Why need the author to add the keyword remember in this @Composable?

user_828896 0
Viewed 58

The Code A is from the project ThemingCodelab, you can see full code here.

I think that the keyword remember is not necessary in Code A.

I have tested the Code B, it seems that I can get the same result just like Code A.

Why need the author to add the keyword remember in this @Composable ?

Code A

@Composable
fun Home() {
    val featured = remember { PostRepo.getFeaturedPost() }
    val posts = remember { PostRepo.getPosts() }
    MaterialTheme {
        Scaffold(
            topBar = { AppBar() }
        ) { innerPadding ->
            LazyColumn(contentPadding = innerPadding) {
                item {
                    Header(stringResource(R.string.top))
                }
                item {
                    FeaturedPost(
                        post = featured,
                        modifier = Modifier.padding(16.dp)
                    )
                }
                item {
                    Header(stringResource(R.string.popular))
                }
                items(posts) { post ->
                    PostItem(post = post)
                    Divider(startIndent = 72.dp)
                }
            }
        }
    }
}

Code B

@Composable
fun Home() {
    val featured =PostRepo.getFeaturedPost() 
    val posts =  PostRepo.getPosts() 
    ...//It's the same with the above code
}
android-jetpack-compose android
1 Answers

You need to use remember to prevent recomputation during recomposition.

Your example works without remember because this view will not recompose while you scroll through it.

But if you use animations, add state variables or use a view model, your view can be recomposed many times(when animating up to once a frame), in which case getting data from the repository will be repeated many times, so you need to use remember to save the result of the computation between recompositions.

So always use remember inside a view builder if the calculations are at least a little heavy, even if right now it looks like the view is not gonna be recomposed.

You can read more about the state in compose in documentation, including this youtube video, which explains the basic principles.

user_35857960
Related