androidx.paging.AsyncPagedListDiffer.txt Maven / Gradle / Ivy
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/*
* Copyright (C) 2017 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package androidx.paging;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.arch.core.executor.ArchTaskExecutor;
import androidx.lifecycle.LiveData;
import androidx.recyclerview.widget.AdapterListUpdateCallback;
import androidx.recyclerview.widget.AsyncDifferConfig;
import androidx.recyclerview.widget.DiffUtil;
import androidx.recyclerview.widget.ListUpdateCallback;
import androidx.recyclerview.widget.RecyclerView;
import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.Executor;
/**
* Helper object for mapping a {@link PagedList} into a
* {@link androidx.recyclerview.widget.RecyclerView.Adapter RecyclerView.Adapter}.
*
* For simplicity, the {@link PagedListAdapter} wrapper class can often be used instead of the
* differ directly. This diff class is exposed for complex cases, and where overriding an adapter
* base class to support paging isn't convenient.
*
* When consuming a {@link LiveData} of PagedList, you can observe updates and dispatch them
* directly to {@link #submitList(PagedList)}. The AsyncPagedListDiffer then can present this
* updating data set simply for an adapter. It listens to PagedList loading callbacks, and uses
* DiffUtil on a background thread to compute updates as new PagedLists are received.
*
* It provides a simple list-like API with {@link #getItem(int)} and {@link #getItemCount()} for an
* adapter to acquire and present data objects.
*
* A complete usage pattern with Room would look like this:
*
* {@literal @}Dao
* interface UserDao {
* {@literal @}Query("SELECT * FROM user ORDER BY lastName ASC")
* public abstract DataSource.Factory<Integer, User> usersByLastName();
* }
*
* class MyViewModel extends ViewModel {
* public final LiveData<PagedList<User>> usersList;
* public MyViewModel(UserDao userDao) {
* usersList = new LivePagedListBuilder<>(
* userDao.usersByLastName(), /* page size {@literal *}/ 20).build();
* }
* }
*
* class MyActivity extends AppCompatActivity {
* {@literal @}Override
* public void onCreate(Bundle savedState) {
* super.onCreate(savedState);
* MyViewModel viewModel = ViewModelProviders.of(this).get(MyViewModel.class);
* RecyclerView recyclerView = findViewById(R.id.user_list);
* final UserAdapter adapter = new UserAdapter();
* viewModel.usersList.observe(this, pagedList -> adapter.submitList(pagedList));
* recyclerView.setAdapter(adapter);
* }
* }
*
* class UserAdapter extends RecyclerView.Adapter<UserViewHolder> {
* private final AsyncPagedListDiffer<User> mDiffer
* = new AsyncPagedListDiffer(this, DIFF_CALLBACK);
* {@literal @}Override
* public int getItemCount() {
* return mDiffer.getItemCount();
* }
* public void submitList(PagedList<User> pagedList) {
* mDiffer.submitList(pagedList);
* }
* {@literal @}Override
* public void onBindViewHolder(UserViewHolder holder, int position) {
* User user = mDiffer.getItem(position);
* if (user != null) {
* holder.bindTo(user);
* } else {
* // Null defines a placeholder item - AsyncPagedListDiffer will automatically
* // invalidate this row when the actual object is loaded from the database
* holder.clear();
* }
* }
* public static final DiffUtil.ItemCallback<User> DIFF_CALLBACK =
* new DiffUtil.ItemCallback<User>() {
* {@literal @}Override
* public boolean areItemsTheSame(
* {@literal @}NonNull User oldUser, {@literal @}NonNull User newUser) {
* // User properties may have changed if reloaded from the DB, but ID is fixed
* return oldUser.getId() == newUser.getId();
* }
* {@literal @}Override
* public boolean areContentsTheSame(
* {@literal @}NonNull User oldUser, {@literal @}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);
* }
* }
* }
*
* @param Type of the PagedLists this differ will receive.
*/
public class AsyncPagedListDiffer {
// updateCallback notifications must only be notified *after* new data and item count are stored
// this ensures Adapter#notifyItemRangeInserted etc are accessing the new data
@SuppressWarnings("WeakerAccess") /* synthetic access */
final ListUpdateCallback mUpdateCallback;
@SuppressWarnings("WeakerAccess") /* synthetic access */
final AsyncDifferConfig mConfig;
@SuppressWarnings("RestrictedApi")
Executor mMainThreadExecutor = ArchTaskExecutor.getMainThreadExecutor();
/**
* Listener for when the current PagedList is updated.
