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Migrate RecyclerView to Lazy list

 1 year ago
source link: https://developer.android.com/jetpack/compose/migrate/migration-scenarios/recycler-view
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RecyclerView is a View component that makes it easy to efficiently display large sets of data. Instead of creating views for each item in the data set, RecyclerView improves the performance of your app by keeping a small pool of views and recycling through them as you scroll through those items.

In Compose, you can use Lazy lists to accomplish the same thing. This page describes how you can migrate your RecyclerView implementation to use Lazy lists in Compose.

Migration steps

To migrate your RecyclerView implementation to Compose, follow these steps:

  1. Comment out or remove the RecyclerView from your UI hierarchy and add a ComposeView to replace it if none is present in the hierarchy yet. This is the container for the Lazy list that you'll add:

          <FrameLayout
              android:layout_width="match_parent"
              android:layout_height="match_parent">

    <!--    <androidx.recyclerview.widget.RecyclerView-->
      <!--            android:id="@+id/recycler_view"-->
      <!--            android:layout_width="match_parent"-->
      <!--            android:layout_height="match_parent />"-->

    <androidx.compose.ui.platform.ComposeView
                  android:id="@+id/compose_view"
                  android:layout_width="match_parent"
                  android:layout_height="match_parent" />

    </FrameLayout>

  2. Determine what type of Lazy list composable you need based on your RecyclerView’s layout manager (see table below). The composable you select will be the top-level composable of the ComposeView you added in the previous step.

    LayoutManager

    Composable

    LinearLayoutManager

    LazyColumn or LazyRow

    GridLayoutManager

    LazyVerticalGrid or LazyHorizontalGrid

    StaggeredGridLayoutManager

    LazyVerticalStaggeredGrid or LazyHorizontalStaggeredGrid

    		 
    // recyclerView.layoutManager = LinearLayoutManager(context)
    composeView.setContent {
        LazyColumn(Modifier.fillMaxSize()) {
            // We use a LazyColumn since the layout manager of the RecyclerView is a vertical LinearLayoutManager
        }
    }

  3. Create a corresponding composable for each view type in your RecyclerView.Adapter implementation. Each view type typically maps to a ViewHolder subclass, though this may not always be the case. These composables will be used as the UI representation for different types of elements in your list:

    @Composable
    fun ListItem(data: MyData, modifier: Modifier = Modifier) {
        Row(modifier.fillMaxWidth()) {
            Text(text = data.name)
            // … other composables required for displaying `data`
        }
    }

    The logic in your RecyclerView.Adapter’s onCreateViewHolder() and onBindViewHolder() methods will be replaced by these composables and the state that you provide them with. In Compose, there is no separation between creating a composable for an item and binding data into it—these concepts are coalesced.

  4. Within the content slot of the Lazy list (the trailing lambda parameter), use the items() function (or an equivalent overload) to iterate through the data for your list. In the itemContent lambda, invoke the appropriate composable item for your data:

    val data = listOf<MyData>(/* ... */)
    composeView.setContent {
        LazyColumn(Modifier.fillMaxSize()) {
            items(data) {
                ListItem(it)
            }
        }
    }

Tip: Provide additional parameters to items() to optimize your list: use the key parameter to provide a unique key for the underlying data so that scroll position will be maintained when items change, or use the contentType parameter to specify a content type for the underlying data (this is a similar concept to RecyclerView's view types) so you can reuse item compositions more efficiently.

Common use cases

Item decorations

RecyclerView has the concept of an ItemDecoration, which you can use to add a special drawing for items in the list. For example, you can add an ItemDecoration to add dividers between items:

val itemDecoration = DividerItemDecoration(recyclerView.context, LinearLayoutManager.VERTICAL)
recyclerView.addItemDecoration(itemDecoration)

Compose does not have an equivalent concept of item decorations. Instead, you can add any UI decorations in the list directly in the composition. For example, to add dividers to the list, you can use the Divider composable after each item:

LazyColumn(Modifier.fillMaxSize()) {
    itemsIndexed(data) { index, d ->
        ListItem(d)
        if (index != data.size - 1) {
            Divider()
        }
    }
}

Item animations

An ItemAnimator can be set on a RecyclerView to animate the appearance of items as changes are made to the adapter. By default, RecyclerView uses DefaultItemAnimator which provides basic animations on remove, add, and move events.

Lazy lists have a similar concept through the animateItemPlacement modifier. See Item animations to learn more.

Additional resources

For more information about migrating a RecyclerView to Compose, see the following resources:

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