gum-tool-variable-grid

name	gum-tool-variable-grid
description	Gum Variables tab and DataUiGrid. Triggers: Variables tab, DataUiGrid control, MemberCategory, InstanceMember, category population, property grid refresh, expansion state persistence.

Gum Variables Tab & DataUiGrid Reference

Overview

The Variables tab displays and edits properties of the selected element, instance, state, or behavior. Built on DataUiGrid (a WPF ItemsControl subclass) from the WpfDataUi library. Categories render as collapsible Expander sections.

Icons rendered inside the Variables grid (unit selectors, alignment, dock/anchor, origin/sizing toggle-button option displays) come from the GumIcon/PathGeometry pipeline. For authoring or replacing them see gum-icons.

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Architecture Layers

[User selects object]
        ↓
[MainVariableGridPlugin] (event subscription)
        ↓
[PropertyGridManager.RefreshDataGrid()]
        ↓
[ElementSaveDisplayer.GetCategories()]
        ↓ (produces List<MemberCategory>)
[DataUiGrid.SetCategories()]
        ↓
[WPF Expander per MemberCategory, rows per InstanceMember]

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Key Files

Purpose	File Path
DataUiGrid control	WpfDataUi/DataUiGrid.cs
DataUiGrid XAML template	WpfDataUi/Themes/Generic.xaml
MemberCategory / InstanceMember models	WpfDataUi/DataTypes/
Gum-specific member subclass	Gum/Plugins/InternalPlugins/VariableGrid/StateReferencingInstanceMember.cs
Plugin wiring selection events	Gum/Plugins/InternalPlugins/VariableGrid/MainVariableGridPlugin.cs
Category population manager	Gum/Plugins/InternalPlugins/VariableGrid/PropertyGridManager.cs
Category factory	Gum/Plugins/InternalPlugins/VariableGrid/ElementSaveDisplayer.cs
Behavior categories	Gum/Plugins/InternalPlugins/VariableGrid/BehaviorShowingLogic.cs
Host UserControl	Gum/Plugins/InternalPlugins/VariableGrid/MainPropertyGrid.xaml(.cs)

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Non-Obvious Behaviors

SetCategories Expansion Preservation

DataUiGrid.SetCategories() captures {name → IsExpanded} from existing categories, replaces the list, then re-applies the saved values by name. Category collapse state persists across selection changes within a session. IsExpanded is Mode=TwoWay in the XAML template so user gestures write back to the model immediately.

Structural Rebuild vs. Partial Refresh

PropertyGridManager.RefreshDataGrid tracks the previous display target (element, state, instances, behavior). If unchanged and force=false, it calls Refresh() to update values without recreating categories. If the target changed, it calls SetCategories with a fresh list from ElementSaveDisplayer. Pass force: true to always rebuild.

Control Recycling (SingleDataUiContainer)

SingleDataUiContainer maintains a static Dictionary<Type, Stack<UserControl>> pool. When a container is removed from the visual tree (Unloaded), its inner displayer control is detached and pushed onto the type-keyed stack. When a new container needs a displayer, CreateInternalControl first checks if the existing control already matches the needed type (reuse in-place — preserves focus), then tries the pool via TryGetFromPool, and only falls back to Activator.CreateInstance if both miss. Pooled controls must clean up stale state when reassigned to a new InstanceMember (e.g., TextBoxDisplay detaches old event handlers, resets error/multiline state, and calls Refresh). SetCategories uses BulkObservableCollection.ReplaceAll (single Reset notification) which triggers WPF to unload old containers (returning controls to the pool) and create new ones (pulling from the pool).

Multi-Select Path

When multiple instances are selected, SetMultipleCategoryLists is used instead of SetCategories. MultiSelectInstanceMember wrappers coordinate synchronized edits across all selected instances and record a single undo after all values are set.

StateReferencingInstanceMember

All members in the Variables tab use StateReferencingInstanceMember (subclass of InstanceMember), not the generic reflection path. Its IsReadOnly returns true when InstanceSave?.Locked == true. Its IsDefault returns true when the value is absent from the selected state (not inherited from defaults).

