Gum Runtime Binding

Two Binding Systems

GraphicalUiElement (GumRuntime/GraphicalUiElement.Binding.cs) — basic binding available on all visuals. FrameworkElement (MonoGameGum/Forms/Controls/) — richer binding on Forms controls, built on top of the GUE system.

FrameworkElement.BindingContext delegates to its Visual.BindingContext — they share one context.

BindingContext

Set on any GraphicalUiElement or FrameworkElement. Cascades automatically to all descendants unless overridden:

root.BindingContext = viewModel;  // all children inherit it
child.BindingContext = other;     // explicit overrides inherited

Subscribes to INotifyPropertyChanged and updates bound UI properties on change.

GraphicalUiElement Binding (Visuals)

Simple string-only binding. No converters, no modes, no path traversal:

element.SetBinding("X", nameof(vm.Position));           // basic
element.SetBinding("Text", nameof(vm.Name), "{0:N0}");  // with format string

PushValueToViewModel() is called from property setters to write back to the VM (always two-way implicitly).

FrameworkElement Binding (Forms)

Three binding styles, all richer than the GUE version.

1. String-based

textBox.SetBinding(nameof(TextBox.Text), nameof(vm.Name));

Shorthand — wraps the string in a default Binding object internally.

2. Explicit Binding object

var binding = new Binding(nameof(vm.IsEnabled))
{
    Mode = BindingMode.OneWay,
    Converter = new BoolToVisibilityConverter(),
    FallbackValue = false
};
checkBox.SetBinding(nameof(CheckBox.IsChecked), binding);

Binding properties: Path, Mode (OneWay/TwoWay/OneWayToSource), UpdateSourceTrigger (Default/PropertyChanged/LostFocus), Converter, ConverterParameter, StringFormat, FallbackValue, TargetNullValue.

3. Lambda / expression tree

// Typed (preferred — compiler-checked, extracts "Child.Text" path):
textBox.SetBinding<MyVm>(nameof(TextBox.Text), vm => vm.Child.Text);

// Parameterless closure:
textBox.SetBinding(nameof(TextBox.Text), () => vm.Child.Text);

Extension methods in FrameworkElementExt.cs. BinderHelpers.ExtractPath() walks the expression tree to produce a dotted path string, then creates a Binding normally. Nested paths (e.g. vm => vm.A.B.C) are fully supported via PropertyPathObserver.

Index-Based Binding (Forms only)

Paths support integer indexer access via [N] syntax. Works in string paths, Binding objects, and lambdas:

// String path
textBox.SetBinding(nameof(TextBox.Text), new Binding("Items[0].Text"));

// Lambda
textBox.SetBinding<MyVm>(nameof(TextBox.Text), vm => vm.Items[0].Text);

// Nested: index in the middle of a path
textBox.SetBinding(nameof(TextBox.Text), new Binding("Child.Items[1].Text"));

All binding features work with indexed paths: modes, converters, StringFormat, FallbackValue, LostFocus trigger.

Collection change notification: PropertyPathObserver subscribes to INotifyCollectionChanged on collections in indexed path segments. When items are added, removed, replaced, or cleared, the binding re-evaluates. Out-of-bounds indexes resolve to null (triggering FallbackValue if set). Currently reacts to ALL collection changes regardless of whether the specific bound index is affected — this is intentionally broad for correctness; a future optimization could filter by index relevance.

Limitations: Dictionary/string key indexing is not supported.

Implementation: BinderHelpers.ParseSegments() splits paths into PathSegment structs (name + optional int index). BuildGetter/BuildSetter emit indexer calls via Expression.MakeIndex or Expression.ArrayIndex. ExtractPath handles MethodCallExpression (get_Item) and IndexExpression nodes from lambdas. PropertyPathObserver uses GetIndexedValue() after property resolution for indexed segments.

Feature Comparison

Feature	GraphicalUiElement	FrameworkElement
String binding	✓	✓
Explicit Binding object	✗	✓
Lambda binding	✗	✓
Nested paths (A.B.C)	✗	✓
Index paths (Items[0].Text)	✗	✓
Binding modes	Implicit TwoWay	Configurable
Converters	✗	✓
FallbackValue / TargetNullValue	✗	✓
UpdateSourceTrigger	Always PropertyChanged	Configurable

Key Files

File	Purpose
GumRuntime/GraphicalUiElement.Binding.cs	GUE binding — BindingContext, SetBinding, PushValueToViewModel
MonoGameGum/Forms/Data/Binding.cs	Binding config class + BindingMode + UpdateSourceTrigger + IValueConverter
MonoGameGum/Forms/Data/NpcBindingExpression.cs	Forms binding engine — UpdateTarget, UpdateSource
MonoGameGum/Forms/Data/PropertyPathObserver.cs	Watches dotted paths, re-hooks on intermediate changes, weak listeners
MonoGameGum/Forms/Data/BinderHelpers.cs	Lambda path extraction, compiled getter/setter delegates
MonoGameGum/Forms/Controls/FrameworkElementExt.cs	Lambda SetBinding extension methods
GumRuntime/BindableGue.cs	Deprecated alias for GraphicalUiElement — do not use

Non-Obvious Behaviors

BindableGue is deprecated. GraphicalUiElement now owns all binding logic. BindableGue exists only as a legacy alias.

Weak listeners in PropertyPathObserver. Forms binding uses weak references to avoid memory leaks on deep paths. GUE binding does not — callers should unsubscribe when disposing.

Lambda extracts path at call time, not at update time. vm => vm.Child.Text becomes the static path "Child.Text". If Child is replaced, PropertyPathObserver re-hooks the listener chain automatically.

ListBox Items is bindable. listBox.SetBinding(nameof(ListBox.Items), nameof(vm.Items)) works and keeps the list in sync with an ObservableCollection on the VM.

UpdateSourceTrigger.Default is project-configurable. Binding.DefaultUpdateTriggers lets a project register a default trigger per control type + property, resolved in NpcBindingExpression.HookUpdateSource before falling back to PropertyChanged. See Default Update Triggers.