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TreeView Specs

This document captures design and implementation notes for TreeView and TreeNode.

Overview

• Status: Implemented
• Primary purpose: Display hierarchical nodes with selection and expand/collapse interaction.
• Key goals:
  • composable node headers (Visual)
  • expandable/collapsible nodes with a visible expander glyph
  • selection as a visible-row index (SelectedIndex)
  • keyboard navigation and mouse selection
  • scroll integration via IScrollable + ScrollModel (vertical and horizontal offsets)
  • optional hierarchy guide lines (tree connectors)
• Non-goals:
  • built-in data model or reflection-based tree construction (users build TreeNode structures)
  • full virtualization (TreeView tracks all expanded nodes; offscreen nodes are still part of the render list)
  • per-node enabled/disabled state in v1 (enable/disable is at the control level)

Implementation notes

• Primary implementation:
  • src/XenoAtom.Terminal.UI/Controls/TreeView.cs
  • src/XenoAtom.Terminal.UI/Controls/TreeNode.cs
  • src/XenoAtom.Terminal.UI/Styling/TreeViewStyle.cs
• Tests:
  • src/XenoAtom.Terminal.UI.Tests/TreeViewTests.cs
  • src/XenoAtom.Terminal.UI.Tests/TreeViewScrollViewerTests.cs
• Demo:
  • samples/ControlsDemo/Demos/TreeViewDemo.cs

Public API surface

TreeView

• TreeView : Visual, IScrollable (sealed)

Layout defaults:

• Focusable = true
• HorizontalAlignment = Align.Stretch
• VerticalAlignment = Align.Stretch

Properties:

• Roots : BindableList<TreeNode> (bindable getter)
  • Owned by the TreeView and uses onAdding/onRemoving hooks to attach/detach nodes.
• SelectedIndex : int (bindable)
  • Index into the current "visible row" list.
  • Coerced to -1 when the tree has no visible rows, otherwise clamped to [0 .. VisibleCount - 1].
• Scroll : ScrollModel (non-bindable property from IScrollable)
  • Exposes scroll state for ScrollViewer integration.

TreeNode

• TreeNode : IVisualElement (sealed)

Properties:

• Header : Visual (required)
• Children : BindableList<TreeNode> (bindable getter)
  • When the node is attached to a TreeView, adding/removing children automatically attaches/detaches their headers.
• Parent : TreeNode? (set automatically when added as a child)
• IsExpanded : bool (bindable)
• Icon : Rune? (bindable)
  • Optional node-provided icon. The final icon is resolved by TreeViewStyle.ResolveIcon(...).
• Data : object? (bindable)
  • Optional data context used by TreeViewStyle.IconResolver.

Notes:

• TreeNode is an IVisualElement because it owns a BindableList<TreeNode>, but it is not itself a Visual. The renderable surface is the Header visual.

Visible rows and child attachment model

TreeView maintains:

• _headers: a VisualList<Visual> containing the header visuals for all nodes currently in the tree
• _visible: a list of visible rows derived from expanded nodes

Node headers are attached once and reused:

• when a node is added (directly or via child list changes), its header is attached to the TreeView visual tree
• when a node is removed, its header is detached

The visible row list is recomputed during PrepareChildren:

• it traverses Roots recursively and includes children only when IsExpanded is true
• it also computes hierarchy line metadata (continuation mask and last-sibling flag)

To avoid allocating temporary sets when toggling visibility, PrepareChildren:

• sets all headers IsVisible = false
• then sets IsVisible = true for headers that appear in _visible

Important: this is not viewport virtualization. All expanded nodes are part of _visible, even if scrolled out of view. Viewport virtualization could be a future improvement.

Scrolling integration

TreeView exposes a ScrollModel via IScrollable.Scroll.

TreeView updates scroll state during arrange:

• Scroll.SetViewport(viewportWidth, viewportHeight)
• Scroll.SetExtent(extentWidth, extentHeight)

Offsets:

• Scroll.OffsetY selects which visible row is rendered at the top.
• Scroll.OffsetX is used to horizontally shift the row chrome and headers.

