androidx.recyclerview.widget.DiffUtil Maven / Gradle / Ivy
package androidx.recyclerview.widget;
import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.annotation.VisibleForTesting;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
/**
* DiffUtil is a utility class that calculates the difference between two lists and outputs a list of update operations that converts the first list into the second one.
*
* It can be used to calculate updates for a RecyclerView Adapter. See {@link ListAdapter} and {@link AsyncListDiffer} which can simplify the use of DiffUtil on a background
* thread.
*
* 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.
*
* Note that DiffUtil, ListAdapter, and AsyncListDiffer require the list to not mutate while in use. This generally means that both the lists themselves and their elements (or at
* least, the properties of elements used in diffing) should not be modified directly. Instead, new lists should be provided any time content changes. It's common for lists passed
* to DiffUtil to share elements that have not mutated, so it is not strictly required to reload all data to use DiffUtil.
*
* 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)
*
* - 100 items and 10 modifications: avg: 0.39 ms, median: 0.35 ms
*
- 100 items and 100 modifications: 3.82 ms, median: 3.75 ms
*
- 100 items and 100 modifications without moves: 2.09 ms, median: 2.06 ms
*
- 1000 items and 50 modifications: avg: 4.67 ms, median: 4.59 ms
*
- 1000 items and 50 modifications without moves: avg: 3.59 ms, median: 3.50 ms
*
- 1000 items and 200 modifications: 27.07 ms, median: 26.92 ms
*
- 1000 items and 200 modifications without moves: 13.54 ms, median: 13.36 ms
*
*
* Due to implementation constraints, the max size of the list can be 2^26.
*
* @see ListAdapter
* @see AsyncListDiffer
*/
public class DiffUtil {
private DiffUtil() {
// utility class, no instance.
}
private static final Comparator SNAKE_COMPARATOR = new Comparator() {
@Override
public int compare(Snake o1, Snake o2) {
int cmpX = o1.x - o2.x;
return cmpX == 0 ? o1.y - o2.y : cmpX;
}
};
// Myers' algorithm uses two lists as axis labels. In DiffUtil's implementation, `x` axis is
// used for old list and `y` axis is used for new list.
/**
* Calculates the list of update operations that can covert one list into the other one.
*
* @param cb
* The callback that acts as a gateway to the backing list data
*
* @return A DiffResult that contains the information about the edit sequence to convert the old list into the new list.
*/
@NonNull
public static DiffResult calculateDiff(@NonNull Callback cb) {
return calculateDiff(cb, true);
}
/**
* Calculates the list of update operations that can covert one list into the other one.
*
* 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 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.
*/
@NonNull
public static DiffResult calculateDiff(@NonNull Callback cb, boolean detectMoves) {
final int oldSize = cb.getOldListSize();
final int newSize = cb.getNewListSize();
final List snakes = new ArrayList<>();
// instead of a recursive implementation, we keep our own stack to avoid potential stack
// overflow exceptions
final List stack = new ArrayList<>();
stack.add(new Range(0, oldSize, 0, newSize));
final int max = oldSize + newSize + Math.abs(oldSize - newSize);
// allocate forward and backward k-lines. K lines are diagonal lines in the matrix. (see the
// paper for details)
// These arrays lines keep the max reachable position for each k-line.
final int[] forward = new int[max * 2];
final int[] backward = new int[max * 2];
// We pool the ranges to avoid allocations for each recursive call.
