What is motion?

Motion is a tool for iOS, macOS, and tvOS developers who want to build user interfaces that feel incredibly fluid and responsive to touch. Instead of relying on standard, predefined animations, it lets you create animations based on real-world physics, like springs and momentum. This is especially useful for building interfaces where users drag, flick, or interact with elements on screen, allowing those elements to react naturally to the speed and direction of a user's gesture. At its core, the library lets a developer set up an animation, give it a target destination, and start it. As the animation runs, it constantly reports back its current value, which the developer uses to update the position or size of an on-screen element. What makes it special is that these animations can be interrupted. If a user grabs an element while it is still moving, the animation seamlessly stops, catches the exact position and speed of the user's touch, and redirects the movement without any jarring jumps or glitches. This project is built for app developers who care deeply about creating highly crafted, delightful user experiences. For example, if you are building a messaging app where a user can drag a chat bubble around the screen, or a photo gallery where swiping causes images to bounce back into place, this tool handles the complex math to make that dragging and bouncing feel perfectly natural. It works alongside standard Apple interface tools and even supports modern SwiftUI apps. One notable thing about the project is how it prioritizes performance. It uses a technology called SIMD, which allows it to process multiple numbers at the exact same time rather than one by one, making the physics calculations extremely fast. However, it makes a specific tradeoff: unlike some built-in Apple animations that run outside the app in a separate protected process, this tool runs directly inside the app. This means developers need to be mindful not to overload their app while these animations are running, treating them with the same care they would a scrolling feed.