* It can be used to calculate updates for a RecyclerView Adapter. *
* DiffUtil uses Eugene W. Myers's difference algorithm to calculate the minimal number of updates * to convert one list into another. Myers's algorithm does not handle items that are moved so * DiffUtil runs a second pass on the result to detect items that were moved. *
* If the lists are large, this operation may take significant time so you are advised to run this * on a background thread, get the {@link DiffResult} then apply it on the RecyclerView on the main * thread. *
* This algorithm is optimized for space and uses O(N) space to find the minimal * number of addition and removal operations between the two lists. It has O(N + D^2) expected time * performance where D is the length of the edit script. *
* If move detection is enabled, it takes an additional O(N^2) time where N is the total number of * added and removed items. If your lists are already sorted by the same constraint (e.g. a created * timestamp for a list of posts), you can disable move detection to improve performance. *
* The actual runtime of the algorithm significantly depends on the number of changes in the list * and the cost of your comparison methods. Below are some average run times for reference: * (The test list is composed of random UUID Strings and the tests are run on Nexus 5X with M) *
* Due to implementation constraints, the max size of the list can be 2^26.
*/
public class DiffUtil {
private DiffUtil() {
// utility class, no instance.
}
private static final Comparator
* If your old and new lists are sorted by the same constraint and items never move (swap
* positions), you can disable move detection which takes
* For example, if your items have unique ids, this method should check their id equality.
*
* @param oldItemPosition The position of the item in the old list
* @param newItemPosition The position of the item in the new list
* @return True if the two items represent the same object or false if they are different.
*/
public abstract boolean areItemsTheSame(int oldItemPosition, int newItemPosition);
/**
* Called by the DiffUtil when it wants to check whether two items have the same data.
* DiffUtil uses this information to detect if the contents of an item has changed.
*
* DiffUtil uses this method to check equality instead of {@link Object#equals(Object)}
* so that you can change its behavior depending on your UI.
* For example, if you are using DiffUtil with a
* {@link android.support.v7.widget.RecyclerView.Adapter RecyclerView.Adapter}, you should
* return whether the items' visual representations are the same.
*
* This method is called only if {@link #areItemsTheSame(int, int)} returns
* {@code true} for these items.
*
* @param oldItemPosition The position of the item in the old list
* @param newItemPosition The position of the item in the new list which replaces the
* oldItem
* @return True if the contents of the items are the same or false if they are different.
*/
public abstract boolean areContentsTheSame(int oldItemPosition, int newItemPosition);
/**
* When {@link #areItemsTheSame(int, int)} returns {@code true} for two items and
* {@link #areContentsTheSame(int, int)} returns false for them, DiffUtil
* calls this method to get a payload about the change.
*
* For example, if you are using DiffUtil with {@link RecyclerView}, you can return the
* particular field that changed in the item and your
* {@link android.support.v7.widget.RecyclerView.ItemAnimator ItemAnimator} can use that
* information to run the correct animation.
*
* Default implementation returns {@code null}.
*
* @param oldItemPosition The position of the item in the old list
* @param newItemPosition The position of the item in the new list
*
* @return A payload object that represents the change between the two items.
*/
@Nullable
public Object getChangePayload(int oldItemPosition, int newItemPosition) {
return null;
}
}
/**
* Snakes represent a match between two lists. It is optionally prefixed or postfixed with an
* add or remove operation. See the Myers' paper for details.
*/
static class Snake {
/**
* Position in the old list
*/
int x;
/**
* Position in the new list
*/
int y;
/**
* Number of matches. Might be 0.
*/
int size;
/**
* If true, this is a removal from the original list followed by {@code size} matches.
* If false, this is an addition from the new list followed by {@code size} matches.
*/
boolean removal;
/**
* If true, the addition or removal is at the end of the snake.
* If false, the addition or removal is at the beginning of the snake.
*/
boolean reverse;
}
/**
* Represents a range in two lists that needs to be solved.
*
* This internal class is used when running Myers' algorithm without recursion.
*/
static class Range {
int oldListStart, oldListEnd;
int newListStart, newListEnd;
public Range() {
}
public Range(int oldListStart, int oldListEnd, int newListStart, int newListEnd) {
this.oldListStart = oldListStart;
this.oldListEnd = oldListEnd;
this.newListStart = newListStart;
this.newListEnd = newListEnd;
}
}
/**
* This class holds the information about the result of a
* {@link DiffUtil#calculateDiff(Callback, boolean)} call.
*
* You can consume the updates in a DiffResult via
* {@link #dispatchUpdatesTo(ListUpdateCallback)} or directly stream the results into a
* {@link RecyclerView.Adapter} via {@link #dispatchUpdatesTo(RecyclerView.Adapter)}.
