Package 

Class DiffUtil

  • 
public class DiffUtil

    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 ListAdapter and 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 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(MN) time where M is the total number of added items and N is the total number of 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.

    • Nested Class Summary

      Nested Classes 
      Modifier and Type Class Description
      public abstract class DiffUtil.Callback

      A Callback class used by DiffUtil while calculating the diff between two lists.
      public abstract class DiffUtil.ItemCallback

      Callback for calculating the diff between two non-null items in a list.

      Callback serves two roles - list indexing, and item diffing. ItemCallback handlesjust the second of these, which allows separation of code that indexes into an array or Listfrom the presentation-layer and content specific diffing code.
      public class DiffUtil.DiffResult

      This class holds the information about the result of a calculateDiff call.

      You can consume the updates in a DiffResult via dispatchUpdatesTo or directly stream the results into a RecyclerView.Adapter via dispatchUpdatesTo.

    • Method Detail

      • calculateDiff

        @NonNull() static DiffUtil.DiffResult calculateDiff(@NonNull() DiffUtil.Callback cb, boolean detectMoves)

        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 (swappositions), you can disable move detection which takes O(N^2) time whereN is the number of added, moved, removed items.

        Parameters:
        cb - The callback that acts as a gateway to the backing list data
        detectMoves - True if DiffUtil should try to detect moved items, false otherwise.