Animate Unity 6 UI Toolkit from C# or from USS — your call. One-liner tweens, declarative `@keyframes`, 32 easing curves, sequences, presets, auto-cleanup. One runtime drives both paths.Unity 6 UI Toolkit has no built-in tween engine and no @keyframes support. You write element.schedule.Execute(...) loops, hand-roll easing math, manage cleanup yourself, and watch tweens leak when an element gets removed mid-animation. Designers can't author motion without writing C#.This asset is the missing animation layer. Animate any UI Toolkit property from a single line of C# — button.TweenScale(1.2f, 0.25f).SetEasing(EaseType.EaseOutBack). Or write declarative @keyframes in USS and watch elements auto-animate the moment a class is applied. Same engine drives both paths, so a curve you pick in C# behaves identically to the same curve named in USS.One-line tweens.element.TweenOpacity(1f, 0.3f).SetEasing(EaseType.EaseOutCubic);button.TweenScale(1.2f, 0.25f).SetEasing(EaseType.EaseOutBack);card.TweenTranslate(0f, -20f, 0.4f).SetDelay(0.1f);await element.FadeIn(0.3f).ToAwaitable();Every property tween returns a TweenHandle for chaining (SetDelay, SetLoops, From, SetRelative, OnComplete, OnKill), control (Play, Pause, Kill, Complete, Restart), and async (ToAwaitable). 30+ animatable properties cover the full UI Toolkit style surface: transforms, sizes, positions, spacing, borders, colors, text, layout.Declarative @keyframes in USS.@keyframes pulse {0% { scale: 1.0 1.0; }50% { scale: 1.05 1.05; }100% { scale: 1.0 1.0; }}.heartbeat-card {animation: pulse 1s ease-in-out infinite;}Full CSS @keyframes syntax — duration, delay, iteration count (including infinite), direction (normal, reverse, alternate, alternate-reverse), fill-mode (forwards, backwards, both), play-state, plus the animation shorthand. Add a class to an element, the animation starts. Remove the class, it stops. No per-element C# wiring.32 named easing curves — shared between C# and USS.button.TweenScale(1.2f, 0.5f).SetEasing(EaseType.EaseOutBack); // from code.bounce-in { animation: pop 0.4s ease-out-back forwards; } /* same curve, from USS */Linear, sine, quad, cubic, quart, quint, expo, circ, back, elastic, bounce, plus damped spring — all 32 reachable as EaseType.X in C# and as kebab-case keywords in USS (ease-out-back, ease-out-bounce, ease-in-out-elastic, spring, …). Same math under both names. Plus full CSS cubic-bezier(x1, y1, x2, y2) and steps(n, start|end) for the cases where the named curves aren't enough.22 presets — fade, slide, scale, shake, pulse, bounce, heartbeat, jiggle, flash, typewriter, and more.One call: card.FadeIn(0.3f), card.SlideInLeft(0.4f), card.ShakeHorizontal(0.5f, 10f), card.Bounce(0.6f), text.TypeWriter(0.05f). Every preset returns a TweenHandle you can await, chain, or control. The USS side ships a parallel preset library — 13 .tw-* classes (.tw-shimmer, .tw-pulse, .tw-glow-border, .tw-spin, .tw-fade-in, .tw-slide-in-left/right/top/bottom, .tw-bounce-in, .tw-shake, .tw-color-cycle, .tw-skeleton-loading, .tw-attention-ring, .tw-float) that drop a polished animation onto any element without writing a @keyframes block yourself.Sequences for composed motion.TweenSequence.Create().Append(card.FadeIn(0.3f)).Append(card.TweenScale(1.05f, 0.2f)).Join(card.TweenBackgroundColor(highlight, 0.2f)).OnComplete(() => Debug.Log("done"));Append for chained, Join for parallel, Insert for time-positioned, plus four stagger directions (first-to-last, last-to-first, center-out, edges-in) for collection animations. Await a whole sequence or any subset with WaitForAll / WaitForAny.Auto-cleanup on element detach.Every tween and every keyframe animation silently disposes itself when its target