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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 android.arch.paging;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.annotation.RestrictTo;
import android.support.v7.recyclerview.extensions.DiffCallback;
import android.support.v7.util.DiffUtil;
import android.support.v7.util.ListUpdateCallback;
/** @hide */
@RestrictTo(RestrictTo.Scope.LIBRARY_GROUP)
class ContiguousDiffHelper {
private ContiguousDiffHelper() {
}
@NonNull
static <T> DiffUtil.DiffResult computeDiff(
final NullPaddedList<T> oldList, final NullPaddedList<T> newList,
final DiffCallback<T> diffCallback, boolean detectMoves) {
if (!oldList.isImmutable()) {
throw new IllegalArgumentException("list must be immutable to safely perform diff");
}
if (!newList.isImmutable()) {
throw new IllegalArgumentException("list must be immutable to safely perform diff");
}
return DiffUtil.calculateDiff(new DiffUtil.Callback() {
@Nullable
@Override
public Object getChangePayload(int oldItemPosition, int newItemPosition) {
T oldItem = oldList.mList.get(oldItemPosition);
T newItem = newList.mList.get(newItemPosition);
if (oldItem == null || newItem == null) {
return null;
}
return diffCallback.getChangePayload(oldItem, newItem);
}
@Override
public int getOldListSize() {
return oldList.mList.size();
}
@Override
public int getNewListSize() {
return newList.mList.size();
}
@Override
public boolean areItemsTheSame(int oldItemPosition, int newItemPosition) {
T oldItem = oldList.mList.get(oldItemPosition);
T newItem = newList.mList.get(newItemPosition);
if (oldItem == newItem) {
return true;
}
if (oldItem == null || newItem == null) {
return false;
}
return diffCallback.areItemsTheSame(oldItem, newItem);
}
@Override
public boolean areContentsTheSame(int oldItemPosition, int newItemPosition) {
T oldItem = oldList.mList.get(oldItemPosition);
T newItem = newList.mList.get(newItemPosition);
if (oldItem == newItem) {
return true;
}
if (oldItem == null || newItem == null) {
return false;
}
return diffCallback.areContentsTheSame(oldItem, newItem);
}
}, detectMoves);
}
private static class OffsettingListUpdateCallback implements ListUpdateCallback {
private final int mOffset;
private final ListUpdateCallback mCallback;
private 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.onRemoved(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 <T> void dispatchDiff(ListUpdateCallback callback,
final NullPaddedList<T> oldList, final NullPaddedList<T> newList,
final DiffUtil.DiffResult diffResult) {
if (oldList.getLeadingNullCount() == 0
&& oldList.getTrailingNullCount() == 0
&& newList.getLeadingNullCount() == 0
&& newList.getTrailingNullCount() == 0) {
// Simple case, dispatch & return
diffResult.dispatchUpdatesTo(callback);
return;
}
// First, remove or insert trailing nulls
final int trailingOld = oldList.getTrailingNullCount();
final int trailingNew = newList.getTrailingNullCount();
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
final int leadingOld = oldList.getLeadingNullCount();
final int leadingNew = newList.getLeadingNullCount();
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);
}
}
}
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