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Using the RecyclerView

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Christopher Clement edited this page Nov 13, 2021 · 210 revisions

Overviews

The RecyclerView is a ViewGroup that renders any adapter-based view in a similar way. It is supposed to be the successor of ListView and GridView. One of the reasons is that RecyclerView has a more extensible framework, especially since it provides the ability to implement both horizontal and vertical layouts. Use the RecyclerView widget when you have data collections whose elements change at runtime based on user action or network events.

If you want to use a RecyclerView, you will need to work with the following:

  • RecyclerView.Adapter - To handle the data collection and bind it to the view
  • LayoutManager - Helps in positioning the items
  • ItemAnimator - Helps with animating the items for common operations such as Addition or Removal of item
RecyclerView

Furthermore, it provides animation support for RecyclerView items whenever they are added or removed, which had been extremely difficult to do with ListView. RecyclerView also begins to enforce the ViewHolder pattern too, which was already a recommended practice but now deeply integrated with this new framework.

For more details, see this detailed overview.

Compared to ListView

RecyclerView differs from its predecessor ListView primarily because of the following features:

  • Required ViewHolder in Adapters - ListView adapters do not require the use of the ViewHolder pattern to improve performance. In contrast, implementing an adapter for RecyclerView requires the use of the ViewHolder pattern for which it uses RecyclerView.Viewholder.
  • Customizable Item Layouts - ListView can only layout items in a vertical linear arrangement and this cannot be customized. In contrast, the RecyclerView has a RecyclerView.LayoutManager that allows any item layouts including horizontal lists or staggered grids.
  • Easy Item Animations - ListView contains no special provisions through which one can animate the addition or deletion of items. In contrast, the RecyclerView has the RecyclerView.ItemAnimator class for handling item animations.
  • Manual Data Source - ListView had adapters for different sources such as ArrayAdapter and CursorAdapter for arrays and database results respectively. In contrast, the RecyclerView.Adapter requires a custom implementation to supply the data to the adapter.
  • Manual Item Decoration - ListView has the android:divider property for easy dividers between items in the list. In contrast, RecyclerView requires the use of a RecyclerView.ItemDecoration object to setup much more manual divider decorations.
  • Manual Click Detection - ListView has a AdapterView.OnItemClickListener interface for binding to the click events for individual items in the list. In contrast, RecyclerView only has support for RecyclerView.OnItemTouchListener which manages individual touch events but has no built-in click handling.

Components of a RecyclerView

LayoutManagers

A RecyclerView needs to have a layout manager and an adapter to be instantiated. A layout manager positions item views inside a RecyclerView and determines when to reuse item views that are no longer visible to the user.

RecyclerView provides these built-in layout managers:

  • LinearLayoutManager shows items in a vertical or horizontal scrolling list.
  • GridLayoutManager shows items in a grid.
  • StaggeredGridLayoutManager shows items in a staggered grid.

To create a custom layout manager, extend the RecyclerView.LayoutManager class.

Here is Dave Smith's talk on the custom layout manager

Notes: In the recent version of the Support Library, if you don't explicitly set the LayoutManager, the RecyclerView will not show! There is a Logcat error though E/RecyclerView: No layout manager attached; skipping layout

RecyclerView.Adapter

RecyclerView includes a new kind of adapter. It’s a similar approach to the ones you already used, but with some peculiarities, such as a required ViewHolder. You will have to override two main methods: one to inflate the view and its view holder, and another one to bind data to the view. The good thing about this is that the first method is called only when we really need to create a new view. No need to check if it’s being recycled.

ItemAnimator

RecyclerView.ItemAnimator will animate ViewGroup modifications such as add/delete/select that are notified to the adapter. DefaultItemAnimator can be used for basic default animations and works quite well. See the section of this guide for more information.

Using the RecyclerView

Using a RecyclerView has the following key steps:

  1. Define a model class to use as the data source
  2. Add a RecyclerView to your activity to display the items
  3. Create a custom row layout XML file to visualize the item
  4. Create a RecyclerView.Adapter and ViewHolder to render the item
  5. Bind the adapter to the data source to populate the RecyclerView

The steps are explained in more detail below.

Defining a Model

Every RecyclerView is backed by a source for data. In this case, we will define a Contact class which represents the data model being displayed by the RecyclerView:

public class Contact {
    private String mName;
    private boolean mOnline;

    public Contact(String name, boolean online) {
        mName = name;
        mOnline = online;
    }

    public String getName() {
        return mName;
    }

    public boolean isOnline() {
        return mOnline;
    }

    private static int lastContactId = 0;

    public static ArrayList<Contact> createContactsList(int numContacts) {
        ArrayList<Contact> contacts = new ArrayList<Contact>();

        for (int i = 1; i <= numContacts; i++) {
            contacts.add(new Contact("Person " + ++lastContactId, i <= numContacts / 2));
        }

        return contacts;
    }
}
class Contact(val name: String, val isOnline: Boolean) {

  companion object {
    private var lastContactId = 0
    fun createContactsList(numContacts: Int) : ArrayList<Contact> {
      val contacts = ArrayList<Contact>()
      for (i in 1..numContacts) {
        contacts.add(Contact("Person " + ++lastContactId, i <= numContacts / 2))
      }
      return contacts
  }
}

Create the RecyclerView within Layout

Inside the desired activity layout XML file in res/layout/activity_users.xml, let's add the RecyclerView from the support library:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <androidx.recyclerview.widget.RecyclerView
        android:id="@+id/rvContacts"
        android:layout_width="0dp"
        android:layout_height="0dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

In the layout, preview we can see the RecyclerView within the activity:

Now the RecyclerView is embedded within our activity layout file. Next, we can define the layout for each item within our list.