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Custom Displayers (how a variable gets non-default UI)

Most variables render with a default control inferred from their type. A variable gets a different control — slider, angle dial, alignment/origin toggles, parent dropdown — through three knobs on InstanceMember:

• PreferredDisplayer (a Type) — selects which WPF control renders the row; SingleDataUiContainer instantiates it. This is what creates e.g. a SliderDisplay.
• PropertiesToSetOnDisplayer (a Dictionary<string, object>) — after the control exists, the container reflectively sets each named property on it. A slider's MinValue/MaxValue (the "range") are just two such pushes onto a control already chosen by PreferredDisplayer — they do not create the slider on their own.
• UiCreated event — for config that must be computed per-instance instead of a constant (see MakeDegreesAngle, and the WpfDataUi sample's per-character MaxValue).

Gum's built-in variables are wired in StandardElementsManager.GumTool.cs (slider for color channels, angle dial, alignment, origin, parent dropdown). That file is where to look or add.

Naming trap: MinWidth/MaxWidth/MinHeight/MaxHeight in StandardElementsManager.cs are real runtime layout clamps — unrelated to a displayer's slider MinValue/MaxValue.

The icon-based displayers (origin/alignment/dock toggles) get their glyphs from the GumIcon pipeline — see gum-icons.

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Refresh Trigger Flow

Selection changed
  → MainVariableGridPlugin.Handle*Selected()
  → PropertyGridManager.RefreshEntireGrid(force: true)
  → RefreshDataGrid(...)
     ├─ Target changed?
     │   yes → ElementSaveDisplayer.GetCategories()
     │          → DataUiGrid.SetCategories()     ← preserves IsExpanded by name
     └─ Target same?
             → DataUiGrid.Refresh()              ← only updates member values

Double-Refresh Guard (Instance Selection)

When an instance is selected, two events fire in sequence: the default state is force-selected first (via PerformAfterSelectInstanceLogic), then the instance-selected event fires. Without a guard, the grid rebuilds twice.

Selection changed (instance)
  → HandleStateSelected()       (state force-selected first)
  → RefreshEntireGrid(force: true) + sets _stateJustRefreshedGrid
  → HandleInstanceSelected()    (fires second)
  → _stateJustRefreshedGrid is true → skip redundant refresh

_stateJustRefreshedGrid is cleared by HandleElementSelected and HandleTreeNodeSelected so it does not suppress legitimate refreshes during unrelated selections.

Variable set by UI:

InstanceMember.AfterSetByUi
  → StateReferencingInstanceMember.NotifyVariableLogic()
  → PropertyGridManager.RefreshEntireGrid(force: false)
  → DataUiGrid.Refresh()   (no structural rebuild needed)

Post-set Selective Refresh (SetVariableLogic)

After a variable is set, SetVariableLogic.RefreshInResponseToVariableChange decides whether the grid needs a structural rebuild, a value-only refresh, or nothing. By default nothing happens — most variable edits only need the value-only refresh that already ran upstream. The decision rule is:

1. Hardcoded name in VariablesRequiringRefresh dictionary (e.g. Parent, BaseType, Font, TextureAddress) — these change which other variables exist on the grid, so they trigger a FullGridRefresh (rebuild + tree view) or FullGridValueRefresh per the dictionary entry.
2. State variable on the element or an instance (detected dynamically via SetVariableLogic.IsStateVariable → VariableSave.IsState) — assigning a categorized state doesn't add/remove variables, but it does change which rows are reference-driven, their IsDefault status, and their subtext. Those are computed at category-build time, so a value-only refresh isn't enough → RefreshVariables(force: true).

State variables can't be in the hardcoded dictionary because their names are dynamic (<CategoryName>State). Use IsStateVariable(unqualifiedMember, parentElement, instance) rather than string-matching.

The trailing-else branch of the dictionary-handling block (RefreshVariables(force: true)) is the "rebuild grid, don't touch the tree" path. It's not named in the VariableRefreshType enum — it's the fallthrough behavior. Watch for it when reading the code.

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Common Patterns

Making a category collapsed by default

Set IsExpanded = false on the MemberCategory before passing to SetCategories. The first time the category appears it uses the incoming value; subsequent appearances restore the user's last state.

Forcing a full grid rebuild

Call PropertyGridManager.RefreshEntireGrid(force: true). The force flag bypasses the same-target optimization and always recreates categories.