Gum Tool Animations

State-based animation authoring in the editor: timeline animations that interpolate between named States over time. NOT the same as runtime AnimationChains (see Landmines).

Where it lives

• Split like the editor tab (phase 80 of the Avalonia migration): the neutral Tool/StateAnimationPlugin.Core (net10.0) holds StateAnimationPluginBase (all event wiring, menu, tab, undo, delete option), the load/save and settings managers, the shared list hotkeys (AnimationTabKeyHandler), and the timeline math (Timeline/TimelineLayout, InterpolationCurve). Each head derives the exported plugin: WPF MainStateAnimationPlugin (Gum/StateAnimationPlugin/, views Views/MainWindow.xaml, Timeline.xaml, StateView.xaml) and Avalonia AvaloniaStateAnimationPlugin (Tool/Gum.Avalonia/Plugins/StateAnimation/, whose TimelineView draws natively in Render). A behavior change to the tab belongs in the core, not in either view. Tab is hidden until View ▸ View Animations.
• VMs (in Gum.Presentation): ElementAnimationsViewModel → AnimationViewModel → AnimatedKeyframeViewModel; the business logic is AnimationTabController.

Data model & serialization

• Persisted as a per-element .ganx sidecar: <ElementName>Animations.ganx next to the element's .gucx/.gusx (path: Managers/AnimationFilePathService.cs). NOT embedded in the element file or .gumx. Load/save: Managers/AnimationCollectionViewModelManager.cs.
• Save classes live in GumDataTypes/SaveClasses/*, namespace Gum.StateAnimation.SaveClasses. ElementAnimationsSave → List<AnimationSave>; each AnimationSave holds three parallel keyframe lists: state keyframes, sub-animation refs, named events.
• A state keyframe binds to a state by name string, "Category/State" convention (uncategorized = no slash). Resolution: AnimationViewModel.GetStateFromCategorizedName.
• Keyframes are cumulative — each combines with prior ones; preview rebuilds via RefreshCumulativeStates.

Named events — FRB-only

"Add Named Event" authors a NamedEventSave (name + time). It round-trips to .ganx, but Gum's own runtime never dispatches it — only FlatRedBall consumes named events. Skip in non-FRB docs.

Runtime relationship (do NOT conflate)

• State animations (this skill) — .ganx, interpolate between states. Runtime driver: GumCommon/Runtime/AnimationController.cs + AnimationRuntime.cs (undocumented elsewhere).
• AnimationChains — .achx, sprite-sheet flipbook on Sprite/NineSlice; see [[gum-runtime-animation-chains]]. Different concept, different file, different runtime.

Landmines

• "Keyframe" is a union of 3 things. AnimatedKeyframeViewModel is a state keyframe OR sub-animation ref OR named event, discriminated by which string is non-empty (StateName → state, else AnimationName → sub-anim, else event). No type enum; fans out into 3 lists on save.
• Misleading names on AnimationSave. States = state keyframes (not element states); Animations = sub-animation keyframes (not child animations).
• Interpolation types come from a NuGet package, not Gum. InterpolationType/Easing (state keyframes only) are FlatRedBall.Glue.StateInterpolation types from the standalone FlatRedBall.InterpolationCore package — an easing/tweening library under the FlatRedBall brand, NOT the FRB engine. The animation data model serializes these into .ganx.
• Sidecar must follow the element. Element rename/duplicate/delete must move the .ganx; wired via RenameManager/DuplicateService/ElementDeleteService in AssignEvents.
• Save is whitelist-filtered. HandleDataChange only re-saves on specific property changes; a new persisted keyframe field won't save unless added there.

Docs

User-facing content today is a 4-part tutorial under docs/gum-tool/tutorials-and-examples/animation-tutorials/. Issue #480 wants a reference section on the Animations tab itself (data model, named events, the tab UI), not tutorial-style.