Creating the Custom Row Layout

Before we create the adapter, let's define the XML layout file that will be used for each row within the list. This item layout for now should contain a horizontal linear layout with a textview for the name and a button to message the person:

This layout file can be created in res/layout/item_contact.xml and will be rendered for each item row. Note that you should be using wrap_content for the layout_height. See this link for more context.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:orientation="horizontal"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:paddingTop="10dp"
        android:paddingBottom="10dp"
        >

    <TextView
        android:id="@+id/contact_name"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_weight="1"
        />

    <Button
        android:id="@+id/message_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:paddingLeft="16dp"
        android:paddingRight="16dp"
        android:textSize="10sp"
        />
</LinearLayout>

With the custom item layout complete, let's create the adapter to populate the data into the recycler view.

Creating the RecyclerView.Adapter

Here we need to create the adapter which will actually populate the data into the RecyclerView. The adapter's role is to convert an object at a position into a list row item to be inserted.

However, with a RecyclerView the adapter requires the existence of a "ViewHolder" object which describes and provides access to all the views within each item row. We can create the basic empty adapter and holder together in ContactsAdapter.java as follows:

// Create the basic adapter extending from RecyclerView.Adapter
// Note that we specify the custom ViewHolder which gives us access to our views
public class ContactsAdapter extends
    RecyclerView.Adapter<ContactsAdapter.ViewHolder> {

    // Provide a direct reference to each of the views within a data item
    // Used to cache the views within the item layout for fast access
    public class ViewHolder extends RecyclerView.ViewHolder {
        // Your holder should contain a member variable
        // for any view that will be set as you render a row
        public TextView nameTextView;
        public Button messageButton;

        // We also create a constructor that accepts the entire item row
        // and does the view lookups to find each subview
        public ViewHolder(View itemView) {
            // Stores the itemView in a public final member variable that can be used
            // to access the context from any ViewHolder instance.
            super(itemView);

            nameTextView = (TextView) itemView.findViewById(R.id.contact_name);
            messageButton = (Button) itemView.findViewById(R.id.message_button);
        }
    }
}
// Create the basic adapter extending from RecyclerView.Adapter
// Note that we specify the custom ViewHolder which gives us access to our views
class ContactsAdapter : RecyclerView.Adapter<ContactsAdapter.ViewHolder>() {
        
    // Provide a direct reference to each of the views within a data item
    // Used to cache the views within the item layout for fast access
    inner class ViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
        // Your holder should contain and initialize a member variable
        // for any view that will be set as you render a row
        val nameTextView = itemView.findViewById<TextView>(R.id.contact_name)
        val messageButton = itemView.findViewById<Button>(R.id.message_button)
    }
}

Now that we've defined the basic adapter and ViewHolder, we need to begin filling in our adapter. First, let's store a member variable for the list of contacts and pass the list in through our constructor:

public class ContactsAdapter extends
    RecyclerView.Adapter<ContactsAdapter.ViewHolder> {

    // ... view holder defined above...

    // Store a member variable for the contacts
    private List<Contact> mContacts;

    // Pass in the contact array into the constructor
    public ContactsAdapter(List<Contact> contacts) {
        mContacts = contacts;
    }
}
class ContactsAdapter (private val mContacts: List<Contact>) : RecyclerView.Adapter<ContactsAdapter.ViewHolder>() {
}

Every adapter has three primary methods: onCreateViewHolder to inflate the item layout and create the holder, onBindViewHolder to set the view attributes based on the data and getItemCount to determine the number of items. We need to implement all three to finish the adapter:

public class ContactsAdapter extends
    RecyclerView.Adapter<ContactsAdapter.ViewHolder> {

    // ... constructor and member variables

    // Usually involves inflating a layout from XML and returning the holder
    @Override
    public ContactsAdapter.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        Context context = parent.getContext();
        LayoutInflater inflater = LayoutInflater.from(context);

        // Inflate the custom layout
        View contactView = inflater.inflate(R.layout.item_contact, parent, false);

        // Return a new holder instance
        ViewHolder viewHolder = new ViewHolder(contactView);
        return viewHolder;
    }

    // Involves populating data into the item through holder
    @Override
    public void onBindViewHolder(ContactsAdapter.ViewHolder holder, int position) {
        // Get the data model based on position
        Contact contact = mContacts.get(position);