*
* @param Type of items in PagedList
*/
public interface PagedListListener {
/**
* Called after the current PagedList has been updated.
*
* @param previousList The previous list, may be null.
* @param currentList The new current list, may be null.
*/
void onCurrentListChanged(
@Nullable PagedList previousList, @Nullable PagedList currentList);
}
private final List> mListeners = new CopyOnWriteArrayList<>();
private boolean mIsContiguous;
private PagedList mPagedList;
private PagedList mSnapshot;
// Max generation of currently scheduled runnable
@SuppressWarnings("WeakerAccess") /* synthetic access */
int mMaxScheduledGeneration;
@SuppressWarnings("WeakerAccess") /* synthetic access */
final PagedList.LoadStateManager mLoadStateManager = new PagedList.LoadStateManager() {
@Override
protected void onStateChanged(@NonNull PagedList.LoadType type,
@NonNull PagedList.LoadState state, @Nullable Throwable error) {
// Don't need to post - PagedList will already have done that
for (PagedList.LoadStateListener listener : mLoadStateListeners) {
listener.onLoadStateChanged(type, state, error);
}
}
};
@SuppressWarnings("WeakerAccess") // synthetic access
PagedList.LoadStateListener mLoadStateListener = new PagedList.LoadStateListener() {
@Override
public void onLoadStateChanged(@NonNull PagedList.LoadType type,
@NonNull PagedList.LoadState state, @Nullable Throwable error) {
mLoadStateManager.onStateChanged(type, state, error);
}
};
@SuppressWarnings("WeakerAccess") // synthetic access
final List mLoadStateListeners =
new CopyOnWriteArrayList<>();
/**
* Convenience for {@code AsyncPagedListDiffer(new AdapterListUpdateCallback(adapter),
* new AsyncDifferConfig.Builder(diffCallback).build();}
*
* @param adapter Adapter that will receive update signals.
* @param diffCallback The {@link DiffUtil.ItemCallback DiffUtil.ItemCallback} instance to
* compare items in the list.
*/
@SuppressWarnings("WeakerAccess")
public AsyncPagedListDiffer(@NonNull RecyclerView.Adapter adapter,
@NonNull DiffUtil.ItemCallback diffCallback) {
mUpdateCallback = new AdapterListUpdateCallback(adapter);
mConfig = new AsyncDifferConfig.Builder<>(diffCallback).build();
}
@SuppressWarnings("WeakerAccess")
public AsyncPagedListDiffer(@NonNull ListUpdateCallback listUpdateCallback,
@NonNull AsyncDifferConfig config) {
mUpdateCallback = listUpdateCallback;
mConfig = config;
}
private PagedList.Callback mPagedListCallback = new PagedList.Callback() {
@Override
public void onInserted(int position, int count) {
mUpdateCallback.onInserted(position, count);
}
@Override
public void onRemoved(int position, int count) {
mUpdateCallback.onRemoved(position, count);
}
@Override
public void onChanged(int position, int count) {
// NOTE: pass a null payload to convey null -> item
mUpdateCallback.onChanged(position, count, null);
}
};
/**
* Get the item from the current PagedList at the specified index.
*
* Note that this operates on both loaded items and null padding within the PagedList.
*
* @param index Index of item to get, must be >= 0, and < {@link #getItemCount()}.
* @return The item, or null, if a null placeholder is at the specified position.
*/
@SuppressWarnings("WeakerAccess")
@Nullable
public T getItem(int index) {
if (mPagedList == null) {
if (mSnapshot == null) {
throw new IndexOutOfBoundsException(
"Item count is zero, getItem() call is invalid");
} else {
return mSnapshot.get(index);
}
}
mPagedList.loadAround(index);
return mPagedList.get(index);
}
/**
* Get the number of items currently presented by this Differ. This value can be directly
* returned to {@link RecyclerView.Adapter#getItemCount()}.
*
* @return Number of items being presented.
*/
@SuppressWarnings("WeakerAccess")
public int getItemCount() {
if (mPagedList != null) {
return mPagedList.size();
}
return mSnapshot == null ? 0 : mSnapshot.size();
}
/**
* Pass a new PagedList to the differ.
