DiffUtil is a utility class that calculates the differences between two lists and outputs the minimal number of updates needed to convert one list into the other. It uses the Myers difference algorithm to efficiently find changes and can detect moved items. When used with a RecyclerView adapter, DiffUtil calculates the differences in the background and applies the results on the main thread to improve performance for large lists.

1. 淺談Diffutil RecyclerView
Chocolabs Android Developer
Louis
2017.3.20 Android Taipei

2. RecyclerView can be improved funter ?
adapter.notifyDataSetChanged();

3. Summary
A. DiffUtil is a utility class that can calculate the difference between two lists and output 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.
B. 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.
C. If the lists are large, this operation may take significant time so you are advised to run this on a
background thread, get the DiffUtil.DiffResult then apply it on the RecyclerView on the main thread.

4. Algorithm
A. 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.
B. 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.
a. 100 items and 10 modifications: avg: 0.39 ms, median: 0.35 ms
b. 100 items and 100 modifications: 3.82 ms, median: 3.75 ms
c. 100 items and 100 modifications without moves: 2.09 ms, median: 2.06 ms
d. 1000 items and 50 modifications: avg: 4.67 ms, median: 4.59 ms

6. extends DiffUtil.Callback

7. Implementation

9. Do we really need this …?

10. For some case
If u want to handle the animation
If u just only need to consider what
exactly data change between two list(same length)

11. Thanks

12. Reference
https://medium.com/wiselteach/diffutils-improving-performance-of-recyclerview-
102b254a9e4a#.4v1o7u22c
https://developer.android.com/reference/android/support/v7/util/DiffUtil.html