        // Set item views based on your views and data model
        TextView textView = holder.nameTextView;
        textView.setText(contact.getName());
        Button button = holder.messageButton;
        button.setText(contact.isOnline() ? "Message" : "Offline");
        button.setEnabled(contact.isOnline());
    }

    // Returns the total count of items in the list
    @Override
    public int getItemCount() {
        return mContacts.size();
    }
}
class ContactsAdapter (private val mContacts: List<Contact>) : RecyclerView.Adapter<ContactsAdapter.ViewHolder>()
{
    // Provide a direct reference to each of the views within a data item
    // Used to cache the views within the item layout for fast access
    inner class ViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView) {
        // Your holder should contain and initialize a member variable
        // for any view that will be set as you render a row
        val nameTextView = itemView.findViewById<TextView>(R.id.contact_name)
        val messageButton = itemView.findViewById<Button>(R.id.message_button)
    }

    // ... constructor and member variables
    // Usually involves inflating a layout from XML and returning the holder
    override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ContactsAdapter.ViewHolder {
        val context = parent.context
        val inflater = LayoutInflater.from(context)
        // Inflate the custom layout
        val contactView = inflater.inflate(R.layout.item_contact, parent, false)
        // Return a new holder instance
        return ViewHolder(contactView)
    }

    // Involves populating data into the item through holder
    override fun onBindViewHolder(viewHolder: ContactsAdapter.ViewHolder, position: Int) { 
        // Get the data model based on position
        val contact: Contact = mContacts.get(position)
        // Set item views based on your views and data model
        val textView = viewHolder.nameTextView
        textView.setText(contact.name)
        val button = viewHolder.messageButton
        button.text = if (contact.isOnline) "Message" else "Offline"
        button.isEnabled = contact.isOnline
    }

    // Returns the total count of items in the list
    override fun getItemCount(): Int {
        return mContacts.size
    }
}

With the adapter completed, all that is remaining is to bind the data from the adapter into the RecyclerView.

Binding the Adapter to the RecyclerView

In our activity, we will populate a set of sample users which should be displayed in the RecyclerView.

public class UserListActivity extends AppCompatActivity {

     ArrayList<Contact> contacts;

     @Override
     protected void onCreate(Bundle savedInstanceState) {
         // ...
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_users);

         // Lookup the recyclerview in activity layout
         RecyclerView rvContacts = (RecyclerView) findViewById(R.id.rvContacts);

         // Initialize contacts
         contacts = Contact.createContactsList(20);
         // Create adapter passing in the sample user data
         ContactsAdapter adapter = new ContactsAdapter(contacts);
         // Attach the adapter to the recyclerview to populate items
         rvContacts.setAdapter(adapter);
         // Set layout manager to position the items
         rvContacts.setLayoutManager(new LinearLayoutManager(this));
         // That's all!
     }
}
class UserListActivity : AppCompatActivity() {
    lateinit var contacts: ArrayList<Contact>
    override fun onCreate(savedInstanceState: Bundle?) { 
        // ...
        // Lookup the recyclerview in activity layout
        val rvContacts = findViewById<View>(R.id.rvContacts) as RecyclerView
        // Initialize contacts
        contacts = Contact.createContactsList(20)
        // Create adapter passing in the sample user data
        val adapter = ContactsAdapter(contacts)
        // Attach the adapter to the recyclerview to populate items
        rvContacts.adapter = adapter
        // Set layout manager to position the items
        rvContacts.layoutManager = LinearLayoutManager(this)
        // That's all!
    }
}

Finally, compile and run the app and you should see something like the screenshot below. If you create enough items and scroll through the list, the views will be recycled and far smoother by default than the ListView widget:

Screenshot

Notifying the Adapter

Unlike ListView, there is no way to add or remove items directly through the RecyclerView adapter. You need to make changes to the data source directly and notify the adapter of any changes. Also, whenever adding or removing elements, always make changes to the existing list. For instance, reinitializing the list of Contacts such as the following will not affect the adapter, since it has a memory reference to the old list:

// do not reinitialize an existing reference used by an adapter
contacts = Contact.createContactsList(5);
// do not reinitialize an existing reference used by an adapter
contacts = Contact.createContactsList(5)

Instead, you need to act directly on the existing reference:

// add to the existing list
contacts.addAll(Contact.createContactsList(5));
// add to the existing list
contacts.addAll(Contact.createContactsList(5))

There are many methods available to use when notifying the adapter of different changes:

Method Description
notifyItemChanged(int pos) Notify that item at the position has changed.
notifyItemInserted(int pos) Notify that item reflected at the position has been newly inserted.
notifyItemRemoved(int pos) Notify that items previously located at the position have been removed from the data set.
notifyDataSetChanged() Notify that the dataset has changed. Use only as last resort.

We can use these from the activity or fragment:

// Add a new contact
contacts.add(0, new Contact("Barney", true));
// Notify the adapter that an item was inserted at position 0
adapter.notifyItemInserted(0);
// Add a new contact
contacts.add(0, Contact("Barney", true))
// Notify the adapter that an item was inserted at position 0
adapter.notifyItemInserted(0)

Every time we want to add or remove items from the RecyclerView, we will need to explicitly inform the adapter of the event. Unlike the ListView adapter, a RecyclerView adapter should not rely on notifyDataSetChanged() since the more granular actions should be used. See the API documentation for more details.

