CODEWITHSUNDEEP
androidandroid-jetpack-composecompose-recomposition
BigDan
BigDan

Reputation: 527

Android Compose Recomposition is not triggered when adding items in a mutable list

I have a list of files in a LazyColumn. User can select multiple files in the list. I have a button floating over that list. The text of the button is "Archive n files" (where n is the number of selected files in the list) I want the text of the button to change every time a file is selcted or unselected in the list.

I use a MutableState<MutableList> that containt the selected files

selections = rememberSaveable { mutableStateOf(mutableListOf()) }

When a user clic on a file in the displayed list, I add or remove an entry in selections

        if (row.isSelectedState.value) {
            selections.value.add(row.idConversation)
        } else {
            selections.value.remove(row.idConversation)
        }

The size of selections is used in the button text

   text =
   if (selections.value.size == 0)
      "Archive"
   else
      String.format("Archive %1$d conversations", selections.value.size)

The problem is that when we add or remove items in selections the text of the button doesn't change, the recomposition is not trigerred base on selections.value.size

I fixed this by using another state

selectionCount = rememberSaveable { mutableStateOf(selections.value.size) }

   text =
   if (selectionCount.value == 0)
      "Archive"
   else
      String.format("Archive %1$d conversations", selectionCount.value)

And update this state every time I add or remove an item in selections

        if (row.isSelectedState.value) {
            selections.value.add(row.idConversation)
        } else {
            selections.value.remove(row.idConversation)
        }
        selectionCount.value = selections.value.size

But I don't like this solution because I have to maintain another state and can forget to update it in futur code where we add or remove items in selections

What can I do for the recomposition of the button to be triggered after adding/removing items in selections ?

Upvotes: 1

Views: 1809

Answers (2)

tasjapr
tasjapr

Reputation: 1240

replace this:

selections = rememberSaveable { mutableStateOf(mutableListOf()) }

with this:

selections = rememberSaveable { mutableStateListOf<Selection>()}

Here is good explanation of difference between mutableStateListOf and mutableStateOf

Upvotes: 2

zaid khan
zaid khan

Reputation: 1012

Create the selection list as

val selections = remember { mutableStateListOf<Selection>()}

Create a variable to save the size

var size = 0

LaunchedUnit(key=selections.size){
  size = selections.size  
}

this LaunchedEffect block will execute everytime selections.size changes, use the size variable wherever you would like to display it.

Upvotes: 0

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