When was the last time you scrolled through a list? Odds are it was today, maybe even seconds ago. Iterating over a list of items is a feature common to many frameworks, and React Native’s FlatList is no different. The FlatList component renders items in a scrollable view without you having to worry about memory consumption and layout management (sort of, we’ll explain later).

The challenge, as many developers can attest to, is getting FlatList to perform on a range of devices without display artifacts like drops in UI frames per second (FPS) and blank items while scrolling fast.

Our React Native Foundations team solved this challenge by creating FlashList, a fast and performant list component that can be swapped into your project with minimal effort. The requirements for FlashList included

• High frame rate: even when using low-end devices, we want to guarantee the user can scroll smoothly, at a consistent 60 FPS or greater.
• No more blank cells: minimize the display of empty items caused by code not being able to render them fast enough as the user scrolls (and causing them to wonder what’s up with their app).
• Developer friendly: we wanted FlashList to be a drop-in replacement for FlatList and also include helpful features beyond what the current alternatives offer.

The FlashList API is five stars. I've been recommending all my friends try FlashList once we open source.

FlatList vs FlashList. This is from the exact same sample. Almost 7.5x better JS FPS!
When items are complex or there are more to update per render, the differences become a lot more obvious.
This is on Moto G, scroll speed is as fast as possible. pic.twitter.com/eT7LGOjHGE

— Talha Naqvi (@naqvitalha) July 13, 2022

Here’s how we approached the FlashList project and its benefits to you.

The Hit List: Why the World Needs Faster Lists

Evolving FlatList was the perfect match between our mission to continuously create powerful components shared across Shopify and solving a difficult technical challenge for React Native developers everywhere. As more apps move from native to React Native, how could we deliver performance that met the needs of today’s data-hungry users while keeping lists simple and easy to use for developers?

Lists are in heavy use across Shopify, in everything from Shop to POS. For Shopify Mobile in particular, where over 90% of the app uses native lists, there was growing demand for a more performant alternative to FlatList as we moved more of our work to React Native. 

What About RecyclerListView?

RecyclerListView is a third-party package optimized for rendering large amounts of data with very good real-world performance. The difficulty lies in using it, as developers must spend a lot of time understanding how it works, playing with it, and requiring significant amounts of code to manage. For example, RecyclerListView needs predicted size estimates for each item and if they aren’t accurate, the UI performance suffers. It also renders the entire layout in JavaScript, which can lead to visible glitches.

When done right, RecyclerListView works very well! But it’s just too difficult to use most of the time. It’s even harder if you have items with dynamic heights that can’t be measured in advance.

So, we decided to build upon RecyclerListView and solve these problems to give the world a new list library that achieved native-like performance and was easy to use.

The Bucket List: Our Approach to Improving React Native Lists

Our React Native Foundations team takes a structured approach to solving specific technical challenges and creates components for sharing across Shopify apps. Once we’ve identified an ambitious problem, we develop a practical implementation plan that includes rigorous development and testing, and an assessment of whether something should be open sourced or not. 

Getting list performance right is a popular topic in the React Native community, and we’d heard about performance degradations when porting apps from native to React Native within Shopify. This was the perfect opportunity for us to create something better. We kicked off the FlashList project to build a better list component that had a similar API to FlatList and boosted performance for all developers. We also heard some skepticism about its value, as some developers rarely saw issues with their lists on newer iOS devices.

The answer here was simple. Our apps are used on a wide range of devices, so developing a performant solution across devices based on a component that was already there made perfect sense.

“We went with a metrics-based approach for FlashList rather than subjective, perception-based feelings. This meant measuring and improving hard performance numbers like blank cells and FPS to ensure the component worked on low-end devices first, with high-end devices the natural byproduct.” - David Cortés, React Native Foundations team

Just replaced Flatlist for Flashlist and wow, that thing is really fast @ShopifyDevs well done 🎉🥳👏🎊

— João Amadeu (@jmamadeu) July 4, 2022

Improved Performance and Memory Utilization

Our goal was to beat the performance of FlatList by orders of magnitude, measured by UI thread FPS and JavaScript FPS. With FlatList, developers typically see frame drops, even with simple lists. With FlashList, we improved upon the FlatList approach of generating new views from scratch on every update by using an existing, already allocated view and refreshing elements within it (that is, recycling cells). We also moved some of the layout operations to native, helping smooth over some of the UI glitches seen when RecyclerListView is provided with incorrect item sizes.