Also, if you are intending to update an existing list, make sure to get the current count of items before making any changes. For instance, a getItemCount() on the adapter should be called to record the first index that will be changed.

// record this value before making any changes to the existing list
int curSize = adapter.getItemCount();

// replace this line with wherever you get new records
ArrayList<Contact> newItems = Contact.createContactsList(20);

// update the existing list
contacts.addAll(newItems);
// curSize should represent the first element that got added
// newItems.size() represents the itemCount
adapter.notifyItemRangeInserted(curSize, newItems.size());
// record this value before making any changes to the existing list
val curSize = adapter.getItemCount()

// replace this line with wherever you get new records
val newItems = Contact.createContactsList(20)

// update the existing list
contacts.addAll(newItems)
// curSize should represent the first element that got added
// newItems.size() represents the itemCount
adapter.notifyItemRangeInserted(curSize, newItems.size)

Diffing Larger Changes

Often times there are cases when changes to your list are more complex (i.e. sorting an existing list) and it cannot be easily determined whether each row changed. In this cases, you would normally have to call notifyDataSetChanged() on the entire adapter to update the entire screen, which eliminates the ability to perform animation sequences to showcase what changed.

Using with ListAdapter

The ListAdapter class simplifies detecting whether an item was inserted, updated, or deleted. You can find more details in this blog post. Note the blog post refers to Support Library v23 that was replaced with AndroidX library. Use this Migration guide to ensure compatibility with the rest of the examples.

First, change your adapter to inherit from a RecyclerView.Adapter to a ListAdapter.

Change from:

public class ContactsAdapter extends RecyclerView.Adapter<ContactsAdapter.ViewHolder> {
class ContactsAdapter : RecyclerView.Adapter<ContactsAdapter.ViewHolder> {

To:

public class ContactsAdapter extends ListAdapter<Contact, ContactsAdapter.ViewHolder> {
class ContactsAdapter : ListAdapter<Contact, ContactsAdapter.ViewHolder> {

Note that a ListAdapter requires an extra generic parameter, which is the type of data managed by this adapter. We also need to declare an item callback:

public static final DiffUtil.ItemCallback<Contact> DIFF_CALLBACK =
        new DiffUtil.ItemCallback<Contact>() {
            @Override
            public boolean areItemsTheSame(Contact oldItem, Contact newItem) {
                return oldItem.getId() == newItem.getId();
            }
            @Override
            public boolean areContentsTheSame(Contact oldItem, Contact newItem) {
                return (oldItem.getName() == newItem.getName() && oldItem.isOnline() == newItem.isOnline());
            }
        };
companion object {
        val DIFF_CALLBACK: DiffUtil.ItemCallback<Contact> = object : DiffUtil.ItemCallback<Contact>() {
            override fun areItemsTheSame(oldItem: Contact, newItem: Contact): Boolean {
                return oldItem.id == newItem.id
            }

            override fun areContentsTheSame(oldItem: Contact, newItem: Contact): Boolean {
                return oldItem.name == newItem.name && oldItem.isOnline == newItem.isOnline
            }
        }
    }

You may notice an error that says "There is no default constructor available in androidx.recyclerview.widget.ListAdapter". The reason is that you will declare an empty constructor and your adapter will also need to invoke this callback method:

public class ContactsAdapter extends ListAdapter<Contact, ContactsAdapter.ViewHolder> {
    public ContactsAdapter() {
        super(DIFF_CALLBACK);
    }
}
class ContactsAdapter : ListAdapter<Contact, ContactsAdapter.ViewHolder>(DIFF_CALLBACK) {

}

Instead of overriding getItemCount(), remove it since the size of the list will be managed by the ListAdapter class:

// remove this section
@Override
public int getItemCount() {
  return mContacts.size();
}
// remove this section
override fun getItemCount(): Int {
    return contacts.size
}

We will also add a helper function to add more contacts. Anytime we wish to add more contacts, will use this method instead. A submitList() function provided by the ListAdapter will trigger the comparison.

public void addMoreContacts(List<Contact> newContacts) {
  mContacts.addAll(newContacts);
  submitList(mContacts); // DiffUtil takes care of the check
}
fun addMoreContacts(newContacts: List<Contact>) {
    contacts.addAll(newContacts)
    submitList(contacts) // DiffUtil takes care of the check
}

Finally, we need to modify the onBindViewHolder to use the getItem() method instead.

Change from:

public void onBindViewHolder(ViewHolder holder, int position) {
   // remove this line
   Contact contact = mContacts.get(position);
override fun onBindViewHolder(holder: ViewHolder, position: Int) {
   // remove this line
   val contact = mContacts.get[position]

To:

public void onBindViewHolder(ViewHolder holder, int position) {
   Contact contact = getItem(position);
override fun onBindViewHolder(holder: ViewHolder, position: Int) {
   val contact = getItem(position)

Using with DiffUtil

The ListAdapter is built on top of the DiffUtil class but requires less boilerplate code. You can see below what the steps are needed to in order to accomplish the same goal. You do not need to follow the steps below if you already using ListAdapter.