element leaves the panel. Stop your scene, swap panels, destroy a UIDocument — there are no leaked update callbacks, no orphaned coroutines, and you didn't write a single cleanup line. Need explicit control? Hold the returned handle and kill it yourself.Two demo scenes included.TweenDemo — every property tween with a toggle button, every preset wired up, a 31-curve easing visualizer animating side-by-side, a sequence builder demo (Append, Join, complex sequences), a stagger demo (8 items × 4 directions), text animations (TypeWriter, CountTo, Scramble), a live performance counter, and a Reset All. Under the easing visualizer: a USS-driven @keyframes sub-section with four cards (pulse, spin, glow, float) that auto-animate from class assignment alone — the same engine, just from USS. LoadingStatesDemo — skeleton placeholders, a spinner, and a pulsing progress bar, all driven from .tw-* classes.Plays well with the rest of the UI Toolkit Components suite.Tween Engine is the animation engine for the entire portfolio. Open Tools > KrookedLilly > Setup to enable integrations with any installed sibling asset — UI Toolkit: Screen Manager, Focus & Navigation, Responsive Layout, Modal & Notifications, Theme Switcher, Form Validation, Data Binding, or ECS Bridge. Each integration ships disabled by default; flip the toggle and the sibling asset's runtime starts using Tween Engine immediately.With UI Toolkit: Screen Manager: four IScreenTransition implementations powered by Tween Engine — slide, scale, fade, and crossfade — snap into Screen Manager's navigation system the moment you enable the integration. Pick a transition per screen or set a default; the tween runtime handles the rest. Without the toggle on, Screen Manager uses its built-in CSS transitions.Works on every platform.Runs under Mono and IL2CPP scripting backends. Uses the standard Unity AOT path that runs on iOS, Android, WebGL, and consoles. Zero-allocation update loop and pooled tween handles keep the GC clean during steady-state animation.Full C# source, no DLLs.XML documentation on every public API. Zero external dependencies.Imperative tween APITweenOpacity, TweenTranslate, TweenScale (uniform + non-uniform), TweenRotate, TweenWidth/TweenHeight/TweenMaxWidth/TweenMaxHeight, TweenLeft/TweenTop/TweenRight/TweenBottom, TweenMargin (uniform + per-side), TweenPadding (uniform + per-side), TweenBorderRadius (uniform + per-corner), TweenBorderWidth (uniform + per-side), TweenBorderColor (uniform + per-side), TweenBackgroundColor, TweenColor, TweenFontSize, TweenFlexGrowEvery method returns TweenHandle; chain .SetEasing(EaseType), .SetEasing(AnimationCurve), .SetEasing(Func), .SetDelay(seconds), .SetLoops(count, LoopType.Restart|PingPong|Yoyo), .From(value), .SetRelative(true), .SetId(object), .OnStart(...), .OnUpdate(...), .OnComplete(...), .OnKill(...), .Play(), .Pause(), .Kill(complete: false), .Complete(), .Restart(), .ToAwaitable(CancellationToken)TweenManager.KillAll(element), TweenManager.KillAll(), TweenManager.GlobalSpeed, TweenManager.GlobalPaused, TweenManager.ActiveTweenCountEasingEaseType enum — 32 values: Linear, in/out/in-out variants for Sine, Quad, Cubic, Quart, Quint, Expo, Circ, Back, Elastic, Bounce, plus SpringEasingFunctions.Evaluate(EaseType, float t) — direct math accessCssTimingFunction.FromEaseType(EaseType) — bridge from EaseType to the USS-side timing-function structCssTimingFunction.CubicBezier(x1, y1, x2, y2) — overshoot-capable (Y unconstrained)CssTimingFunction.Steps(count, StepsJumpTerm.End|Start)USS keyword surface: CSS spec (linear, ease, ease-in, ease-out, ease-in-out, cubic-bezier(...), steps(...)) plus kebab-case Penner extensions (ease-in-sine, ease-out-quad, ease-in-out-cubic, ease-out-back, ease-in-bounce, ease-out-elastic, spring, …)SequencesTweenSequence.Create() — fluent builderAppend(handle) — chain after the current endJoin(handle) — start in parallel with the previous appendInsert(timeOffset, handle) — explicit time-positionedAppendInterval(seconds) — gap in a chainStagger.From(elements, makeHandle, interval, StaggerMode.FirstToLast|LastToFirst|CenterOut|EdgesIn)Awaitable, killable, pausable as one unitPresetsFadeIn, FadeOut, FadeSlide, ScaleIn, ScaleOut, ScaleOutBounce, SlideOutLeft/Right/Top/Bottom, Bounce, Pulse, Flash, Heartbeat, Jiggle, Shake, ShakeVertical, Collapse, ExpandText presets: TypeWriter, CountTo, ScrambleUSS preset library: .tw-shimmer, .tw-pulse, .tw-glow-border, .tw-spin, .tw-fade-in, .tw-fade-out, .tw-slide-in-left/right/top/bottom, .tw-bounce-in, .tw-shake, .tw-color-cycle, .tw-skeleton-loading, .tw-attention-ring, .tw-floatAll preset USS lives in Presets/keyframe-presets.uss; load via or Keyframes.RegisterPresets()@keyframes engineFull CSS @keyframes block syntax + animation shorthand parser14 animatable USS properties: opacity, translate, scale, rotate, width/height/min-/max-, margin (uniform + per-side), padding (uniform + per-side), border-radius (uniform + per-corner), border-width (uniform + per-side), border-color (uniform + per-side), background-color, color, font-size, left/top/right/bottom, flex-grow/flex-shrinkKeyframes.RegisterStyleSheet(text, sourcePath) / Keyframes.RegisterStyleSheet(textAsset) — register @keyframes blocks from any USS sourceKeyframes.RegisterPresets() — register the bundled preset libraryKeyframeAnimator manipulator — auto-starts / -stops animations as .tw-* classes are added or removedKeyframeAnimator.ScanDescendants(root) — attach manipulators to every descendant with a .tw-* class in one callC# escape hatch: element.PlayAnimation("name", duration, delay, iterationCount, timingFunction, fillMode), PauseAnimation, ResumeAnimation, StopAnimation, StopAllAnimations, IsAnimatingLifecycle events bubble up the tree: AnimationStartEvent, AnimationEndEvent, AnimationCancelEventOne runtime: every @keyframes animation registers as a TweenHandle with TweenManager, so the same update loop, easing dispatch, and auto-cleanup pipeline drives both authoring pathsCross-asset integrationsTools > KrookedLilly > Setup is the one-stop unified matrix window for enabling integrations with every installed sibling asset. One toggle per integration; integrations ship disabled and the customer flips what they want.Supported siblings: UI Toolkit: Screen Manager, Focus & Navigation, Responsive Layout, Modal & Notifications, Theme Switcher, Form Validation, Data Binding, ECS Bridge.EditorUnified Setup window (Tools > KrookedLilly > Setup) — enable/disable Tween Engine integrations with any installed sibling assetIncluded demosTweenDemo — every property tween + every preset + 31-curve easing visualizer + sequences + stagger + text animations + a USS-driven @keyframes sub-sectionLoadingStatesDemo — skeleton, spinner, pulsing progress bar via .tw-* classesCompatibilityUnity 6+ (6000.0 and newer)UI Toolkit (com.unity.modules.uielements)Mono + IL2CPP (Standalone verified; iOS / Android / WebGL / consoles supported via standard AOT path)Full C# source, no DLLsXML documentation on all public APIsZero external dependenciesAI (Claude Code) was used as a development assistant throughout the package creation process. This includes code generation, architecture design, writing unit tests, documentation authoring, and debugging. All AI-generated code was reviewed, tested, and validated by the developer. The final package is 100% human-supervised C# source code with no AI runtime components.