*/
public static class DiffResult {
/**
* While reading the flags below, keep in mind that when multiple items move in a list,
* Myers's may pick any of them as the anchor item and consider that one NOT_CHANGED while
* picking others as additions and removals. This is completely fine as we later detect
* all moves.
*
* Below, when an item is mentioned to stay in the same "location", it means we won't
* dispatch a move/add/remove for it, it DOES NOT mean the item is still in the same
* position.
*/
// item stayed the same.
private static final int FLAG_NOT_CHANGED = 1;
// item stayed in the same location but changed.
private static final int FLAG_CHANGED = FLAG_NOT_CHANGED << 1;
// Item has moved and also changed.
private static final int FLAG_MOVED_CHANGED = FLAG_CHANGED << 1;
// Item has moved but did not change.
private static final int FLAG_MOVED_NOT_CHANGED = FLAG_MOVED_CHANGED << 1;
// Ignore this update.
// If this is an addition from the new list, it means the item is actually removed from an
// earlier position and its move will be dispatched when we process the matching removal
// from the old list.
// If this is a removal from the old list, it means the item is actually added back to an
// earlier index in the new list and we'll dispatch its move when we are processing that
// addition.
private static final int FLAG_IGNORE = FLAG_MOVED_NOT_CHANGED << 1;
// since we are re-using the int arrays that were created in the Myers' step, we mask
// change flags
private static final int FLAG_OFFSET = 5;
private static final int FLAG_MASK = (1 << FLAG_OFFSET) - 1;
// The Myers' snakes. At this point, we only care about their diagonal sections.
private final List
* This class also flags whether an item has been changed or not.
*
* DiffUtil does this pre-processing so that if it is running on a big list, it can be moved
* to background thread where most of the expensive stuff will be calculated and kept in
* the statuses maps. DiffResult uses this pre-calculated information while dispatching
* the updates (which is probably being called on the main thread).
*/
private void findMatchingItems() {
int posOld = mOldListSize;
int posNew = mNewListSize;
// traverse the matrix from right bottom to 0,0.
for (int i = mSnakes.size() - 1; i >= 0; i--) {
final Snake snake = mSnakes.get(i);
final int endX = snake.x + snake.size;
final int endY = snake.y + snake.size;
if (mDetectMoves) {
while (posOld > endX) {
// this is a removal. Check remaining snakes to see if this was added before
findAddition(posOld, posNew, i);
posOld--;
}
while (posNew > endY) {
// this is an addition. Check remaining snakes to see if this was removed
// before
findRemoval(posOld, posNew, i);
posNew--;
}
}
for (int j = 0; j < snake.size; j++) {
// matching items. Check if it is changed or not
final int oldItemPos = snake.x + j;
final int newItemPos = snake.y + j;
final boolean theSame = mCallback
.areContentsTheSame(oldItemPos, newItemPos);
final int changeFlag = theSame ? FLAG_NOT_CHANGED : FLAG_CHANGED;
mOldItemStatuses[oldItemPos] = (newItemPos << FLAG_OFFSET) | changeFlag;
mNewItemStatuses[newItemPos] = (oldItemPos << FLAG_OFFSET) | changeFlag;
}
posOld = snake.x;
posNew = snake.y;
}
}
private void findAddition(int x, int y, int snakeIndex) {
if (mOldItemStatuses[x - 1] != 0) {
return; // already set by a latter item
}
findMatchingItem(x, y, snakeIndex, false);
}
private void findRemoval(int x, int y, int snakeIndex) {
if (mNewItemStatuses[y - 1] != 0) {
return; // already set by a latter item
}
findMatchingItem(x, y, snakeIndex, true);
}
/**
* Finds a matching item that is before the given coordinates in the matrix
* (before : left and above).
*
* @param x The x position in the matrix (position in the old list)
* @param y The y position in the matrix (position in the new list)
* @param snakeIndex The current snake index
* @param removal True if we are looking for a removal, false otherwise
*
* @return True if such item is found.
*/
private boolean findMatchingItem(final int x, final int y, final int snakeIndex,
final boolean removal) {
final int myItemPos;
int curX;
int curY;
if (removal) {
myItemPos = y - 1;
curX = x;
curY = y - 1;
} else {
myItemPos = x - 1;
curX = x - 1;
curY = y;
}
for (int i = snakeIndex; i >= 0; i--) {
final Snake snake = mSnakes.get(i);
final int endX = snake.x + snake.size;
final int endY = snake.y + snake.size;
if (removal) {
// check removals for a match
for (int pos = curX - 1; pos >= endX; pos--) {
if (mCallback.areItemsTheSame(pos, myItemPos)) {
// found!