*
* If a PagedList 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
* {@link ListUpdateCallback}), and the new PagedList will be swapped in as the
* {@link #getCurrentList() current list}.
*
* @param pagedList The new PagedList.
*/
public void submitList(@Nullable final PagedList pagedList) {
submitList(pagedList, null);
}
/**
* Pass a new PagedList to the differ.
*
* If a PagedList 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
* {@link ListUpdateCallback}), and the new PagedList will be swapped in as the
* {@link #getCurrentList() current list}.
*
* The commit callback can be used to know when the PagedList is committed, but note that it
* may not be executed. If PagedList B is submitted immediately after PagedList A, and is
* committed directly, the callback associated with PagedList A will not be run.
*
* @param pagedList The new PagedList.
* @param commitCallback Optional runnable that is executed when the PagedList is committed, if
* it is committed.
*/
@SuppressWarnings("ReferenceEquality")
public void submitList(@Nullable final PagedList pagedList,
@Nullable final Runnable commitCallback) {
if (pagedList != null) {
if (mPagedList == null && mSnapshot == null) {
mIsContiguous = pagedList.isContiguous();
} else {
if (pagedList.isContiguous() != mIsContiguous) {
throw new IllegalArgumentException("AsyncPagedListDiffer cannot handle both"
+ " contiguous and non-contiguous lists.");
}
}
}
// incrementing generation means any currently-running diffs are discarded when they finish
final int runGeneration = ++mMaxScheduledGeneration;
if (pagedList == mPagedList) {
// nothing to do (Note - still had to inc generation, since may have ongoing work)
if (commitCallback != null) {
commitCallback.run();
}
return;
}
final PagedList previous = (mSnapshot != null) ? mSnapshot : mPagedList;
if (pagedList == null) {
int removedCount = getItemCount();
if (mPagedList != null) {
mPagedList.removeWeakCallback(mPagedListCallback);
mPagedList.removeWeakLoadStateListener(mLoadStateListener);
mPagedList = null;
} else if (mSnapshot != null) {
mSnapshot = null;
}
// dispatch update callback after updating mPagedList/mSnapshot
mUpdateCallback.onRemoved(0, removedCount);
onCurrentListChanged(previous, null, commitCallback);
return;
}
if (mPagedList == null && mSnapshot == null) {
// fast simple first insert
mPagedList = pagedList;
mPagedList.addWeakLoadStateListener(mLoadStateListener);
pagedList.addWeakCallback(null, mPagedListCallback);
// dispatch update callback after updating mPagedList/mSnapshot
mUpdateCallback.onInserted(0, pagedList.size());
onCurrentListChanged(null, pagedList, commitCallback);
return;
}
if (mPagedList != null) {
// first update scheduled on this list, so capture mPages as a snapshot, removing
// callbacks so we don't have resolve updates against a moving target
mPagedList.removeWeakCallback(mPagedListCallback);
mPagedList.removeWeakLoadStateListener(mLoadStateListener);
mSnapshot = (PagedList) mPagedList.snapshot();
mPagedList = null;
}
if (mSnapshot == null || mPagedList != null) {
throw new IllegalStateException("must be in snapshot state to diff");
}
final PagedList oldSnapshot = mSnapshot;
final PagedList newSnapshot = (PagedList) pagedList.snapshot();
mConfig.getBackgroundThreadExecutor().execute(new Runnable() {
@Override
public void run() {
final DiffUtil.DiffResult result;
result = PagedStorageDiffHelper.computeDiff(
oldSnapshot.mStorage,
newSnapshot.mStorage,
mConfig.getDiffCallback());
mMainThreadExecutor.execute(new Runnable() {
@Override
public void run() {
if (mMaxScheduledGeneration == runGeneration) {
latchPagedList(pagedList, newSnapshot, result,
oldSnapshot.mLastLoad, commitCallback);
}
}
});
}
});
}
@SuppressWarnings("WeakerAccess") /* synthetic access */
void latchPagedList(
@NonNull PagedList newList,
@NonNull PagedList diffSnapshot,
@NonNull DiffUtil.DiffResult diffResult,
int lastAccessIndex,
@Nullable Runnable commitCallback) {
if (mSnapshot == null || mPagedList != null) {
throw new IllegalStateException("must be in snapshot state to apply diff");
}
PagedList previousSnapshot = mSnapshot;
mPagedList = newList;
mPagedList.addWeakLoadStateListener(mLoadStateListener);
mSnapshot = null;
// dispatch update callback after updating mPagedList/mSnapshot
PagedStorageDiffHelper.dispatchDiff(mUpdateCallback,
previousSnapshot.mStorage, newList.mStorage, diffResult);
newList.addWeakCallback(diffSnapshot, mPagedListCallback);
if (!mPagedList.isEmpty()) {
// Transform the last loadAround() index from the old list to the new list by passing it
// through the DiffResult. This ensures the lastKey of a positional PagedList is carried
// to new list even if no in-viewport item changes (AsyncPagedListDiffer#get not called)
// Note: we don't take into account loads between new list snapshot and new list, but
// this is only a problem in rare cases when placeholders are disabled, and a load
// starts (for some reason) and finishes before diff completes.