The DiffUtil class, which was added in the v24.2.0 of the support library, helps compute the difference between the old and new list. This class uses the same algorithm used for computing line changes in source code (the diff utility program), so it usually fairly fast. It is recommended however for larger lists that you execute this computation in a background thread.

To use the DiffUtil class, you need to first implement a class that implements the DiffUtil.Callback that accepts the old and new list:

public class ContactDiffCallback extends DiffUtil.Callback {

    private List<Contact> mOldList;
    private List<Contact> mNewList;

    public ContactDiffCallback(List<Contact> oldList, List<Contact> newList) {
        this.mOldList = oldList;
        this.mNewList = newList;
    }
    @Override
    public int getOldListSize() {
        return mOldList.size();
    }

    @Override
    public int getNewListSize() {
        return mNewList.size();
    }

    @Override
    public boolean areItemsTheSame(int oldItemPosition, int newItemPosition) {
        // add a unique ID property on Contact and expose a getId() method
        return mOldList.get(oldItemPosition).getId() == mNewList.get(newItemPosition).getId();
    }

    @Override
    public boolean areContentsTheSame(int oldItemPosition, int newItemPosition) {
        Contact oldContact = mOldList.get(oldItemPosition);
        Contact newContact = mNewList.get(newItemPosition);

        return oldContact.getName() == newContact.getName() && oldContact.isOnline() == newContact.isOnline()
    }
}
class ContactDiffCallback(
    private val mOldList: List<Contact>,
    private val mNewList: List<Contact>
) : DiffUtil.Callback() {

    override fun getOldListSize() = mOldList.size

    override fun getNewListSize() = mNewList.size

    override fun areItemsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean {
        // add a unique ID property on Contact and expose a getId() method
        return mOldList[oldItemPosition].id == mNewList[newItemPosition].id
    }

    override fun areContentsTheSame(oldItemPosition: Int, newItemPosition: Int): Boolean {
        val oldContact = mOldList[oldItemPosition]
        val newContact = mNewList[newItemPosition]

        return oldContact.name == newContact.name && oldContact.isOnline == newContact.isOnline
    }

}

Next, you would implement a swapItems() method on your adapter to perform the diff and then invoke dispatchUpdates() to notify the adapter whether the element was inserted, removed, moved, or changed:

public class ContactsAdapter extends
        RecyclerView.Adapter<ContactsAdapter.ViewHolder> {

  public void swapItems(List<Contact> contacts) {
        // compute diffs
        final ContactDiffCallback diffCallback = new ContactDiffCallback(this.mContacts, contacts);
        final DiffUtil.DiffResult diffResult = DiffUtil.calculateDiff(diffCallback);

        // clear contacts and add
        this.mContacts.clear();
        this.mContacts.addAll(contacts);

        diffResult.dispatchUpdatesTo(this); // calls adapter's notify methods after diff is computed
    }
}
class ContactsAdapter(contacts: List<Contact>) : RecyclerView.Adapter<ContactsAdapter.ViewHolder>() {
    
    fun swapItems(contacts: List<Contact>) {
        // compute diffs
        val diffCallback = ContactDiffCallback(this.contacts, contacts)
        val diffResult = DiffUtil.calculateDiff(diffCallback)

        // clear contacts and add
        this.contacts.clear()
        this.contacts.addAll(contacts)

        diffResult.dispatchUpdatesTo(this) // calls adapter's notify methods after diff is computed
    }
}

For a working example, see this sample code.

Scrolling to New Items

If we are inserting elements to the front of the list and wish to maintain the position at the top, we can set the scroll position to the 1st element:

adapter.notifyItemInserted(0);
rvContacts.scrollToPosition(0);   // index 0 position
adapter.notifyItemInserted(0)
rvContacts.scrollToPosition(0)   // index 0 position

If we are adding items to the end and wish to scroll to the bottom as items are added, we can notify the adapter that an additional element has been added and can call smoothScrollToPosition() on the RecyclerView:

adapter.notifyItemInserted(contacts.size() - 1);  // contacts.size() - 1 is the last element position
rvContacts.scrollToPosition(mAdapter.getItemCount() - 1); // update based on adapter
adapter.notifyItemInserted(contacts.size() - 1)  // contacts.size() - 1 is the last element position
rvContacts.scrollToPosition(mAdapter.getItemCount() - 1) // update based on adapter

Implementing Endless Scrolling

To implement fetching more data and appending to the end of the list as the user scrolls towards the bottom, use the addOnScrollListener() from the RecyclerView and add an onLoadMore method leveraging the EndlessScrollViewScrollListener document in the guide.

Configuring the RecyclerView

The RecyclerView is quite flexible and customizable. Several of the options available are shown below.