final List rangePool = new ArrayList<>();
while (!stack.isEmpty()) {
final Range range = stack.remove(stack.size() - 1);
final Snake snake = diffPartial(cb, range.oldListStart, range.oldListEnd, range.newListStart,
range.newListEnd, forward, backward, max);
if (snake != null) {
if (snake.size > 0) {
snakes.add(snake);
}
// offset the snake to convert its coordinates from the Range's area to global
snake.x += range.oldListStart;
snake.y += range.newListStart;
// add new ranges for left and right
final Range left = rangePool.isEmpty() ? new Range() : rangePool.remove(rangePool.size() - 1);
left.oldListStart = range.oldListStart;
left.newListStart = range.newListStart;
if (snake.reverse) {
left.oldListEnd = snake.x;
left.newListEnd = snake.y;
} else {
if (snake.removal) {
left.oldListEnd = snake.x - 1;
left.newListEnd = snake.y;
} else {
left.oldListEnd = snake.x;
left.newListEnd = snake.y - 1;
}
}
stack.add(left);
// re-use range for right
// noinspection UnnecessaryLocalVariable
final Range right = range;
if (snake.reverse) {
if (snake.removal) {
right.oldListStart = snake.x + snake.size + 1;
right.newListStart = snake.y + snake.size;
} else {
right.oldListStart = snake.x + snake.size;
right.newListStart = snake.y + snake.size + 1;
}
} else {
right.oldListStart = snake.x + snake.size;
right.newListStart = snake.y + snake.size;
}
stack.add(right);
} else {
rangePool.add(range);
}
}
// sort snakes
Collections.sort(snakes, SNAKE_COMPARATOR);
return new DiffResult(cb, snakes, forward, backward, detectMoves);
}
private static Snake diffPartial(Callback cb, int startOld, int endOld, int startNew, int endNew, int[] forward,
int[] backward, int kOffset) {
final int oldSize = endOld - startOld;
final int newSize = endNew - startNew;
if (endOld - startOld < 1 || endNew - startNew < 1) {
return null;
}
final int delta = oldSize - newSize;
final int dLimit = (oldSize + newSize + 1) / 2;
Arrays.fill(forward, kOffset - dLimit - 1, kOffset + dLimit + 1, 0);
Arrays.fill(backward, kOffset - dLimit - 1 + delta, kOffset + dLimit + 1 + delta, oldSize);
final boolean checkInFwd = delta % 2 != 0;
for (int d = 0; d <= dLimit; d++) {
for (int k = -d; k <= d; k += 2) {
// find forward path
// we can reach k from k - 1 or k + 1. Check which one is further in the graph
int x;
final boolean removal;
if (k == -d || (k != d && forward[kOffset + k - 1] < forward[kOffset + k + 1])) {
x = forward[kOffset + k + 1];
removal = false;
} else {
x = forward[kOffset + k - 1] + 1;
removal = true;
}
// set y based on x
int y = x - k;
// move diagonal as long as items match
while (x < oldSize && y < newSize && cb.areItemsTheSame(startOld + x, startNew + y)) {
x++;
y++;
}
forward[kOffset + k] = x;
if (checkInFwd && k >= delta - d + 1 && k <= delta + d - 1) {
if (forward[kOffset + k] >= backward[kOffset + k]) {
Snake outSnake = new Snake();
outSnake.x = backward[kOffset + k];
outSnake.y = outSnake.x - k;
outSnake.size = forward[kOffset + k] - backward[kOffset + k];
outSnake.removal = removal;
outSnake.reverse = false;
return outSnake;
}
}
}
for (int k = -d; k <= d; k += 2) {
// find reverse path at k + delta, in reverse
final int backwardK = k + delta;
int x;
final boolean removal;
if (backwardK == d + delta || (backwardK != -d + delta
&& backward[kOffset + backwardK - 1] < backward[kOffset + backwardK + 1])) {
x = backward[kOffset + backwardK - 1];
removal = false;
} else {
x = backward[kOffset + backwardK + 1] - 1;
removal = true;
}
// set y based on x
int y = x - backwardK;
// move diagonal as long as items match
while (x > 0 && y > 0 && cb.areItemsTheSame(startOld + x - 1, startNew + y - 1)) {
x--;
y--;
}
backward[kOffset + backwardK] = x;
if (!checkInFwd && k + delta >= -d && k + delta <= d) {
if (forward[kOffset + backwardK] >= backward[kOffset + backwardK]) {
Snake outSnake = new Snake();
outSnake.x = backward[kOffset + backwardK];
outSnake.y = outSnake.x - backwardK;
outSnake.size = forward[kOffset + backwardK] - backward[kOffset + backwardK];
outSnake.removal = removal;
outSnake.reverse = true;
return outSnake;
}
}
}
}
throw new IllegalStateException("DiffUtil hit an unexpected case while trying to calculate"
+ " the optimal path. Please make sure your data is not changing during the" + " diff calculation.");
}
/**
* A Callback class used by DiffUtil while calculating the diff between two lists.
