dmitriev]

So when there are 2 last items in the list, they are both clickable, but as soon as you remove one there should be only one item left that is not clickable correct?

[Nikhil Parab]

yep

[Sergey Dmitriev]

Ok, could you show your

fun removeItem(...) {
    viewModelScope.launch {
        ...
    }
}

And how you apply Mutex in particular

[Nikhil Parab]

private val estimateMutex = Mutex()

fun removeItem(...) {
    if (quantity < 0) return

    viewModelScope.launch {
        estimateMutex.withLock {

         }
     }
}

[Sergey Dmitriev]

Move quantity check under withLock because now it passes for both clicks

[Sergey Dmitriev]

If I understand correctly what quantity means 😅

[Sergey Dmitriev]

fun removeItem(...) {
    viewModelScope.launch {
        estimateMutex.withLock {
               if (quantity < 0) return
               // do something
               quantity = x
         }
     }
}

[Sergey Dmitriev]

And of course be careful with changing (and reading) quantity outside of Mutex

[Nikhil Parab]

see, in the remove item the quatity is being sent as 0 as default, so that the fun just return & ends it's execution, it unrelated to the logic or removing the item, just a check for minus qtys

[Sergey Dmitriev]

Then put a different check in there like

fun removeItem(...) {
    viewModelScope.launch {
        estimateMutex.withLock {
             // Check if the item can be removed
         }
     }
}

[Sergey Dmitriev]

Or for an easy fix you can probably just do

if (mutex.isLocked) return

[Jonathan]

I don’t think the quantity check is correct. Its checking when the quantity is less than zero which should never happen. Should it be if (quantity <= 0) return or if (quantity < 1) return

[Jonathan]

I believe the removeItem(…) always executes and never returns early.

[Sergey Dmitriev]

it unrelated to the logic or removing the item, just a check for minus qtys

It’s unrelated

I think the problem here is that no logic guards the execution of removal
2 clicks schedule 2 coroutines to execute, and wrapping them with Mutex is not enough — doing so will only guarantee their execution in sequence, one after another, no racing conditions — however still both will be executed

Since UI can send remove requests “uncontrollably” (using two, three, fours… fingers) there should be something that will check the validity of every new request

[Nikhil Parab]

Yes, tried adding a validation in viewModel fun itself, and now I don't know why, but the condition is working, Thanks