RecyclerView

The RecyclerView class supports the display of a collection of data. You supply the data and define how each item looks, and the RecyclerView library dynamically creates the elements when they’re needed.

RecyclerView recycles those individual elements. When an item scrolls off the screen, RecyclerView doesn’t destroy its view. Instead, RecyclerView reuses the view for new items that have scrolled onscreen. This reuse vastly improves performance, improving your app’s responsiveness and reducing power consumption.

Key classes

Several different classes work together to build your dynamic list.

RecyclerView is the ViewGroup that contains the views corresponding to your data. It’s a view itself, so you add RecyclerView into your layout the way you would add any other UI element.
Each individual element in the list is defined by a view holder object. When the view holder is created, it doesn’t have any data associated with it. After the view holder is created, the RecyclerView binds it to its data. You define the view holder by extending RecyclerView.ViewHolder.
The RecyclerView requests those views, and binds the views to their data, by calling methods in the adapter. You define the adapter by extending RecyclerView.Adapter.
The layout manager arranges the individual elements in your list. You can use one of the layout managers provided by the RecyclerView library, or you can define your own. Layout managers are all based on the library’s LayoutManager abstract class.

Steps for implementing your RecyclerView

Decide what the list or grid is going to look like. Ordinarily you’ll be able to use one of the RecyclerView library’s standard layout managers.
Design how each element in the list is going to look and behave. Based on this design, extend the ViewHolder class. Your version of ViewHolder provides all the functionality for your list items. Your view holder is a wrapper around a View, and that view is managed by RecyclerView.
Define the Adapter that associates your data with the ViewHolder views.

Modifying the layout

The RecyclerView uses a layout manager to position the individual items on the screen and determine when to reuse item views that are no longer visible to the user. To reuse (or recycle) a view, a layout manager may ask the adapter to replace the contents of the view with a different element from the dataset. Recycling views in this manner improves performance by avoiding the creation of unnecessary views or performing expensive findViewById() lookups. The Android Support Library includes three standard layout managers, each of which offers many customization options:

LinearLayoutManager arranges the items in a one-dimensional list. Using a RecyclerView with LinearLayoutManager provides functionality like the older ListView layout.
GridLayoutManager arranges the items in a two-dimensional grid, like the squares on a checkerboard. Using a RecyclerView with GridLayoutManager provides functionality like the older GridView layout.
StaggeredGridLayoutManager arranges the items in a two-dimensional grid, with each column slightly offset from the one before, like the stars in an American flag.

If none of these layout managers suits your needs, you can create your own by extending the RecyclerView.LayoutManager abstract class.

Implementing your adapter and view holder

Once you’ve determined your layout, you need to implement your Adapter and ViewHolder. These two classes work together to define how your data is displayed. The ViewHolder is a wrapper around a View that contains the layout for an individual item in the list. The Adapter creates ViewHolder objects as needed, and also sets the data for those views. The process of associating views to their data is called binding.

When you define your adapter, you need to override three key methods:

onCreateViewHolder(): RecyclerView calls this method whenever it needs to create a new ViewHolder. The method creates and initializes the ViewHolder and its associated View, but does not fill in the view’s contents—the ViewHolder has not yet been bound to specific data.
onBindViewHolder(): RecyclerView calls this method to associate a ViewHolder with data. The method fetches the appropriate data and uses the data to fill in the view holder’s layout. For example, if the RecyclerView dislays a list of names, the method might find the appropriate name in the list and fill in the view holder’s TextView widget.
getItemCount(): RecyclerView calls this method to get the size of the data set. For example, in an address book app, this might be the total number of addresses. RecyclerView uses this to determine when there are no more items that can be displayed.

Here’s a typical example of a simple adapter with a nested ViewHolder that displays a list of data (Kotlin):

class CustomAdapter(private val dataSet: Array<String>) :
        RecyclerView.Adapter<CustomAdapter.ViewHolder>() {

    /**
     * Provide a reference to the type of views that you are using
     * (custom ViewHolder).
     */
    class ViewHolder(view: View) : RecyclerView.ViewHolder(view) {
        val textView: TextView

        init {
            // Define click listener for the ViewHolder's View.
            textView = view.findViewById(R.id.textView)
        }
    }

    // Create new views (invoked by the layout manager)
    override fun onCreateViewHolder(viewGroup: ViewGroup, viewType: Int): ViewHolder {
        // Create a new view, which defines the UI of the list item
        val view = LayoutInflater.from(viewGroup.context)
                .inflate(R.layout.text_row_item, viewGroup, false)

        return ViewHolder(view)
    }

    // Replace the contents of a view (invoked by the layout manager)
    override fun onBindViewHolder(viewHolder: ViewHolder, position: Int) {

        // Get element from your dataset at this position and replace the
        // contents of the view with that element
        viewHolder.textView.text = dataSet[position]
    }

    // Return the size of your dataset (invoked by the layout manager)
    override fun getItemCount() = dataSet.size

}

The layout for the each view item is defined in an XML layout file, as usual. In this case, the app has a text_row_item.xml file like this:

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="@dimen/list_item_height"
    android:layout_marginLeft="@dimen/margin_medium"
    android:layout_marginRight="@dimen/margin_medium"
    android:gravity="center_vertical">

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/element_text"/>
</FrameLayout>

Links

https://developer.android.com/guide/topics/ui/layout/recyclerview
https://developer.android.com/guide/topics/ui/layout/recyclerview-custom

Next questions

How does RecyclerView differ from ListView

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