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in my ViewModel:

private val _itemList = mutableStateListOf<Post>()
val itemList: List<Post> = _itemList

fun likePost(newPost: Post){
        val index = _itemList.indexOf(newPost)
        _itemList[index] = _itemList[index].copy(isLiked = true)
}

Here my Post data class:

data class Post(
    val id: Int,
    val name: String, 
    val isLiked: Boolean = false,
)

And here my Composable:

val postList = viewModel.itemList

LazyRow(content = {

    items(postList.size) { i ->
        val postItem = postList[i]
        PostItem(
            name = postItem.name,
            isLiked = postItem.isLiked,
            likePost = { viewModel.likePost(postItem)}
        )

    }
})

The change does not update in the UI instantly, I first have to scroll the updated item out of the screen so it recomposes or switch to another Screen and go back to see the change.

The problem is in your viewmodel. You need to do a deep copy first then set the isLiked. fun likePost(newPost: Post){ val index = _itemList.indexOf(newPost) _itemList[index] = _itemList[index].copy() _itemList[index].isLiked = true }