*/
public abstract static class Callback {
/**
* Returns the size of the old list.
*
* @return The size of the old list.
*/
public abstract int getOldListSize();
/**
* Returns the size of the new list.
*
* @return The size of the new list.
*/
public abstract int getNewListSize();
/**
* Called by the DiffUtil to decide whether two object represent the same Item.
*
* 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 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 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;
}
}
/**
* Callback for calculating the diff between two non-null items in a list.
*
* {@link Callback} serves two roles - list indexing, and item diffing. ItemCallback handles just the second of these, which allows separation of code that indexes into an
* array or List from the presentation-layer and content specific diffing code.
*
* @param
* Type of items to compare.
*/
public abstract static class ItemCallback {
/**
* Called to check whether two objects represent the same item.
*
* For example, if your items have unique ids, this method should check their id equality.
*
* Note: {@code null} items in the list are assumed to be the same as another {@code null} item and are assumed to not be the same as a non-{@code null} item. This callback
* will not be invoked for either of those cases.
*
* @param oldItem
* The item in the old list.
* @param newItem
* The item in the new list.
* @return True if the two items represent the same object or false if they are different.
*
* @see Callback#areItemsTheSame(int, int)
*/
public abstract boolean areItemsTheSame(@NonNull T oldItem, @NonNull T newItem);
/**
* Called to check whether two items have the same data.
*
* This information is used to detect if the contents of an item have changed.
*
* 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 RecyclerView.Adapter RecyclerView.Adapter}, you should return whether the items' visual representations are the
* same.
*
* This method is called only if {@link #areItemsTheSame(T, T)} returns {@code true} for these items.
*
* Note: Two {@code null} items are assumed to represent the same contents. This callback will not be invoked for this case.
*
* @param oldItem
* The item in the old list.
* @param newItem
* The item in the new list.
* @return True if the contents of the items are the same or false if they are different.
*
* @see Callback#areContentsTheSame(int, int)
*/
public abstract boolean areContentsTheSame(@NonNull T oldItem, @NonNull T newItem);
/**
* When {@link #areItemsTheSame(T, T)} returns {@code true} for two items and {@link #areContentsTheSame(T, T)} returns false for them, this method is called 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 RecyclerView.ItemAnimator
* ItemAnimator} can use that information to run the correct animation.
*
* Default implementation returns {@code null}.
*
* @see Callback#getChangePayload(int, int)
*/
@SuppressWarnings({ "WeakerAccess", "unused" })
@Nullable
public Object getChangePayload(@NonNull T oldItem, @NonNull T newItem) {
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 {
/**
* Signifies an item not present in the list.
*/
public static final int NO_POSITION = -1;
/**
* 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 mSnakes;
// The list to keep oldItemStatuses. As we traverse old items, we assign flags to them
// which also includes whether they were a real removal or a move (and its new index).
private final int[] mOldItemStatuses;
// The list to keep newItemStatuses. As we traverse new items, we assign flags to them
// which also includes whether they were a real addition or a move(and its old index).
private final int[] mNewItemStatuses;
// The callback that was given to calcualte diff method.
