Android Dev Summit, October 23-24: two days of technical content, directly from the Android team. Sign-up for livestream updates.

AsyncListDiffer

public class AsyncListDiffer
extends Object

java.lang.Object
   ↳ androidx.recyclerview.widget.AsyncListDiffer<T>


Helper for computing the difference between two lists via DiffUtil on a background thread.

It can be connected to a RecyclerView.Adapter, and will signal the adapter of changes between sumbitted lists.

For simplicity, the ListAdapter wrapper class can often be used instead of the AsyncListDiffer directly. This AsyncListDiffer can be used for complex cases, where overriding an adapter base class to support asynchronous List diffing isn't convenient.

The AsyncListDiffer can consume the values from a LiveData of List and present the data simply for an adapter. It computes differences in list contents via DiffUtil on a background thread as new Lists are received.

Use getCurrentList() to access the current List, and present its data objects. Diff results will be dispatched to the ListUpdateCallback immediately before the current list is updated. If you're dispatching list updates directly to an Adapter, this means the Adapter can safely access list items and total size via getCurrentList().

A complete usage pattern with Room would look like this:

 @Dao
 interface UserDao {
     @Query("SELECT * FROM user ORDER BY lastName ASC")
     public abstract LiveData<List<User>> usersByLastName();
 }

 class MyViewModel extends ViewModel {
     public final LiveData<List<User>> usersList;
     public MyViewModel(UserDao userDao) {
         usersList = userDao.usersByLastName();
     }
 }

 class MyActivity extends AppCompatActivity {
     @Override
     public void onCreate(Bundle savedState) {
         super.onCreate(savedState);
         MyViewModel viewModel = new ViewModelProvider(this).get(MyViewModel.class);
         RecyclerView recyclerView = findViewById(R.id.user_list);
         UserAdapter adapter = new UserAdapter();
         viewModel.usersList.observe(this, list -> adapter.submitList(list));
         recyclerView.setAdapter(adapter);
     }
 }

 class UserAdapter extends RecyclerView.Adapter<UserViewHolder> {
     private final AsyncListDiffer<User> mDiffer = new AsyncListDiffer(this, DIFF_CALLBACK);
     @Override
     public int getItemCount() {
         return mDiffer.getCurrentList().size();
     }
     public void submitList(List<User> list) {
         mDiffer.submitList(list);
     }
     @Override
     public void onBindViewHolder(UserViewHolder holder, int position) {
         User user = mDiffer.getCurrentList().get(position);
         holder.bindTo(user);
     }
     public static final DiffUtil.ItemCallback<User> DIFF_CALLBACK
             = new DiffUtil.ItemCallback<User>() {
         @Override
         public boolean areItemsTheSame(
                 @NonNull User oldUser, @NonNull User newUser) {
             // User properties may have changed if reloaded from the DB, but ID is fixed
             return oldUser.getId() == newUser.getId();
         }
         @Override
         public boolean areContentsTheSame(
                 @NonNull User oldUser, @NonNull User newUser) {
             // NOTE: if you use equals, your object must properly override Object#equals()
             // Incorrectly returning false here will result in too many animations.
             return oldUser.equals(newUser);
         }
     }
 }

Summary

Nested classes

interface AsyncListDiffer.ListListener<T>

Listener for when the current List is updated. 

Public constructors

AsyncListDiffer(Adapter adapter, ItemCallback<T> diffCallback)

Convenience for AsyncListDiffer(new AdapterListUpdateCallback(adapter), new AsyncDifferConfig.Builder().setDiffCallback(diffCallback).build());

AsyncListDiffer(ListUpdateCallback listUpdateCallback, AsyncDifferConfig<T> config)

Create a AsyncListDiffer with the provided config, and ListUpdateCallback to dispatch updates to.

Public methods

void addListListener(ListListener<T> listener)

Add a ListListener to receive updates when the current List changes.

List<T> getCurrentList()

Get the current List - any diffing to present this list has already been computed and dispatched via the ListUpdateCallback.

void removeListListener(ListListener<T> listener)

Remove a previously registered ListListener.

void submitList(List<T> newList)

Pass a new List to the AdapterHelper.

void submitList(List<T> newList, Runnable commitCallback)

Pass a new List to the AdapterHelper.

Inherited methods

Public constructors

AsyncListDiffer

public AsyncListDiffer (Adapter adapter, 
                ItemCallback<T> diffCallback)

Convenience for AsyncListDiffer(new AdapterListUpdateCallback(adapter), new AsyncDifferConfig.Builder().setDiffCallback(diffCallback).build());

Parameters
adapter Adapter: Adapter to dispatch position updates to.

diffCallback ItemCallback: ItemCallback that compares items to dispatch appropriate animations when

AsyncListDiffer

public AsyncListDiffer (ListUpdateCallback listUpdateCallback, 
                AsyncDifferConfig<T> config)

Create a AsyncListDiffer with the provided config, and ListUpdateCallback to dispatch updates to.

Parameters
listUpdateCallback ListUpdateCallback: Callback to dispatch updates to.

config AsyncDifferConfig: Config to define background work Executor, and DiffUtil.ItemCallback for computing List diffs.

Public methods

addListListener

public void addListListener (ListListener<T> listener)

Add a ListListener to receive updates when the current List changes.

Parameters
listener ListListener: Listener to receive updates.

getCurrentList

public List<T> getCurrentList ()

Get the current List - any diffing to present this list has already been computed and dispatched via the ListUpdateCallback.

If a null List, or no List has been submitted, an empty list will be returned.

The returned list may not be mutated - mutations to content must be done through submitList(List).

Returns
List<T> current List.

removeListListener

public void removeListListener (ListListener<T> listener)

Remove a previously registered ListListener.

Parameters
listener ListListener: Previously registered listener.

submitList

public void submitList (List<T> newList)

Pass a new List to the AdapterHelper. Adapter updates will be computed on a background thread.

If a List is already present, a diff will be computed asynchronously on a background thread. When the diff is computed, it will be applied (dispatched to the ListUpdateCallback), and the new List will be swapped in.

Parameters
newList List: The new List.

submitList

public void submitList (List<T> newList, 
                Runnable commitCallback)

Pass a new List to the AdapterHelper. Adapter updates will be computed on a background thread.

If a List is already present, a diff will be computed asynchronously on a background thread. When the diff is computed, it will be applied (dispatched to the ListUpdateCallback), and the new List will be swapped in.

The commit callback can be used to know when the List is committed, but note that it may not be executed. If List B is submitted immediately after List A, and is committed directly, the callback associated with List A will not be run.

Parameters
newList List: The new List.

commitCallback Runnable: Optional runnable that is executed when the List is committed, if it is committed.