Скачать или смотреть Solving the UICollectionView Prefetching Issue with Compositional Layouts

Discover how to enable `UICollectionViewDataSourcePrefetching` in your UICollectionView with compositional layouts. Learn about fixes and workarounds for estimated heights.
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Solving the UICollectionView Prefetching Issue with Compositional Layouts

When working with UICollectionView in iOS development, many developers have encountered a frustrating problem: prefetching has stopped functioning as expected when using compositional layouts. This issue can significantly impact app performance, especially when displaying a large number of items dynamically. In this guide, we'll dive into understanding this problem and explore practical solutions to get your prefetching back on track.

The Problem: Prefetching Disables

What is Prefetching?

Prefetching in UICollectionView allows for the loading of data for upcoming cells before they are displayed on screen. This is crucial for creating smooth, fast-scrolling experiences in your apps. When prefetching is disabled, users experience delays and a laggy interface, which can lead to frustration.

The Specific Issue with Compositional Layouts

Let's take a closer look at a simple implementation provided by a fellow developer. They set up a UICollectionView with a compositional layout, expecting it to handle dynamic cell heights seamlessly. However, they noticed that the collectionView(_:prefetchItemsAt:) method was only called once when the collection view was first displayed, and never again during scrolling.

Here's a brief explanation of the sample code they shared:

[[See Video to Reveal this Text or Code Snippet]]

In the initial implementation, this function was called only once, highlighting the issue at hand.

The Solution: Fixing Prefetching

Investigating the Cause

Upon further examination, the developer found that the issue lay in how height dimensions were set in the layout. Specifically, using .estimated(44) for the heightDimension in the compositional layout was causing prefetching to fail. By changing this to .absolute(44), prefetching began to work again.

However, this comes at a cost—using an absolute height defeats the purpose of having flexible, dynamic cell heights capable of displaying multi-line text.

Apple's Feedback and Resolution

In a turn of events, an update from Apple indicated that this issue had been resolved in later versions of Xcode. Testing in Xcode 13.1 showed that prefetching now works correctly even with estimated height dimensions. For those who may still be using older versions of Xcode or iOS, here's a workaround to consider:

Fallback to Flow Layout: As noted in the original post, switching to a more traditional flow layout can resolve prefetching issues by allowing for calculated individual cell heights. This approach can ensure smoother performance while maintaining prefetching capabilities.

Summary of Solutions

To summarize, here are the potential steps you can take to solve the prefetching issue in your UICollectionView:

Update Xcode: First and foremost, ensure that you are running the latest version of Xcode (recommended: 13.1 or later) to benefit from fixes related to prefetching.

Adjust Layout: If updating is not an option, consider switching from compositional layouts to a flow layout, where you can manually calculate the height of each cell.

Experiment with Heights: If you still prefer using compositional layouts, adjust the height setting. Test using both absolute and estimated to see how it affects performance.

Conclusion

Working with UICollectionView and compositional layouts can significantly enhance the UI of your iOS applications, especially when handling dynamic content. Although prefetching issues can be a hurdle, updates from Apple have made strides toward a more seamless development experience. By keeping your Xcode updated and knowing how to troubleshoot or work around potential issues, you can ensure your app re