private final Callback mCallback;
private final int mOldListSize;
private final int mNewListSize;
private final boolean mDetectMoves;
/**
* @param callback
* The callback that was used to calculate the diff
* @param snakes
* The list of Myers' snakes
* @param oldItemStatuses
* An int[] that can be re-purposed to keep metadata
* @param newItemStatuses
* An int[] that can be re-purposed to keep metadata
* @param detectMoves
* True if this DiffResult will try to detect moved items
*/
DiffResult(Callback callback, List snakes, int[] oldItemStatuses, int[] newItemStatuses,
boolean detectMoves) {
mSnakes = snakes;
mOldItemStatuses = oldItemStatuses;
mNewItemStatuses = newItemStatuses;
Arrays.fill(mOldItemStatuses, 0);
Arrays.fill(mNewItemStatuses, 0);
mCallback = callback;
mOldListSize = callback.getOldListSize();
mNewListSize = callback.getNewListSize();
mDetectMoves = detectMoves;
addRootSnake();
findMatchingItems();
}
/**
* We always add a Snake to 0/0 so that we can run loops from end to beginning and be done when we run out of snakes.
*/
private void addRootSnake() {
Snake firstSnake = mSnakes.isEmpty() ? null : mSnakes.get(0);
if (firstSnake == null || firstSnake.x != 0 || firstSnake.y != 0) {
Snake root = new Snake();
root.x = 0;
root.y = 0;
root.removal = false;
root.size = 0;
root.reverse = false;
mSnakes.add(0, root);
}
}
/**
* This method traverses each addition / removal and tries to match it to a previous removal / addition. This is how we detect move operations.
*
* 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);
}
/**
* Given a position in the old list, returns the position in the new list, or {@code NO_POSITION} if it was removed.
*
* @param oldListPosition
* Position of item in old list
*
* @return Position of item in new list, or {@code NO_POSITION} if not present.
*
* @see #NO_POSITION
* @see #convertNewPositionToOld(int)
*/
public int convertOldPositionToNew(int oldListPosition) {
if (oldListPosition < 0 || oldListPosition >= mOldListSize) {
throw new IndexOutOfBoundsException("Index out of bounds - passed position = " + oldListPosition
+ ", old list size = " + mOldListSize);
}
final int status = mOldItemStatuses[oldListPosition];
if ((status & FLAG_MASK) == 0) {
return NO_POSITION;
} else {
return status >> FLAG_OFFSET;
}
}
/**
* Given a position in the new list, returns the position in the old list, or {@code NO_POSITION} if it was removed.
*
* @param newListPosition
* Position of item in new list
*
* @return Position of item in old list, or {@code NO_POSITION} if not present.
*
* @see #NO_POSITION
* @see #convertOldPositionToNew(int)
*/
public int convertNewPositionToOld(int newListPosition) {
if (newListPosition < 0 || newListPosition >= mNewListSize) {
throw new IndexOutOfBoundsException("Index out of bounds - passed position = " + newListPosition
+ ", new list size = " + mNewListSize);
}
final int status = mNewItemStatuses[newListPosition];
if ((status & FLAG_MASK) == 0) {
return NO_POSITION;
} else {
return status >> FLAG_OFFSET;
}
}
/**
* 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 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.
*
*
* List oldList = mAdapter.getData();
* DiffResult result = DiffUtil.calculateDiff(new MyCallback(oldList, newList));
* mAdapter.setData(newList);
* result.dispatchUpdatesTo(mAdapter);
*
*
* 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 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 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.
* @see AdapterListUpdateCallback
*/
public void dispatchUpdatesTo(@NonNull final RecyclerView.Adapter adapter) {
dispatchUpdatesTo(new AdapterListUpdateCallback(adapter));
}
/**
* Dispatches update operations to the given Callback.
