Gum Forms Itemscontrol

ItemsControl and ListBox Reference

Key Files

The Two Collections

Items and ListBoxItems are separate and can get out of sync.

• Items (IList, default ObservableCollection<object>) — the logical data collection. Can hold anything: strings, view models, ListBoxItem instances, or any FrameworkElement / GraphicalUiElement.
• ListBoxItems (ReadOnlyCollection<ListBoxItem>) — the visual row controls actually shown. Wraps ListBoxItemsInternal (a List<ListBoxItem>).

In normal usage (adding data objects to Items) they stay in sync. They diverge in several cases — see Desync Gotchas below.

Data Flow: Items → InnerPanel → ListBoxItems

Adding to Items triggers a two-stage pipeline:

1. HandleItemsCollectionChanged — responds to Items. Creates or locates a visual and inserts it into InnerPanel.Children.
2. HandleInnerPanelCollectionChanged — responds to InnerPanel.Children. Calls HandleCollectionNewItemCreated(frameworkElement, index).
3. HandleCollectionNewItemCreated (ListBox override) — if the item is a ListBoxItem, inserts it into ListBoxItemsInternal and calls AssignListBoxEvents.

HandleCollectionNewItemCreated is NOT called directly from step 1. It is only triggered by InnerPanel firing its own CollectionChanged. This indirection is intentional.

What Gets Created Per Item Type

HandleItemsCollectionChanged dispatches based on what was added to Items:

ListBox overrides CreateNewItemFrameworkElement with additional logic:

Templates

There are two template types with different roles:

VisualTemplate — produces a GraphicalUiElement (visual-first).

• Constructed with a Type (must have (bool, bool) or no-arg constructor), Func<GraphicalUiElement>, Func<object, GraphicalUiElement>, or Func<object, bool, GraphicalUiElement>.
• Used by CreateNewVisual. When constructed from a Type, calls it with (true, false) — createFormsInternally:false prevents the visual from creating its own Forms object, since the ListBox will wrap it.
• Set on ItemsControl.VisualTemplate. Changing it clears and rebuilds all visuals.

FrameworkElementTemplate — produces a FrameworkElement (forms-first).

• Constructed with a Type or Func<FrameworkElement>.
• Used by CreateNewItemFrameworkElement. For ListBox, the result must be a ListBoxItem subclass or an exception is thrown.
• Set on ItemsControl.FrameworkElementTemplate.

Global fallback — if neither template is set, DefaultFormsTemplates[typeof(ListBoxItem)] is used (set during app initialization). This is the normal path for default apps.

Setting a template clears and rebuilds all existing items — both VisualTemplate and FrameworkElementTemplate setters call ClearVisualsInternal() and replay the Items collection.

Desync Gotchas

1. Adding a non-ListBoxItem FrameworkElement to Items

When a Button, CheckBox, or other FrameworkElement is added to Items, its Visual is inserted into InnerPanel. HandleInnerPanelCollectionChanged fires, but asGue.FormsControlAsObject is the Button (not a ListBoxItem), so HandleCollectionNewItemCreated is called with a Button. ListBox's override only inserts into ListBoxItemsInternal if the item is ListBoxItem, so the Button is silently skipped. Items.Count increases but ListBoxItems.Count does not.

2. Adding directly to InnerPanel.Children

HandleInnerPanelCollectionChanged fires and can populate ListBoxItemsInternal if the child's FormsControlAsObject is a ListBoxItem. But Items is never updated — it stays at 0 (or whatever it was). This is the case when a ListBox's visual is constructed by the Gum tool with pre-filled children.

3. Gum tool pre-filled ListBox (ReactToVisualChanged recovery)

If a ListBox Visual arrives with children already in InnerPanel and Items.Count == 0, ReactToVisualChanged in ListBox iterates InnerPanel.Children, adds ListBoxItem instances to both Items and ListBoxItemsInternal, and calls AssignListBoxEvents. This recovery only runs once at construction; it does not stay in sync afterward.

4. Index alignment assumption

HandleItemSelected resolves the data object via Items[ListBoxItemsInternal.IndexOf(listBoxItem)]. If Items and ListBoxItems have drifted (any of the above cases), selection silently fails or selects the wrong item — the check clickedIndex >= Items.Count causes an early return.

5. AssignListBoxEvents idempotency

ListBoxItem.AssignListBoxEvents is guarded by hasHadListBoxEventsAssigned. A ListBoxItem that bypasses normal item creation (e.g., added to InnerPanel directly without going through Items) may or may not have its events assigned. If events are missing, the item renders but clicking it produces no selection change.

Selection

SelectedItems is an ObservableCollection<object> (not replaceable, only modified). SelectedObject and SelectedIndex are convenience properties that read/write the first entry in SelectedItems.

SyncIsSelectedFromSelectedItems walks ListBoxItemsInternal and reconciles IsSelected on each item. It runs whenever SelectedItems changes or SelectedObject/SelectedIndex are set.

SelectionMode controls click behavior: Single (default), Multiple (each click toggles), Extended (Ctrl/Shift modifier keys). Gamepad/keyboard input always uses single-selection behavior regardless of mode.

DisplayMemberPath

When set, DisplayMemberPath causes listBoxItem.UpdateToObject(property_value_as_string) instead of UpdateToObject(the_object_itself). This applies both on initial creation and when DisplayMemberPath is changed after items are already loaded.