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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.recyclerview.widget.DiffUtil;
import androidx.recyclerview.widget.ListUpdateCallback;
/**
* Methods for computing and applying DiffResults between PagedLists.
*
* To minimize the amount of diffing caused by placeholders, we only execute DiffUtil in a reduced
* 'diff space' - in the range (computeLeadingNulls..size-computeTrailingNulls).
*
* This allows the diff of a PagedList, e.g.:
* 100 nulls, placeholder page, (empty page) x 5, page, 100 nulls
*
* To only inform DiffUtil about single loaded page in this case, by pruning all other nulls from
* consideration.
*
* @see PagedStorage#computeLeadingNulls()
* @see PagedStorage#computeTrailingNulls()
*/
class PagedStorageDiffHelper {
private PagedStorageDiffHelper() {
}
static DiffUtil.DiffResult computeDiff(
final PagedStorage oldList,
final PagedStorage newList,
final DiffUtil.ItemCallback diffCallback) {
final int oldOffset = oldList.computeLeadingNulls();
final int newOffset = newList.computeLeadingNulls();
final int oldSize = oldList.size() - oldOffset - oldList.computeTrailingNulls();
final int newSize = newList.size() - newOffset - newList.computeTrailingNulls();
return DiffUtil.calculateDiff(new DiffUtil.Callback() {
@Nullable
@Override
public Object getChangePayload(int oldItemPosition, int newItemPosition) {
T oldItem = oldList.get(oldItemPosition + oldOffset);
T newItem = newList.get(newItemPosition + newList.getLeadingNullCount());
if (oldItem == null || newItem == null) {
return null;
}
return diffCallback.getChangePayload(oldItem, newItem);
}
@Override
public int getOldListSize() {
return oldSize;
}
@Override
public int getNewListSize() {
return newSize;
}
@Override
public boolean areItemsTheSame(int oldItemPosition, int newItemPosition) {
T oldItem = oldList.get(oldItemPosition + oldOffset);
T newItem = newList.get(newItemPosition + newList.getLeadingNullCount());
if (oldItem == newItem) {
return true;
}
//noinspection SimplifiableIfStatement
if (oldItem == null || newItem == null) {
return false;
}
return diffCallback.areItemsTheSame(oldItem, newItem);
}
@Override
public boolean areContentsTheSame(int oldItemPosition, int newItemPosition) {
T oldItem = oldList.get(oldItemPosition + oldOffset);
T newItem = newList.get(newItemPosition + newList.getLeadingNullCount());
if (oldItem == newItem) {
return true;
}
//noinspection SimplifiableIfStatement
if (oldItem == null || newItem == null) {
return false;
}
return diffCallback.areContentsTheSame(oldItem, newItem);
}
}, true);
}
private static class OffsettingListUpdateCallback implements ListUpdateCallback {
private final int mOffset;
private final ListUpdateCallback mCallback;
OffsettingListUpdateCallback(int offset, ListUpdateCallback callback) {
mOffset = offset;
mCallback = callback;
}
@Override
public void onInserted(int position, int count) {
mCallback.onInserted(position + mOffset, count);
}
@Override
public void onRemoved(int position, int count) {
mCallback.onRemoved(position + mOffset, count);
}
@Override
public void onMoved(int fromPosition, int toPosition) {
mCallback.onMoved(fromPosition + mOffset, toPosition + mOffset);
}
@Override
public void onChanged(int position, int count, Object payload) {
mCallback.onChanged(position + mOffset, count, payload);
}
}
/**
* TODO: improve diffing logic
*
* This function currently does a naive diff, assuming null does not become an item, and vice
* versa (so it won't dispatch onChange events for these). It's similar to passing a list with
* leading/trailing nulls in the beginning / end to DiffUtil, but dispatches the remove/insert
* for changed nulls at the beginning / end of the list.
*
* Note: if lists mutate between diffing the snapshot and dispatching the diff here, then we
* handle this by passing the snapshot to the callback, and dispatching those changes
* immediately after dispatching this diff.
*/
static void dispatchDiff(ListUpdateCallback callback,
final PagedStorage oldList,
final PagedStorage newList,
final DiffUtil.DiffResult diffResult) {
final int trailingOld = oldList.computeTrailingNulls();
final int trailingNew = newList.computeTrailingNulls();
final int leadingOld = oldList.computeLeadingNulls();
final int leadingNew = newList.computeLeadingNulls();
if (trailingOld == 0
&& trailingNew == 0
&& leadingOld == 0
&& leadingNew == 0) {
// Simple case, dispatch & return
diffResult.dispatchUpdatesTo(callback);
return;
}
// First, remove or insert trailing nulls
if (trailingOld > trailingNew) {
int count = trailingOld - trailingNew;
callback.onRemoved(oldList.size() - count, count);
} else if (trailingOld < trailingNew) {
callback.onInserted(oldList.size(), trailingNew - trailingOld);
}
// Second, remove or insert leading nulls
if (leadingOld > leadingNew) {
callback.onRemoved(0, leadingOld - leadingNew);
} else if (leadingOld < leadingNew) {
callback.onInserted(0, leadingNew - leadingOld);
}
// apply the diff, with an offset if needed
if (leadingNew != 0) {
diffResult.dispatchUpdatesTo(new OffsettingListUpdateCallback(leadingNew, callback));
} else {
diffResult.dispatchUpdatesTo(callback);
}
}
/**
* Given an oldPosition representing an anchor in the old data set, computes its new position
* after the diff, or a guess if it no longer exists.
*/
static int transformAnchorIndex(@NonNull DiffUtil.DiffResult diffResult,
@NonNull PagedStorage oldList, @NonNull PagedStorage newList, final int oldPosition) {
final int oldOffset = oldList.computeLeadingNulls();
// diffResult's indices starting after nulls, need to transform to diffutil indices
// (see also dispatchDiff(), which adds this offset when dispatching)
int diffIndex = oldPosition - oldOffset;
final int oldSize = oldList.size() - oldOffset - oldList.computeTrailingNulls();
// if our anchor is non-null, use it or close item's position in new list
if (diffIndex >= 0 && diffIndex < oldSize) {
// search outward from old position for position that maps
for (int i = 0; i < 30; i++) {
int positionToTry = diffIndex + (i / 2 * (i % 2 == 1 ? -1 : 1));
// reject if (null) item was not passed to DiffUtil, and wouldn't be in the result
if (positionToTry < 0 || positionToTry >= oldList.getStorageCount()) {
continue;
}
int result = diffResult.convertOldPositionToNew(positionToTry);
if (result != -1) {
// also need to transform from diffutil output indices to newList
return result + newList.getLeadingNullCount();
}
}
}
// not anchored to an item in new list, so just reuse position (clamped to newList size)
return Math.max(0, Math.min(oldPosition, newList.size() - 1));
}
}