*
* These updates are atomic such that the first update call affects 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(@NonNull 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 postponedUpdates = new ArrayList<>();
int posOld = mOldListSize;
int posNew = mNewListSize;
for (int snakeIndex = mSnakes.size() - 1; snakeIndex >= 0; snakeIndex--) {
final Snake snake = mSnakes.get(snakeIndex);
final int snakeSize = snake.size;
final int endX = snake.x + snakeSize;
final int endY = snake.y + snakeSize;
if (endX < posOld) {
dispatchRemovals(postponedUpdates, batchingCallback, endX, posOld - endX, endX);
}
if (endY < posNew) {
dispatchAdditions(postponedUpdates, batchingCallback, endX, posNew - endY, endY);
}
for (int i = snakeSize - 1; i >= 0; i--) {
if ((mOldItemStatuses[snake.x + i] & FLAG_MASK) == FLAG_CHANGED) {
batchingCallback.onChanged(snake.x + i, 1,
mCallback.getChangePayload(snake.x + i, snake.y + i));
}
}
posOld = snake.x;
posNew = snake.y;
}
batchingCallback.dispatchLastEvent();
}
private static PostponedUpdate removePostponedUpdate(List updates, int pos, boolean removal) {
for (int i = updates.size() - 1; i >= 0; i--) {
final PostponedUpdate update = updates.get(i);
if (update.posInOwnerList == pos && update.removal == removal) {
updates.remove(i);
for (int j = i; j < updates.size(); j++) {
// offset other ops since they swapped positions
updates.get(j).currentPos += removal ? 1 : -1;
}
return update;
}
}
return null;
}
private void dispatchAdditions(List postponedUpdates, ListUpdateCallback updateCallback,
int start, int count, int globalIndex) {
if (!mDetectMoves) {
updateCallback.onInserted(start, count);
return;
}
for (int i = count - 1; i >= 0; i--) {
int status = mNewItemStatuses[globalIndex + i] & FLAG_MASK;
switch (status) {
case 0: // real addition
updateCallback.onInserted(start, 1);
for (PostponedUpdate update : postponedUpdates) {
update.currentPos += 1;
}
break;
case FLAG_MOVED_CHANGED:
case FLAG_MOVED_NOT_CHANGED:
final int pos = mNewItemStatuses[globalIndex + i] >> FLAG_OFFSET;
final PostponedUpdate update = removePostponedUpdate(postponedUpdates, pos, true);
// the item was moved from that position
// noinspection ConstantConditions
updateCallback.onMoved(update.currentPos, start);
if (status == FLAG_MOVED_CHANGED) {
// also dispatch a change
updateCallback.onChanged(start, 1, mCallback.getChangePayload(pos, globalIndex + i));
}
break;
case FLAG_IGNORE: // ignoring this
postponedUpdates.add(new PostponedUpdate(globalIndex + i, start, false));
break;
default:
throw new IllegalStateException(
"unknown flag for pos " + (globalIndex + i) + " " + Long.toBinaryString(status));
}
}
}
private void dispatchRemovals(List postponedUpdates, ListUpdateCallback updateCallback,
int start, int count, int globalIndex) {
if (!mDetectMoves) {
updateCallback.onRemoved(start, count);
return;
}
for (int i = count - 1; i >= 0; i--) {
final int status = mOldItemStatuses[globalIndex + i] & FLAG_MASK;
switch (status) {
case 0: // real removal
updateCallback.onRemoved(start + i, 1);
for (PostponedUpdate update : postponedUpdates) {
update.currentPos -= 1;
}
break;
case FLAG_MOVED_CHANGED:
case FLAG_MOVED_NOT_CHANGED:
final int pos = mOldItemStatuses[globalIndex + i] >> FLAG_OFFSET;
final PostponedUpdate update = removePostponedUpdate(postponedUpdates, pos, false);
// the item was moved to that position. we do -1 because this is a move not
// add and removing current item offsets the target move by 1
// noinspection ConstantConditions
updateCallback.onMoved(start + i, update.currentPos - 1);
if (status == FLAG_MOVED_CHANGED) {
// also dispatch a change
updateCallback.onChanged(update.currentPos - 1, 1,
mCallback.getChangePayload(globalIndex + i, pos));
}
break;
case FLAG_IGNORE: // ignoring this
postponedUpdates.add(new PostponedUpdate(globalIndex + i, start + i, true));
break;
default:
throw new IllegalStateException(
"unknown flag for pos " + (globalIndex + i) + " " + Long.toBinaryString(status));
}
}
}
@VisibleForTesting
List getSnakes() {
return mSnakes;
}
}
/**
* Represents an update that we skipped because it was a move.
*
* 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;
}
}
}