Performance

We can also enable optimizations if the items are static and will not change for significantly smoother scrolling:

recyclerView.setHasFixedSize(true);
recyclerView.setHasFixedSize(true)

Layouts

The positioning of the items is configured using the layout manager. By default, we can choose between LinearLayoutManager, GridLayoutManager, and StaggeredGridLayoutManager. Linear displays items either vertically or horizontally:

// Setup layout manager for items with orientation
// Also supports `LinearLayoutManager.HORIZONTAL`
LinearLayoutManager layoutManager = new LinearLayoutManager(this, LinearLayoutManager.VERTICAL, false);
// Optionally customize the position you want to default scroll to
layoutManager.scrollToPosition(0);
// Attach layout manager to the RecyclerView
recyclerView.setLayoutManager(layoutManager);
// Setup layout manager for items with orientation
// Also supports `LinearLayoutManager.HORIZONTAL`
val layoutManager = LinearLayoutManager(this, LinearLayoutManager.VERTICAL, false)
// Optionally customize the position you want to default scroll to
layoutManager.scrollToPosition(0)
// Attach layout manager to the RecyclerView
recyclerView.layoutManager = layoutManager

Displaying items in a grid or staggered grid works similarly:

// First param is number of columns and second param is orientation i.e Vertical or Horizontal
StaggeredGridLayoutManager gridLayoutManager =
    new StaggeredGridLayoutManager(2, StaggeredGridLayoutManager.VERTICAL);
// Attach the layout manager to the recycler view
recyclerView.setLayoutManager(gridLayoutManager);
// First param is number of columns and second param is orientation i.e Vertical or Horizontal
val gridLayoutManager = 
    StaggeredGridLayoutManager(2, StaggeredGridLayoutManager.VERTICAL)
// Attach the layout manager to the recycler view
recyclerView.layoutManager = gridLayoutManager

For example, a staggered grid might look like:

Screenshot

We can build our own custom layout managers as outlined there.

Decorations

We can decorate the items using various decorators attached to the recyclerview such as the DividerItemDecoration:

RecyclerView.ItemDecoration itemDecoration = new
    DividerItemDecoration(this, DividerItemDecoration.VERTICAL);
recyclerView.addItemDecoration(itemDecoration);
val itemDecoration: ItemDecoration =
    DividerItemDecoration(this, DividerItemDecoration.VERTICAL)
recyclerView.addItemDecoration(itemDecoration)

This decorator displays dividers between each item within the list as illustrated below:

Screenshot

Grid Spacing Decorations

Decorators can also be used for adding consistent spacing around items displayed in a grid layout or staggered grid. Copy over this SpacesItemDecoration.java decorator into your project and apply to a RecyclerView using the addItemDecoration method. Refer to this staggered grid tutorial for a more detailed outline.

Animators

RecyclerView supports custom animations for items as they enter, move, or get deleted using ItemAnimator. The default animation effects is defined by DefaultItemAnimator, and the complex implementation (see source code) shows that the logic necessary to ensure that animation effects are performed in a specific sequence (remove, move, and add).

Currently, the fastest way to implement animations with RecyclerView is to use third-party libraries. The third-party recyclerview-animators library contains a lot of animations that you can use without needing to build your own. Simply edit your app/build.gradle:

repositories {
    mavenCentral()
}

dependencies {
    implementation 'jp.wasabeef:recyclerview-animators:4.0.2'
}

Next, we can use any of the defined animators to change the behavior of our RecyclerView:

recyclerView.setItemAnimator(new SlideInUpAnimator());
recyclerView.itemAnimator = SlideInUpAnimator()

For example, here's scrolling through a list after customizing the animation:

Screenshot

For a further look into defining custom item animators, check out this custom RecyclerView item animation post.

New ItemAnimator interface

There is also a new interface for the ItemAnimator interface. The old interface has now been deprecated to SimpleItemAnimator . This library adds a ItemHolderInfo class, which appears to be similar to the MoveInfo class defined by DefaultItemAnimator but used more generically to pass state information between animation transition states. It is likely that the next version of DefaultItemAnimator will be simplified to use this new class and revised interface.

Heterogeneous Views

See this guide if you want to inflate multiple types of rows inside a single RecyclerView:

ss2

This is useful for feeds which contain various different types of items within a single list.

Handling Touch Events

RecyclerView allows us to handle touch events with:

recyclerView.addOnItemTouchListener(new RecyclerView.OnItemTouchListener() {
    
    @Override
    public boolean onInterceptTouchEvent(RecyclerView recycler, MotionEvent event) {
        return false;
    }

    @Override
    public void onTouchEvent(RecyclerView recycler, MotionEvent event) {
        // Handle on touch events here
    }

    @Override
    public void onRequestDisallowInterceptTouchEvent(boolean disallowIntercept) {

    }
});
recyclerView.addOnItemTouchListener(object : OnItemTouchListener {
    override fun onInterceptTouchEvent(rv: RecyclerView, e: MotionEvent): Boolean {
        return false
    }

    override fun onTouchEvent(rv: RecyclerView, e: MotionEvent) {
        // Handle on touch events here
    }

   override fun onRequestDisallowInterceptTouchEvent(disallowIntercept: Boolean) {
        
   }
})

Snap to Center Effect

In certain cases, we might want a horizontal RecyclerView that allows the user to scroll through a list of items. As the user scrolls, we might want items to "snap to center" as they are revealed. Such as in this example:

LinearSnapHelper

To achieve this snapping to center effect as the user scrolls we can use the built-in LinearSnapHelper as follows:

SnapHelper snapHelper = new LinearSnapHelper();
snapHelper.attachToRecyclerView(recyclerView);
val snapHelper = LinearSnapHelper()
snapHelper.attachToRecyclerView(recyclerView)

For more sophisticated snapping behavior, read more about customizing these helpers and review related sample code here.

