Inertia

Inertia is a keyframe animation editor for animating the UI you already built. You wrap the views you want to move, run your app inside the editor, drag those views around on a timeline, and Inertia writes the result to a MessagePack file your app loads.

There is no separate rendering surface and no exported video. The thing you animate in the editor is the real view in your real app.

Three runtimes read the animations the editor writes:

Runtime	Package	Editor target
SwiftUI	Inertia (Swift package)	iOS — a live iOS Simulator
Jetpack Compose	com.github.hpennington:inertia-compose	Android — a running emulator
React	inertia-react + inertia-base	Web — your dev server in a web view

They implement the same model against the same file format. Choosing a runtime is the page to read if you want to know where they still differ — and they do, in ways worth knowing before you start.

• Install a runtime

  Add the package to your app and wrap your root view.

• Quickstart

  Get a view moving in about ten minutes.

• Use the editor

  Record keyframes against a live app.

• Animation file format

  What the editor writes, and what the runtimes read.

How it fits together

In editor mode the editor hosts a local WebSocket server and your app connects to it. The app reports its Inertia-tagged view hierarchy, and the editor pushes animation schemas back as you edit, so what you see running is the animation as it currently stands.

In release mode none of that is running. The container loads animation.inertia for itself — out of the app bundle in SwiftUI, over HTTP in React — and plays it on its own clock. In SwiftUI the WebSocket client is gated on the same dev flag, so a shipped build never dials out. (The Compose runtime does not yet have a release path; see Choosing a runtime.)

What you can animate

Each tagged view gets a track of keyframes over five values:

Value	Meaning
translate	[x, y] offset, as a fraction of the container's size
scale	Uniform scale factor (1.0 is unchanged)
rotate	Rotation in degrees, anchored top-left
rotateCenter	Rotation in degrees, anchored at the view's center
opacity	0.0 transparent through 1.0 opaque

See Animatable values for the details, including why translate is normalized rather than in points.

Platform support

The editor is a macOS app. What it drives depends on the target you pick in its framework picker:

Target	Runtime requirement	How the editor reaches it
iOS	iOS 17+ / Swift 5.9+	An iOS Simulator, driven through simctl
Android	minSdk 26, Compose	A running emulator, streamed over adb
Web	React 18.3	Your dev server, loaded in a WKWebView

The iOS 17 floor is not arbitrary: the SwiftUI runtime plays tracks through KeyframeAnimator, an iOS 17 API. The Compose and React runtimes sample their tracks themselves and have no equivalent floor.

Where to go next

Start with Choosing a runtime to see what each one can do today, then Installation. If you would rather read about the workflow before touching your project, the editor tour is the shortest route to understanding what Inertia actually does.