February 23, 2024

A brand new rendering engine with vital efficiency enhancements powered by Core Animation

By: Cal Stephens

Lottie is Airbnb’s cross-platform, open source library for rendering vector movement graphics. We use Lottie extensively at Airbnb, and it additionally powers animations in hundreds of different apps all through the business.

Instance Lottie animations included in Airbnb’s iOS app

Right now we’re releasing Lottie 4.0 for iOS. This main new launch brings vital efficiency enhancements to all Lottie animations, with a model new rendering engine powered by Core Animation.

Utilizing Lottie at scale for a few years, we’ve realized lots about its efficiency traits in real-world use circumstances. We discovered that it was comparatively widespread for Lottie animations to drop frames in a few of our extra advanced screens. To know why, we first have to try how Lottie beforehand rendered animations.

Earlier variations of Lottie performed animations on the app’s principal thread, successfully utilizing a CADisplayLink. As soon as per body, Lottie would execute code on the primary thread to advance the progress of the animation and re-render its content material. This meant that animations would devour 5–20%+ of the CPU whereas taking part in, leaving fewer CPU cycles obtainable for the remainder of the app:

Taking part in an animation with Lottie 3.5.0, utilizing the unique principal thread rendering engine

This additionally meant that animations wouldn’t replace when the primary thread was busy. This might trigger animations to drop frames or freeze solely, which ends up in a poor person expertise:

Lottie animations dropping frames when the primary thread is overloaded

These points are inherent limitations of utilizing a main-thread-bound rendering structure.

On iOS, probably the most performant and power-efficient solution to play animations is through the use of Core Animation. This technique framework renders animations out-of-process with GPU {hardware} acceleration. Animation playback is managed by a separate system course of known as the “render server”. This implies Core Animation-powered animations don’t contribute to the CPU utilization of the app course of itself, and may proceed even when its principal thread is blocked or busy.

All through 2022, we’ve been engaged on a brand new rendering engine implementation for Lottie constructed on prime of Core Animation. For every of the layers within the animation JSON file, the brand new engine builds a CALayer and applies CAAnimations with keyframes for the layer’s animated properties. Lottie passes these animation keyframes off to Core Animation, which takes care of truly rendering them on-screen and updating the animation every body.

This new engine eliminates the CPU overhead from taking part in a Lottie animation, and successfully ensures that Lottie animations will animate easily at 60 or 120 fps whatever the app’s CPU load.

Taking part in an animation with Lottie 4.0, utilizing the brand new Core Animation rendering engine

Since animations rendered by the brand new engine don’t execute any code on the app’s principal thread, apps now have extra sources obtainable for different performance. That is particularly invaluable when operating duties with excessive CPU load. For instance, the Airbnb app shows a Lottie animation when beginning up for the primary time. We ran an experiment right here and located that switching to the brand new rendering engine reduces our app’s whole launch time, whereas additionally bettering the frame-rate and UX of the startup animation.

We first introduced the Core Animation rendering engine in Lottie 3.4.0 earlier this 12 months, behind an opt-in characteristic flag. We’ve been utilizing the brand new engine by default for all Lottie animations within the Airbnb app for over six months, and have been onerous at work fixing points reported by early-adopters in the neighborhood.

Beginning in right now’s Lottie 4.0 launch for iOS, the Core Animation rendering engine is enabled by default for all apps utilizing Lottie, with no further work or migration required by app builders. It is a main milestone that we’ve been working in the direction of for a very long time, and we hope it helps increase the bar for animation high quality and efficiency even larger all through the business!

Lottie 4.0 for iOS additionally consists of a number of vital enhancements contributed by members of the group:

  • Help for dotLottie animation files, that are a lot smaller in dimension than customary JSON recordsdata
  • A brand new animation decoding implementation that’s ~2x quicker than the earlier Codable-based implementation

You possibly can be taught extra about Lottie, and our dedication to open supply, in earlier posts we’ve revealed:

All in favour of working at Airbnb? Try these open roles:

Staff Software Engineer, Wishlists

Staff Software Engineer, Guests & Hosts

Many due to Eric Horacek for first proposing this venture and reviewing 100+ pull requests over the previous 12 months. Additionally due to Brandon Withrow, the unique writer of Lottie, plus the many other contributors who’ve helped out over time.