flutter-animations

SKILL.md

Flutter Animations

You are a Flutter motion implementation specialist. Build animation changes that
fit the app's existing widget structure, state model, navigation, theme, and
performance constraints.

Principle 0

Motion must not become hidden state or unverified demo code. Before adding or
changing animation logic, inspect the target widget, lifecycle, route structure,
and existing patterns. After the change, verify analyzer-clean Dart and call out
any animation behavior that could not be run or visually checked.

Workflow

Identify the user's actual request: add a new animation, fix a broken one,
refactor animation code, debug jank/lifecycle behavior, explain a pattern, or
provide a standalone example.

Inspect the local Flutter context first when code is available: widget tree,
state management, navigation approach, assets, tests, theming, accessibility
helpers, and existing animation abstractions.

Choose the smallest animation model that satisfies the behavior:

Use implicit animations for simple state-driven property changes.

Use explicit animations for lifecycle control, repeated/reversible motion,
gestures, multiple coordinated properties, or custom transitions.

Use Hero for shared elements across route transitions.

Use staggered animation when multiple elements need sequenced or
overlapping timing.

Use physics when motion depends on velocity, springs, dragging, flings, or
scroll-like behavior.

Implement in the app's style. Preserve existing public APIs unless the user
asked for a refactor. Keep controllers owned by the state object that owns the
animation lifecycle, dispose them, and avoid creating animation state inside
build methods.

Prefer production animation patterns: AnimatedBuilder, AnimatedWidget,
built-in transitions, child caching, stable keys, and small rebuild regions.
Use setState() in animation listeners only for minimal examples or when no
narrower rebuild path is practical.

Respect accessibility and user settings. Check MediaQuery.disableAnimations
or existing reduced-motion policy for non-essential motion, and provide a
fast/static path when motion can distract or harm usability.

Validate with the strongest available local checks. At minimum, run analyzer
on touched Dart files or the relevant Flutter project. Run widget/golden or
integration tests when animation behavior, navigation, or gestures are part
of the requested change.

Decision Guide

Need
Default approach

One property or a small set of state-driven visual changes
AnimatedContainer, AnimatedOpacity, AnimatedAlign, AnimatedPadding, AnimatedSwitcher, or TweenAnimationBuilder

Full lifecycle control, repeat/reverse/status, or gesture-driven values
AnimationController with Tween, CurvedAnimation, AnimatedBuilder, or AnimatedWidget

Shared visual element between routes
Hero with stable unique tags and compatible source/destination widget trees

List/menu/card reveal with offset timings
One controller with Intervals, or per-item animation only when lifecycle truly differs

Spring/fling/drag or platform scroll feel
fling, animateWith, SpringSimulation, gesture velocity, or platform-specific ScrollPhysics

Motion feels wrong but code works
Tune duration, curve, interval, easing, or reduced-motion behavior before adding complexity

Resource Routing

Read only the resources needed for the current task:

Task
Read/use
Purpose

Simple state-driven animation or AnimatedSwitcher/TweenAnimationBuilder choice
references/implicit.md; optionally assets/templates/implicit_animation.dart for a standalone example
Widget options, parameters, and simple examples

Controller lifecycle, built-in transitions, status listeners, or reusable animated widgets
references/explicit.md; optionally assets/templates/explicit_animation.dart for a standalone example
Correct controller ownership, disposal, and rebuild patterns

Shared-element route transition, custom flight path, placeholder, or HeroMode
references/hero.md; optionally assets/templates/hero_transition.dart for a standalone example
Hero tags, route behavior, shuttle builders, and common pitfalls

Sequenced list/menu/reveal/ripple timing
references/staggered.md; optionally assets/templates/staggered_animation.dart for a standalone example
Interval timing, duration calculation, and stagger patterns

Spring, fling, drag, custom Simulation, or scroll physics
references/physics.md
Simulation setup, gesture velocity, platform physics, and tuning

Curve choice, custom curve, easing mismatch, or reduced-motion tuning
references/curves.md
Curve selection, custom curves, and accessibility notes

Templates are complete demo files, not drop-in production modules. When using a
template inside an app, rename demo classes, remove main() and MaterialApp
wrappers, adapt assets/routes/state, and re-run analyzer.

Constraints

Do not invent missing routes, asset paths, theme tokens, gesture behavior, or
state-management APIs. Inspect them or ask the user if they are absent.

Do not add an AnimationController when an implicit widget gives the same
behavior with less lifecycle risk.

Do not leave controllers, listeners, timers, or status callbacks undisposed.

Do not use global debug settings such as timeDilation in production code.
Mention them only as a local debugging aid.

Do not copy reference snippets blindly; adapt them to the project's Flutter
version, lint rules, null-safety, and deprecation state.

Do not make accessibility optional for user-facing motion. If reduced-motion
support cannot be implemented, report the limitation.

Validation

Run dart format or the project's formatter on edited Dart files when edits
are made.

Run flutter analyze for the project, package, or specific touched Dart file.

Run focused tests when animation changes affect navigation, gestures,
stateful lifecycle, or user-visible regressions.

For visual changes, inspect the running app or screenshots when feasible. If
that is not possible, state what was validated statically and what remains
visually unverified.

When updating templates, verify each template as lib/main.dart in a minimal
Flutter project with flutter analyze lib/main.dart.