diffutil.dart

Calculate the difference between two lists.

Heavily inspired bei Android's DiffUtil class, the code was adopted for Dart.

Uses Myers algorithm internally. For an explanation of how the algorithm works internally, for details I recommend this great series of articles: https://blog.jcoglan.com/2017/02/12/the-myers-diff-algorithm-part-1/

What is this good for?

There are often situations, where an app displays a list of items, which are fetched from an external data source like a server endpoint or a database, and updates are not sent as delta, but as a whole new list.

It can be useful to take the old list and the new list and calculate the difference between those two, for example when animating the insertion and removal of new items in the displayed list (See diffutil_sliverlist).

This package does exactly that: It takes two lists and calculates the difference (or to be more accurate: edit script) between those two lists as list of Insert, Remove, Change (only when using custom delegates) and Move (when enabled) operations.

Usage

Calculating diffs:

Simple usage:

final diffResult = calculateListDiff<int>([1, 2 ,3], [1, 3, 4]);

Custom equality:

final diffResult = calculateListDiff<YourClassWithId>(oldList, newList, (o1, o2) => o1.id == o2.id);

If you don't want to use plain old Dart lists (for example if you're using built_value or kt.dart), and don't want to convert your custom list to standard lists, you can use the calculateDiff function and implement your own DiffDelegate easily.

Or use calculateCustomListDiff and CustomListDiffDelegate.

Move detection is disabled by default.

Using the result:

Call .getUpdates() on the diffResult to get a List of DiffUpdate objects. These are sealed classes of type Insert, Remove, Change or Move. Move operations are only calculated if calculateListDiff was called with detectMoves: true

The order of these operations matters! They describe how to turn the previous list into the new list, when applied in the order as they are returned.

  for (final update in diffResult.getUpdates())
    update.when(
      insert: (pos, count) => print("inserted $count on $pos"),
      remove: (pos, count) => print("removed $count on $pos"),
      change: (pos, payload) => print("changed on $pos with payload $payload"),
      move: (from, to) => print("move $from to $to"),
    );

By default, Insert and Remove Operations are batched. (e.g. multiple consecutive inserts or removes are represented by a single Insert/Remove object with a count field > 1). If you want to turn off edit script batching, call getUpdates(batch: false). This means, every Insert and Remove operation will have a count of 1 and the edit script of [] and [1, 2] will be

[Insert(position: 0, count : 1), Insert(position: 0, count :1 )]

instead of

[Insert(position: 0, count: 2)]

Updates with data

If you need the concrete items that have been inserted/removed/changed/moved, call getUpdatesWithData().

     diffutil.calculateListDiff([1, 2, 3], [1, 0, 3]).getUpdatesWithData();

returns

      [
        DataRemove(position: 1, data: 2),
        DataInsert(position: 1, data: 0)
      ];

The result of getUpdatesWithData() cannot be batched. You can use the resulting Iterable<DataDiffUpdate> like this:

 for (final update in updates) {
      update.when(
        insert: (pos, data) => print('insert $pos $data'),
        remove: (pos, data) => print('remove $pos $data'),
        change: (pos, oldData, newData) => print('change on $pos from $oldData to $newData'),
        move: (from, to, data) => print('move $data from $from to $to'),
      );
    }

Note that if you implement you own DiffDelegate and call calculateDiff() directly, the DiffDelegate also needs to implement IndexableItemDiffDelegate if you want to call getUpdatesWithData().

Change Operations

For data objects that have some meaningful identity (like an unique id field), you can enable the calculation of Change operations. For this, you need to implement your own DiffDelegate and override the areItemsTheSame method.

Let's say we have a class called DataObject which is defined like this:

class DataObject {
  final int id;
  final int? payload;

  DataObject({required this.id, this.payload});
}

The delegate can be implemented like this:

class DataObjectListDiff extends diffutil.ListDiffDelegate<DataObject> {
  DataObjectListDiff(List<DataObject> oldList, List<DataObject> newList)
      : super(oldList, newList);

  @override
  bool areContentsTheSame(int oldItemPosition, int newItemPosition) {
    return equalityChecker(oldList[oldItemPosition], newList[newItemPosition]);
  }

  @override
  bool areItemsTheSame(int oldItemPosition, int newItemPosition) {
    return oldList[oldItemPosition].id == newList[newItemPosition].id;
  }
}

With this, Change Operations are emitted if the id of an object stays the same between oldList and newList, but it's data changes.

Performance metrics:

Same as Android's DiffUtil:

• O(N) space
• O(N + D^2) time performance where D is the length of the edit script.
• additional O(N^2) time where N is the total number of added and removed items if move detection is enabled

The edit script is the smallest set of operations needed to transform the first list into the second list.