In the WWDC 2019 session of Advances in UI Data Sources, Apple introduced Diffable Data Source to avoid the complexity and some exceptions like below when updating a table/collection view.

Terminating app due to uncaught exception ‘NSInternalInconsistencyException’, reason: ‘Invalid update: invalid number of sections. The number of sections contained in the collection view after the update (10) must be equal to the number of sections contained in the collection view before the update (10), plus or minus the number of sections inserted or deleted (0 inserted, 1 deleted).’


Let’s look at the declaration of UICollectionViewDiffableDataSource.

class UICollectionViewDiffableDataSource<SectionIdentifierType,ItemIdentifierType> : NSObject
where SectionIdentifierType : Hashable, ItemIdentifierType : Hashable

The requirement is the same for other Diffable Data Source classes. We need two types that implemented Hashable, one for the section, one for the cell. Diffable Data Source uses them to determine which kind of update ( insertion , deletion , and rearrangement ) should be done.

Then create the instance of Diffable Data Source for Our table/collection view with a CellProvider.

// The instance will set itself as the data source by this initialization
let dataSource = UICollectionViewDiffableDataSource
    <Section, Item>(collectionView: collectionView) {
        collectionView, indexPath, item in
        // Setup our cell

After the preparations above, we can update our table/collection view by one line code.

dataSource.apply(snapshot, animatingDifferences: true)

But what about the snapshot?
It’s just as its name a snapshot of our table/collection view, we can get the current snapshot by dataSource.snapshot() then edit it or just create a new one.

var snapshot = NSDiffableDataSourceSnapshot<Section, Item>()

Then just apply it to the data source.
The data source will do all complexity works for us and perform proper animations if we need it.


The usage is simple, but there are also some obstructs we have to face.

iOS 13/macOS 10.15 or Later

The biggest obstruct we have to face since we can just update our table/collection view with reloadData() without animation and exceptions caused by the animation.

No documentation

There is no documentation at all for now, so it’s hard to solve issues we didn’t expect.

No Support for Cell Content Change Animation

Like I mentioned above, it only handles the animation for insertion, deletion, and rearrangement. If the new cell is not equal to the old cell, the animation will be a deletion for the old cell and an inserting for the new cell. Otherwise, the animation will be just a rearrangement for the cell without any cell content change.

There is a new protocol in Xcode 11 named Identifiable, it seems suitable to combine with Hashable to determine a cell content change by == && newItem != oldItem. But there is nothing change after I implement Identifiable. I am not sure if I have done something wrong since there is no documentation at all.


It’s time to use Diffable Data Source if our project is targeting iOS 13/macOS 10.15 and using a static data source which has no content change for any item.

Otherwise, we need to implement the data source to accomplish our requirements or use an alternative that has accomplished our requirements.