SnappyRecyclerView

For a more manual approach, we can create a custom extension to RecyclerView called SnappyRecyclerView which will snap items to center as the user scrolls:

  1. Copy over the code from SnappyRecyclerView.java to your project.

  2. Configure your new SnappyRecyclerView with a horizontal LinearLayoutManager:

    LinearLayoutManager layoutManager = new LinearLayoutManager(this, LinearLayoutManager.HORIZONTAL, false);
snappyRecyclerView.setLayoutManager(layoutManager);
    val layoutManager = LinearLayoutManager(this, LinearLayoutManager.HORIZONTAL, false)
snappyRecyclerView.layoutManager = layoutManager

  3. Attach your adapter to the RecyclerView to populate the data into the horizontal list as normal.

  4. You can access the currently "snapped" item position with snappyRecyclerView.getFirstVisibleItemPosition().

That's all, you should be set for a snap-to-center horizontal scrolling list!

Attaching Click Handlers to Items

If you'd like to perform an action whenever a user clicks on any item in your RecyclerView, you'll need to perform that action within a handler.

Below are three ways you can attach a handler to listen to clicks on a RecyclerView. Note that this can be used to recognize clicks on items, but not for recognizing clicks on individual buttons or other elements within your items...

Attaching Click Listeners with Decorators

The easiest solution for handling a click on an item in a RecyclerView is to add a decorator class such as this clever ItemClickSupport decorator and then implement the following code in your Activity or Fragment code:

public class PostsFragment extends Fragment {
    // ...

    @Override
    public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
        super.onViewCreated(view, savedInstanceState);

        // Leveraging ItemClickSupport decorator to handle clicks on items in our recyclerView
        ItemClickSupport.addTo(rvPosts).setOnItemClickListener(
                new ItemClickSupport.OnItemClickListener() {
                    @Override
                    public void onItemClicked(RecyclerView recyclerView, int position, View v) {
                        // do stuff
                    }
                }
        );

    }

    // ...
}
class PostsFragment : Fragment() {
    // ...

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)

        // Leveraging ItemClickSupport decorator to handle clicks on items in our recyclerView
        ItemClickSupport.addTo(rvPosts).setOnItemClickListener { recyclerView, position, v ->
            // do stuff
        }
    }

    // ...
}

This technique was originally outlined in this article. Under the covers, this is wrapping the interface pattern described in detail below.

So, if you apply this code above, you do not need the Simple Click Handler within ViewHolder described below.

Simple Click Handler within ViewHolder

Another solution for setting up item click handlers within a RecyclerView is to add code to your Adapter instead...

Unlike ListView which has the setOnItemClickListener method, RecyclerView does not have special provisions for attaching click handlers to items. So, to achieve a similar effect manually (instead of using the decorator utility above), we can attach click events within the ViewHolder inside our adapter:

public class ContactsAdapter extends RecyclerView.Adapter<ContactsAdapter.ViewHolder> {
    // ...

    // Used to cache the views within the item layout for fast access
    public class ViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener {
        public TextView tvName;
        public TextView tvHometown;
        private Context context;

        public ViewHolder(Context context, View itemView) {
            super(itemView);
            this.tvName = (TextView) itemView.findViewById(R.id.tvName);
            this.tvHometown = (TextView) itemView.findViewById(R.id.tvHometown);
            // Store the context
            this.context = context;
            // Attach a click listener to the entire row view
            itemView.setOnClickListener(this);
        }

        // Handles the row being being clicked
        @Override
        public void onClick(View view) {
            int position = getAbsoluteAdapterPosition(); // gets item position
            if (position != RecyclerView.NO_POSITION) { // Check if an item was deleted, but the user clicked it before the UI removed it
                User user = users.get(position);
                // We can access the data within the views
                Toast.makeText(context, tvName.getText(), Toast.LENGTH_SHORT).show();
            }
        }
    }

    // ...
}
class ContactsAdapter : RecyclerView.Adapter<ContactsAdapter.ViewHolder>() {
    // ...

