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The Tetris Diffing Competition

This is a comparison of diffing frameworks for UIKit table views and collection views. I wrote a blog post called Collection View Tetris, where I presented a fun hack: Tetris implemented using a quite ordinary Collection View and its default animation. This requires that all possible animations are implemented – inserts, deletes and moves of items and sections, including moving items from one section to another.

After writing that, I got curious: how do existing diffing libs meet the task of being the diffing engine from Collection View Tetris? I did this little comparison of existing diffing libs in September 2018. Then, in 2019, Apple announced their own diffable data sources to be released with iOS 13. I recommend the WWDC talk "Advances in UI Data Sources" for a description of this API.

The (slightly ad hoc) rules

There is a whole bunch of diffing frameworks available for iOS, but to be eligble for the Tetris Diffing Competition, they have to meet the following:

  • There has to be a diffing API that handles both items and sections.
  • There has to also be a method for running the whole animation – as you can read about in my blog post, supporting all of this is non-trivial and requires more than the diffing itself.
  • I have to understand how to use it in reasonable time.

Three of the libraries I found in 2018 seemed to fit the criteria. In 2019, UIKit itself was added.

The results

Framework Version Result Comment
UIKit 13 beta 2 🎉 Works just as intended.
DifferenceKit 1.1.3 🎉 Works just as intended.
Dwifft 0.9 😐 Works, but does not seem to animate moves.
Differ 1.4.3 😞 Does not work.

Out of the third party frameworks, the clear winner here is DifferenceKit, which was the only library that truly passed the test. Whether or not you want to take this as an indicator of general quality of the framework, I'm not sure – but I happen to think that this library does seem like a good choice for your general purpose diffing needs. It has a flexible and powerful API, uses a performant algorithm and is well documented.

There are some things I think could be improved. Notably, it does not allow you to pass on a completion handler to the performBatchUpdates methods. This seem to be something it has in common with many other of these frameworks; indeed, not even UIKit's UICollectionViewDiffableDataSource

My advice would be, for any app that is able to use iOS 13 as deployment target, to use UIKit's own diffable data sources. Until then, use DifferenceKit.

Oh, I should mention that there is one more diffing framework that is up to the task – my own SKRBatchUpdates, which is what led me to do this Tetris thing. It is, however, not packaged for general consumtion as a framework – any of these other links here will quite probably better fulfill your diffing needs.

Other diffing frameworks

Here's another nice thing about DifferenceKit: its README has a nice comparison of available diffing frameworks. So I will refer you to that. It may very well be another one that best matches your needs.

Prove me wrong with a Pull Request!

This repository contains an app where you can test Collection View Tetris with each of the tested frameworks. If you find that I'm being unfair towards some framework, using it wrong, or if there's just something missing – please file a pull request. The competition is always up for re-evaluation.

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I'm sure your diffing library is great, but can you play Tetris with it?

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