lottie_fixup

Fixes Lottie/Bodymovin exports that crash or freeze the lottie Flutter package: audio layers, empty precomps, and expressions that lottie doesn't execute (loopOut()/ loopIn(), wiggle(), random(), time-based motion, cross-layer links).

Features

Stops the audio-layer crash — After Effects audio layers ("ty": 6) ship without the transform block lottie expects, which throws Null check operator used on a null value. This package strips them.
Bakes loopOut()/loopIn() expressions into real keyframes, so looping animations don't freeze after their first cycle (lottie doesn't execute expressions). All four After Effects loop modes are supported in both directions — 'cycle', 'pingpong', 'offset', 'continue' — the latter two on any numeric property (position, scale, rotation, opacity...) — plus the duration-based loopOutDuration()/loopInDuration() variants in 'cycle'/'pingpong' mode.
Bakes other expressions, on a never-animated property or one that's already keyframed (the expression's result is authoritative, same as After Effects — the original curve is only available through value/ valueAtTime()): continuous time-based motion (e.g. time * 180 for constant rotation), if/else branching with comparisons/booleans, local var bindings, cross-layer links (thisComp.layer('Name').transform.position, copied exactly when that's the whole expression on a never-animated property, sampled when combined with other math or via .valueAtTime(t)), the Math.* namespace, linear()/ease()/easeIn()/easeOut()/clamp(), add()/sub()/mul()/div()/value, posterizeTime(), and random()/wiggle() (a deterministic, seeded approximation — After Effects' own noise/PRNG can't be reproduced bit-for-bit, but this is reproducible across builds and beats a frozen property). A wiggle()-only expression on a shape path wiggles each vertex independently.
Prunes empty precomps and now-unreferenced assets left behind by the fixes above.
Use it at load time (drop-in decoder, no build step) or ahead of time (CLI, zero runtime cost).

Getting started

Add the dependency:

dependencies:
  lottie_fixup: ^0.3.0

Usage

At load time — no build step

Drop fixupLottieDecoder into any lottie loading API that takes a decoder:

import 'package:lottie/lottie.dart';
import 'package:lottie_fixup/lottie_fixup.dart';

Lottie.asset('assets/character.json', decoder: fixupLottieDecoder)

Safe to apply unconditionally, even to files already fixed ahead of time — fix is a no-op when there's nothing left to do. This adds a JSON decode/walk/re-encode once per composition load, not per frame. For larger files, pass backgroundLoading: true to move that work off the UI isolate:

Lottie.asset(
  'assets/character.json',
  decoder: fixupLottieDecoder,
  backgroundLoading: true,
)

Ahead of time — CLI

For an animation that ships in every build and never changes, fix it once and skip the runtime cost entirely:

dart pub global activate lottie_fixup
lottie_fixup diagnose assets/animations/*.json   # report only, no changes
lottie_fixup fix assets/animations/*.json        # fix in place

Library

import 'dart:convert';
import 'dart:io';
import 'package:lottie_fixup/lottie_fixup.dart';

final file = File('animation.json');
final doc = jsonDecode(file.readAsStringSync()) as Map<String, dynamic>;

final result = fix(doc); // mutates doc in place
if (result.changed) {
  file.writeAsStringSync(jsonEncode(doc));
}

Configuration: opting out of approximations

A few parts of expression baking are an approximation or a judgment call rather than an exact match to what After Effects would render — see Features above. BakeOptions lets you turn any of them off individually; every option defaults to true (bake everything), and turning one off only ever makes baking more conservative — the affected expressions are reported as unsupported instead of altered:

Option	Default	Turn off to...
`bakeRandomAndWiggle`	`true`	Leave `random()`/`wiggle()` unbaked (also disables `bakeShapePathWiggle`).
`bakeOnKeyframedProperties`	`true`	Only bake never-keyframed (`"a": 0`) properties, matching versions before 0.3.0.
`bakeShapePathWiggle`	`true`	Leave `wiggle()` on a shape path unbaked.
`bakeApproximateEasing`	`true`	Leave `ease()`/`easeIn()`/`easeOut()` unbaked (`linear()` is unaffected — it's an exact formula, not an approximation).

It plugs into every entry point:

const options = BakeOptions(bakeOnKeyframedProperties: false);

// Library
fix(doc, options: options);
diagnose(rawJson, doc, options: options); // pass the same options you'll fix() with

// At load time
Lottie.asset(
  'assets/character.json',
  decoder: fixupLottieDecoderWithOptions(options),
)

# CLI
lottie_fixup fix --no-keyframed-properties assets/animations/*.json

What this does not fix

Expressions this package's small evaluator doesn't understand (e.g. effect(...), a reference into a nested comp, for/while loops or user-defined functions) are reported (diagnose, or FixResult.propertyBake.skippedExpressions) but left untouched.
'offset'/'continue' on a non-numeric value (a shape path, for example, rather than position/scale/rotation/opacity) is reported rather than baked, since those modes work by doing arithmetic directly on the value.
A non-wiggle() expression on a shape path (arithmetic directly on a path value) is reported rather than baked — After Effects doesn't support that either.
A duration variant (loopOutDuration/loopInDuration) whose duration is shorter than the keyframed segment itself is reported rather than baked — that would need interpolating a cut point in the middle of the real animation, which isn't implemented.
A property that calls both loopIn and loopOut (a manual "loop both ways" expression) is reported rather than guessed at.
random()/wiggle() are baked as a plausible approximation, not a bit-exact match to After Effects — see Features above.
A layer missing ks for a reason other than being an audio layer is flagged (SanitizeResult.layersMissingTransform) rather than silently removed, since that could be a real authoring mistake worth checking by hand.

Additional information

If this package saved you a debugging session, consider buying me a coffee.