This is an attempt at implementing the solution outlined in Dave DeLong’s article, Edit distance and edit steps.
A Changeset
describes the minimal edits required to go from one Collection
of Equatable
elements to another.
It has been written primarily to be used in conjunction with UITableView
and UICollectionView
data sources by detecting additions, deletions, substitutions, and moves between the two sets of data. But it can also be used to compute more general changes between two data sets.
The following code computes the minimal edits of the canonical example, going from the String
collections “kitten” to “sitting”:
let changeset = Changeset(source: "kitten", target: "sitting")
print(changeset)
// 'kitten' -> 'sitting':
// replace with s at offset 0
// replace with i at offset 4
// insert g at offset 6
The following assertion would then succeed:
let edits = [
Changeset<String>.Edit(operation: .substitution, value: "s", destination: 0),
Changeset<String>.Edit(operation: .substitution, value: "i", destination: 4),
Changeset<String>.Edit(operation: .insertion, value: "g", destination: 6),
]
assert(changeset.edits == edits)
If you don’t want the overhead of Changeset
itself, which also stores the source and target collections, you can call edits
directly (here with example data from Apple’s Table View Programming Guide for iOS):
let source = ["Arizona", "California", "Delaware", "New Jersey", "Washington"]
let target = ["Alaska", "Arizona", "California", "Georgia", "New Jersey", "Virginia"]
let edits = Changeset.edits(from: source, to: target)
print(edits)
// [insert Alaska at offset 0, replace with Georgia at offset 2, replace with Virginia at offset 4]
Note that Changeset uses offsets, not indices, to refer to elements in the collections. This is mainly because Swift collections aren’t guaranteed to use zero-based integer indices. See discussion in issue #37 for more details.
The offset values can be used directly in the animation blocks of beginUpdates
/endUpdates
on UITableView
and performBatchUpdates
on UICollectionView
in that Changeset
follows the principles explained under Batch Insertion, Deletion, and Reloading of Rows and Sections in Apple’s guide.
In short; first all deletions and substitutions are made, relative to the source collection, then, relative to the resulting collection, insertions. A move is just a deletion followed by an insertion.
In the iOS framework, two convenience extensions (one on UITableView
and one on UICollectionView
) have been included to make animated table/collection view updates a breeze. Just call update
, like this:
tableView.update(with: changeset.edits)
By default a Changeset
uses ==
to compare elements, but you can write your own comparator, illustrated below, where the occurence of an “a” always triggers a change:
let alwaysChangeA: (Character, Character) -> Bool = {
if $0 == "a" || $1 == "a" {
return false
} else {
return $0 == $1
}
}
let changeset = Changeset(source: "ab", target: "ab", comparator: alwaysChangeA)
As a result, the changeset will consist of a substitution of the “a” (to another “a”):
let expectedEdits: [Changeset<String>.Edit] = [Changeset.Edit(operation: .substitution, value: "a", destination: 0)]
assert(changeset.edits == expectedEdits)
One possible use of this is when a cell in a UITableView
or UICollectionView
shouldn’t animate when they change.
The Xcode project also contains a target to illustrate the usage in an app:
This uses the extensions mentioned above to animate transitions based on the edits of a Changeset
.
This project is available under The MIT License.
Copyright © 2015-18, Joachim Bondo. See LICENSE file.