Tree

The Tree widget displays hierarchical data with expand/collapse, keyboard navigation, and optional multi-selection with cascade behavior.

Basic Usage

Create a tree using the Tree() extension method with a builder callback. Use t.Item() to create items, optionally with nested children.

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Key features:

• Use Icon() to add an emoji or character prefix
• Use Expanded() to show children by default
• Use Children() to nest static child items
• Navigate with arrow keys, Enter to activate, Space to expand/collapse

Lazy Loading

For dynamic data, use OnExpanding() to load children asynchronously when the user expands an item. A spinner automatically displays during loading.

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How it works:

• OnExpanding() is called the first time an item is expanded
• The item shows a spinner instead of the expand indicator while loading
• Children are cached after loading (subsequent expands don't reload)

Multi-Select with Cascade Selection

Enable multi-select mode to show checkboxes. When enabled, selecting a parent automatically selects all children (cascade selection).

csharp

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Selection behavior:

• Click checkbox or press Space to toggle selection
• Selecting a parent selects all descendants
• Deselecting a child shows indeterminate state ([-]) on the parent
• OnSelectionChanged fires with the list of selected items

Data Binding

Bind a tree to a data source using the generic Tree<T>() overload. Use Data<T>() to associate typed data with items, and GetData<T>() to retrieve it in event handlers.

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Type-safe data access:

• Data<T>(value) — Associate data when creating the item
• GetData<T>() — Retrieve data in handlers (throws if type mismatch)
• TryGetData<T>(out value) — Non-throwing alternative
• GetDataOrDefault<T>() — Returns default if not set

Keyboard Navigation

Key	Action
↑/↓	Move focus between items
←/→	Collapse/Expand focused item
Enter	Activate the focused item
Space	Toggle expand (or selection in multi-select mode)
Tab	Move to next focusable widget

API Reference

TreeWidget

Method	Description
MultiSelect(bool)	Enable checkboxes with cascade selection
OnSelectionChanged(handler)	Called when selection changes
OnItemActivated(handler)	Called when an item is activated (Enter)

TreeItemWidget

Method	Description
Icon(string)	Set the icon/emoji prefix
Children(items...)	Set static child items
Expanded(bool)	Set initial expanded state
Selected(bool)	Set initial selection state
Loading(bool)	Show loading spinner
Data<T>(value)	Associate typed data
OnExpanding(handler)	Lazy-load children when expanding
OnExpanded(handler)	Called after expansion completes
OnCollapsed(handler)	Called when item is collapsed
OnActivated(handler)	Called when item is activated

TreeItemNode (in event handlers)

Method	Description
GetData<T>()	Get typed data (throws if wrong type)
TryGetData<T>(out T)	Try to get typed data
GetDataOrDefault<T>()	Get data or default value

Related Widgets

• List — Flat scrollable lists without hierarchy
• Table — Tabular data with columns
• Checkbox — Standalone checkbox widget