    // Used to cache the views within the item layout for fast access
    // Store the context
    class ViewHolder(private val context: Context, view: View) : RecyclerView.ViewHolder(view), View.OnClickListener {
        val tvName: TextView = itemView.findViewById(R.id.tvName)
        val tvHometown: TextView = itemView.findViewById(R.id.tvHometown)

        init {
            // Attach a click listener to the entire row view
            itemView.setOnClickListener(this)
        }

        // Handles the row being being clicked
        override fun onClick(v: View?) {
            val position = absoluteAdapterPosition // gets item position
            if (position != RecyclerView.NO_POSITION) { // Check if an item was deleted, but the user clicked it before the UI removed it
                val user = users[position]
                // We can access the data within the views
                Toast.makeText(context, tvName.text, Toast.LENGTH_SHORT).show()
            }
        }
    }

    // ...
}

If we want the item to show a "selected" effect when pressed, we can set the android:background of the root layout for the row to ?android:attr/selectableItemBackground:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="horizontal" android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="?android:attr/selectableItemBackground">
  <!-- ... -->
</LinearLayout>

This creates the following effect:

Screenshot

Attaching Click Handlers using Listeners

In certain cases, you'd want to setup click handlers for views within the RecyclerView but define the click logic within the containing Activity or Fragment (i.e bubble up events from the adapter). To achieve this, create a custom listener within the adapter and then fire the events upwards to an interface implementation defined within the parent:

public class ContactsAdapter extends RecyclerView.Adapter<ContactsAdapter.ViewHolder> {
    // ...

    /***** Creating OnItemClickListener *****/

    // Define listener member variable
    private OnItemClickListener listener;
    // Define the listener interface
    public interface OnItemClickListener {
        void onItemClick(View itemView, int position);
    }
    // Define the method that allows the parent activity or fragment to define the listener
    public void setOnItemClickListener(OnItemClickListener listener) {
        this.listener = listener;
    }

    public class ViewHolder extends RecyclerView.ViewHolder implements View.OnClickListener {
        public TextView tvName;
        public TextView tvHometown;

        public ViewHolder(final View itemView) {
            super(itemView);
            this.tvName = (TextView) itemView.findViewById(R.id.tvName);
            this.tvHometown = (TextView) itemView.findViewById(R.id.tvHometown);
            // Setup the click listener
            itemView.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    // Triggers click upwards to the adapter on click
                    if (listener != null) {
                        int position = getAdapterPosition();
                        if (position != RecyclerView.NO_POSITION) {
                            listener.onItemClick(itemView, position);
                        }
                    }
                }
            });
        }
    }

    // ...
}

Then we can attach a click handler to the adapter with:

// In the activity or fragment
ContactsAdapter adapter = ...;
adapter.setOnItemClickListener(new ContactsAdapter.OnItemClickListener() {
    @Override
    public void onItemClick(View view, int position) {
        String name = users.get(position).name;
        Toast.makeText(UserListActivity.this, name + " was clicked!", Toast.LENGTH_SHORT).show();
    }
});

See this detailed stackoverflow post which describes how to setup item-level click handlers when using RecyclerView.

Implementing Pull to Refresh

The SwipeRefreshLayout should be used to refresh the contents of a RecyclerView via a vertical swipe gesture. See our detailed RecyclerView with SwipeRefreshLayout guide for a step-by-step tutorial on implementing pull to refresh.

Swipe Detection

RecyclerView has an OnFlingListener method that can be used to implement custom fling behavior. Download this RecyclerViewSwipeListener and you can handle custom swipe detection by adding this class to your RecyclerView:

RecyclerView rvMyList;

rvMyList.setOnFlingListener(new RecyclerViewSwipeListener(true) {
   @Override
   public void onSwipeDown() {
      Toast.makeText(MainActivity.this, "swipe down", Toast.LENGTH_SHORT).show();
   }

   @Override
   public void onSwipeUp() {
      Toast.makeText(MainActivity.this, "swipe up", Toast.LENGTH_SHORT).show();
   }
});

Using Uncover library

If your use RecyclerView to implement 'infinite scrolling' over output of the web service, or some other resource that requires slow background calls with multiple items at once, it may be reasonable to try the Uncover library from Maven central:

dependencies {
    compile ('io.github.andviane:uncover:2.0.1@aar')
}

This library requires you to implement the primary data fetcher, mediating between fast single item UI-thread calls on model and slow chunked calls on the background on your fetcher:

    final RecyclerView recyclerView = (RecyclerView) findViewById(R.id.recycler);
    final UncoveringDataModel<String> model = new UncoveringDataModel<>();

    model.setPrimaryDataProvider(new PrimaryDataProvider<String>() {

      @Override
      public PrimaryResponse fetch(PrimaryRequest primaryRequest) {
        Log.i("Fetch", "Service call to fetch items" + 
          primaryRequest.getFrom() + "- " + primaryRequest.getTo());
        ...
        ArrayList<String> data = new ArrayList<String>();
        ...
        return new PrimaryResponse<String>(data, Integer.MAX_VALUE);
      }
    });

    RecyclerView.Adapter adapter = new RecyclerView.Adapter() {
      public int getItemCount() {
        return model.size();
      ...
    };
    model.install(recyclerView, adapter);

The model.install glues model, view and adapter into working implementation. When the user swipes forward quickly, the library skips unneeded fetches of data between the new and old position, last requested data are fetched first, and any pending requests are dropped from the queue if they data are no longer visible.

References

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