This streamlined approach boosted performance to 60 FPS or greater—even on low-end devices!

Simple list comparison (Twitter clone) @ShopifyEng FlashList vs @reactnative FlatList
on a low end Android

On the left, trying my best to scroll the FlatList 😅: the scroll locks because JS is dead! ☠️
Right side: no locking! ✅

Will post #performance metrics next week 📈 pic.twitter.com/IW8Ly5c1jD

— Alexandre Moureaux (@almouro) July 1, 2022

We applied a similar strategy to improve memory utilization. Say you have a Twitter feed with 200-300 tweets per page, FlatList starts rendering with a large number of items to ensure they’re available as the user scrolls up or down. FlashList, in comparison, requires a much smaller buffer which reduces memory footprint, improves load times, and keeps the app significantly more responsive. The default buffer is just 250 pixels.

Both these techniques, along with other optimizations, help FlashList achieve its goal of no more blank cells, as the improved render times can keep up with user scrolling on a broader range of devices. 

Shopify Mobile is using FlashList as the default and the Shop team moved search to FlashList. Multiple teams have seen major improvements and are confident in moving the rest of their screens.

These performance improvements included extensive testing and benchmarking on Android devices to ensure we met the needs of a range of capabilities. A developer may not see blank items or choppy lists on the latest iPhone but that doesn’t mean it’ll work the same on a lower-end device. Ensuring FlashList was tested and working correctly on more cost-effective devices made sure that it would work on the higher-end ones too.

Developer Friendly

If you know how FlatList works, you know how FlashList works. It’s easy to learn, as FlashList uses almost the same API as FlatList, and has new features designed to eliminate any worries about layout and performance, so you can focus on your app’s value.

Shotgun's main feature is its feed, so ensuring consistent and high-quality performance has always been crucial. That's why using FlashList was a no brainer. I love that compared to the classic FlatList, you can scrollToIndex to an index that is not within your view. This allowed us to quickly develop our new event calendar list, where users can jump between dates to see when and where there are events.

The fact that FlashList's API is exactly the same as FlatList is an application that will make it very easy to move your project to FlashList. In addition, they have added a few features, for example: You can see how long it takes to load the list thanks to the onLoad feature. pic.twitter.com/Izkwyte2Xj

— V. Furkan Güner (@vfurkanguner) July 4, 2022

It takes seconds to swap your existing list implementation from FlatList to FlashList. All you need to do is change the component name and optionally add the estimatedItemSize prop, as you can see in this example from our documentation:

Powerful Features

Going beyond the standard FlatList props, we added new features based on common scenarios and developer feedback. Here are three examples:

• getItemType: improves render performance for lists that have different types of items, like text vs. image, by leveraging separate recycling pools based on type.

• Flexible column spans: developers can create grid layouts where each item’s column span can be set explicitly or with a size estimate (using overrideItemLayout), providing flexibility in how the list appears.
• Performance metrics: as we moved many layout functions to native, FlashList can extract key metrics for developers to measure and troubleshoot performance, like reporting on blank items and FPS. This guide provides more details.

Documentation and Samples

We provide a number of resources to help you get started with FlashList:

• FlashList developer portal
• FlashList documentation
• Example app using FlashList
• How to use estimatedItemSize
• How to write performant components

Flash Forward with FlashList

Accelerate your React Native list performance by installing FlashList now. It’s already deployed within Shopify Mobile, Shop, and POS and we’re working on known issues and improvements. We recommend starring the FlashList repo and can’t wait to hear your feedback!

Want to read more about the process of launching this open source project? Check out our follow-up post, What We Learned from Open-Sourcing FlashList.

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