final boolean theSame = mCallback.areContentsTheSame(pos, myItemPos);
final int changeFlag = theSame ? FLAG_MOVED_NOT_CHANGED
: FLAG_MOVED_CHANGED;
mNewItemStatuses[myItemPos] = (pos << FLAG_OFFSET) | FLAG_IGNORE;
mOldItemStatuses[pos] = (myItemPos << FLAG_OFFSET) | changeFlag;
return true;
}
}
} else {
// check for additions for a match
for (int pos = curY - 1; pos >= endY; pos--) {
if (mCallback.areItemsTheSame(myItemPos, pos)) {
// found
final boolean theSame = mCallback.areContentsTheSame(myItemPos, pos);
final int changeFlag = theSame ? FLAG_MOVED_NOT_CHANGED
: FLAG_MOVED_CHANGED;
mOldItemStatuses[x - 1] = (pos << FLAG_OFFSET) | FLAG_IGNORE;
mNewItemStatuses[pos] = ((x - 1) << FLAG_OFFSET) | changeFlag;
return true;
}
}
}
curX = snake.x;
curY = snake.y;
}
return false;
}
/**
* Dispatches the update events to the given adapter.
*
* For example, if you have an {@link android.support.v7.widget.RecyclerView.Adapter Adapter}
* that is backed by a {@link List}, you can swap the list with the new one then call this
* method to dispatch all updates to the RecyclerView.
*
* Note that the RecyclerView requires you to dispatch adapter updates immediately when you
* change the data (you cannot defer {@code notify*} calls). The usage above adheres to this
* rule because updates are sent to the adapter right after the backing data is changed,
* before RecyclerView tries to read it.
*
* On the other hand, if you have another
* {@link android.support.v7.widget.RecyclerView.AdapterDataObserver AdapterDataObserver}
* that tries to process events synchronously, this may confuse that observer because the
* list is instantly moved to its final state while the adapter updates are dispatched later
* on, one by one. If you have such an
* {@link android.support.v7.widget.RecyclerView.AdapterDataObserver AdapterDataObserver},
* you can use
* {@link #dispatchUpdatesTo(ListUpdateCallback)} to handle each modification
* manually.
*
* @param adapter A RecyclerView adapter which was displaying the old list and will start
* displaying the new list.
*/
public void dispatchUpdatesTo(final RecyclerView.Adapter adapter) {
dispatchUpdatesTo(new ListUpdateCallback() {
@Override
public void onInserted(int position, int count) {
adapter.notifyItemRangeInserted(position, count);
}
@Override
public void onRemoved(int position, int count) {
adapter.notifyItemRangeRemoved(position, count);
}
@Override
public void onMoved(int fromPosition, int toPosition) {
adapter.notifyItemMoved(fromPosition, toPosition);
}
@Override
public void onChanged(int position, int count, Object payload) {
adapter.notifyItemRangeChanged(position, count, payload);
}
});
}
/**
* Dispatches update operations to the given Callback.
*
* These updates are atomic such that the first update call effects every update call that
* comes after it (the same as RecyclerView).
*
* @param updateCallback The callback to receive the update operations.
* @see #dispatchUpdatesTo(RecyclerView.Adapter)
*/
public void dispatchUpdatesTo(ListUpdateCallback updateCallback) {
final BatchingListUpdateCallback batchingCallback;
if (updateCallback instanceof BatchingListUpdateCallback) {
batchingCallback = (BatchingListUpdateCallback) updateCallback;
} else {
batchingCallback = new BatchingListUpdateCallback(updateCallback);
// replace updateCallback with a batching callback and override references to
// updateCallback so that we don't call it directly by mistake
//noinspection UnusedAssignment
updateCallback = batchingCallback;
}
// These are add/remove ops that are converted to moves. We track their positions until
// their respective update operations are processed.
final List
* When an update is skipped, it is tracked as other updates are dispatched until the matching
* add/remove operation is found at which point the tracked position is used to dispatch the
* update.
*/
private static class PostponedUpdate {
int posInOwnerList;
int currentPos;
boolean removal;
public PostponedUpdate(int posInOwnerList, int currentPos, boolean removal) {
this.posInOwnerList = posInOwnerList;
this.currentPos = currentPos;
this.removal = removal;
}
}
}
O(N^2) time where
* N is the number of added, moved, removed items.
*
* @param cb The callback that acts as a gateway to the backing list data
* @param detectMoves True if DiffUtil should try to detect moved items, false otherwise.
*
* @return A DiffResult that contains the information about the edit sequence to convert the
* old list into the new list.
*/
public static DiffResult calculateDiff(Callback cb, boolean detectMoves) {
final int oldSize = cb.getOldListSize();
final int newSize = cb.getNewListSize();
final List
* List oldList = mAdapter.getData();
* DiffResult result = DiffUtil.calculateDiff(new MyCallback(oldList, newList));
* mAdapter.setData(newList);
* result.dispatchUpdatesTo(mAdapter);
*
*