int newPosition = PagedStorageDiffHelper.transformAnchorIndex(
diffResult, previousSnapshot.mStorage, diffSnapshot.mStorage, lastAccessIndex);
// Trigger load in new list at this position, clamped to list bounds.
// This is a load, not just an update of last load position, since the new list may be
// incomplete. If new list is subset of old list, but doesn't fill the viewport, this
// will likely trigger a load of new data.
mPagedList.loadAround(Math.max(0, Math.min(mPagedList.size() - 1, newPosition)));
}
onCurrentListChanged(previousSnapshot, mPagedList, commitCallback);
}
private void onCurrentListChanged(
@Nullable PagedList previousList,
@Nullable PagedList currentList,
@Nullable Runnable commitCallback) {
for (PagedListListener listener : mListeners) {
listener.onCurrentListChanged(previousList, currentList);
}
if (commitCallback != null) {
commitCallback.run();
}
}
/**
* Add a PagedListListener to receive updates when the current PagedList changes.
*
* @param listener Listener to receive updates.
*
* @see #getCurrentList()
* @see #removePagedListListener(PagedListListener)
*/
public void addPagedListListener(@NonNull PagedListListener listener) {
mListeners.add(listener);
}
/**
* Remove a previously registered PagedListListener.
*
* @param listener Previously registered listener.
* @see #getCurrentList()
* @see #addPagedListListener(PagedListListener)
*/
public void removePagedListListener(@NonNull PagedListListener listener) {
mListeners.remove(listener);
}
/**
* Add a LoadStateListener to observe the loading state of the current PagedList.
*
* As new PagedLists are submitted and displayed, the listener will be notified to reflect
* current REFRESH, START, and END states.
*
* @param listener Listener to receive updates.
*
* @see #removeLoadStateListListener(PagedList.LoadStateListener)
*/
public void addLoadStateListener(@NonNull PagedList.LoadStateListener listener) {
if (mPagedList != null) {
mPagedList.addWeakLoadStateListener(listener);
} else {
listener.onLoadStateChanged(PagedList.LoadType.REFRESH, mLoadStateManager.getRefresh(),
mLoadStateManager.getRefreshError());
listener.onLoadStateChanged(PagedList.LoadType.START, mLoadStateManager.getStart(),
mLoadStateManager.getStartError());
listener.onLoadStateChanged(PagedList.LoadType.END, mLoadStateManager.getEnd(),
mLoadStateManager.getEndError());
}
mLoadStateListeners.add(listener);
}
/**
* Remove a previously registered PagedListListener.
*
* @param listener Previously registered listener.
* @see #getCurrentList()
* @see #addPagedListListener(PagedListListener)
*/
public void removeLoadStateListListener(@NonNull PagedList.LoadStateListener listener) {
mLoadStateListeners.remove(listener);
if (mPagedList != null) {
mPagedList.removeWeakLoadStateListener(listener);
}
}
/**
* Returns the PagedList currently being displayed by the differ.
*
* This is not necessarily the most recent list passed to {@link #submitList(PagedList)},
* because a diff is computed asynchronously between the new list and the current list before
* updating the currentList value. May be null if no PagedList is being presented.
*
* @return The list currently being displayed, may be null.
*/
@SuppressWarnings("WeakerAccess")
@Nullable
public PagedList getCurrentList() {
if (mSnapshot != null) {
return mSnapshot;
}
return mPagedList;
}
}