Gum Runtime Variable References

Runtime Variable References

Overview

Variable references defined in the Gum tool can be re-evaluated at runtime. The primary use case is theming: modify centralized style values in code, propagate them across all elements, then create UI.

Key API

ApplyAllVariableReferences (GumRuntime)

Extension method on GumProjectSave in ElementSaveExtensions.GumRuntime.cs. Iterates all elements (standards, components, screens) and applies variable references on every state including category states (e.g., ColorCategory). Uses topological sort so dependencies are applied first — if B references A, A is applied before B. Handles circular dependencies gracefully (appends them at the end).

No Roslyn dependency — lives in GumRuntime/GumCommon, available to all platforms.

GumExpressionService (Gum.Expressions NuGet)

Located in Runtimes/GumExpressions/. Provides Roslyn-based expression evaluation for arithmetic in variable references (Width + 10, Width * 2). Optional — without it, only simple dot-path lookups work (OtherInstance.Width).

GumExpressionService.Initialize() sets ElementSaveExtensions.CustomEvaluateExpression to a Roslyn-based evaluator. The evaluator is EvaluatedSyntax, which was extracted from the Gum tool into this project. Conditional (ternary), comparison (==, !=, <, >, <=, >=), and logical (&&, ||, !) operators all flow through this same path — they work at runtime when Gum.Expressions is wired.

Two Apply Overloads (ElementSaveExtensions)

• ApplyVariableReferences(ElementSave, StateSave) — writes hard values into the StateSave. Use before creating UI.
• ApplyVariableReferences(GraphicalUiElement, StateSave) — sets properties on live runtime visuals via SetProperty.

Architecture

GumCommon (no Roslyn)
    ↑
Gum.Expressions (adds Roslyn) — optional NuGet
    ↑           ↑
Gum Tool    Game (opt-in)

The decoupling mechanism is ElementSaveExtensions.CustomEvaluateExpression — a static Func<StateSave, string, string, GraphicalUiElement?, object> delegate. When null, falls back to RecursiveVariableFinder (simple lookups only). When set by GumExpressionService.Initialize(), uses Roslyn for full expression support. The 4th argument is the live, already-laid-out GraphicalUiElement being applied against (when one exists) — it lets a reference resolve the runtime-computed Absolute* properties, which don't exist on authored StateSave data at all (see gum-tool-variable-references for the resolution mechanism). ApplyVariableReferences(GraphicalUiElement, StateSave) supplies its own top-level element automatically; ApplyVariableReferences(ElementSave, StateSave) only resolves Absolute* references when called with its optional liveRoot argument.

After applying variable references, call GraphicalUiElement.RefreshStyles() or GumService.Default.RefreshStyles() to push the updated values to live visuals. For a deep dive into how this works end-to-end, see the gum-variable-deep-dive skill.

global::Localization.CurrentLanguage

Reserved identifier (int, mirrors ILocalizationService.CurrentLanguage) resolved in EvaluatedSyntax against Gum.Localization.LocalizationRuntimeState.Current — a GumCommon-level static so Gum.Expressions can read it without depending on any platform runtime. CustomSetPropertyOnRenderable.LocalizationService (per-platform-compiled) forwards to it. A CurrentLanguage-dependent reference needs an explicit ApplyAllVariableReferences() + RefreshStyles() after a language switch, same as any other reference — CurrentLanguageChanged does not trigger re-evaluation on its own.