Selection and scroll:

• when SelectedIndex changes, TreeView schedules "ensure selected visible"
• if a viewport is already known (Scroll.ViewportHeight > 0), it updates the scroll offset immediately
• otherwise it performs the ensure-visible step during the next arrange

Integration with ScrollViewer:

• ScrollViewer can drive the offsets by setting its own offsets; TreeView reflects those through its scroll model.
• TreeView handles mouse wheel events itself (scroll by 1 row), and this works when hosted in a ScrollViewer because the ScrollViewer reads offsets from the scroll model.

Binding model loop avoidance (ScrollVersion)

ScrollModel.Version is a bindable property that changes when viewport/extent/offset change.

TreeView both:

• reads scroll state during layout/render
• and writes scroll state during arrange (SetViewport/SetExtent/SetOffset)

To avoid "read then write the same bindable in one tracking context" errors, TreeView uses the standard pattern:

• in PrepareChildren, copy Scroll.Version into a private bindable ScrollVersion
• in ArrangeCore and RenderOverride, read ScrollVersion (not Scroll.Version)

This decouples the binding dependency read from the writes performed during arrange.

TreeView also stores MeasuredContentWidth as a private bindable so arrange can re-run when content width changes.

Layout and rendering

Measure

TreeView measures based on the visible row list and header desired widths:

• each visible node header is measured with infinite max width and height 1
• the control computes a "prefix width" per row:
  • selection marker glyph width (FocusMarkerGlyph)
  • indentation (depends on depth and whether hierarchy lines are enabled)
  • expander glyph width (or a space when the node has no children)
  • a fixed 1-cell gap
  • icon glyph width
  • SpaceBetweenGlyphAndText gap
• the max row width is prefix + header.DesiredSize.Width

The measured content width is stored in MeasuredContentWidth.

Height:

• desired height is max(1, VisibleCount) (one row per visible node)

Vertical scrollbar width reservation:

• if the height is bounded and the desired height exceeds the available height, TreeView adds a "reserved width" equal to ScrollViewerStyle.ScrollBarThickness.
• This is intended to reduce layout shifts when a TreeView is hosted in a ScrollViewer that will display a vertical scrollbar.

Measure returns flexible hints (grow/shrink enabled) so it can be used as a resizable panel.

Arrange

Arrange:

• sets the scroll viewport to the arranged size
• sets the scroll extent to:
  • height: visible row count
  • width: max(arranged width, MeasuredContentWidth)
• clamps offsets to valid range
• applies ensure-selected-visible if needed

Each visible header is arranged to a 1-row rectangle at:

• y = Bounds.Y + (rowIndex - Scroll.OffsetY)
• x = Bounds.X + prefixWidth - Scroll.OffsetX

Render

TreeView renders the row chrome (not the header content):

• fills the whole bounds with spaces (Style.None baseline)
• then for each viewport row:
  • fills the entire row width with the resolved row style
  • draws the selection marker glyph in the first cell
  • optionally draws hierarchy guide lines using HierarchyLines (when enabled and indent size >= 2)
  • draws the expander glyph and icon glyph
  • leaves remaining row cells for the header visuals to render (children render after the parent)

Hierarchy line rendering uses a continuation mask to decide which ancestor levels draw a vertical connector.

Input behavior

TreeView does not register commands yet; interactions are implemented directly in key/pointer handlers.

Keyboard

• Navigation: Up/Down, Home/End, PageUp/PageDown
• Expansion:
  • Left: collapse selected node (if it has children)
  • Right: expand selected node (if it has children)
  • Enter/Space: toggle expand/collapse (if it has children)

There is no type-to-search in v1.

Mouse

• Left click selects the clicked row.
• Clicking on the expander glyph cell toggles expand/collapse.
  • The expander hit test is based on the computed prefix position and includes horizontal scrolling (OffsetX).

Wheel

Mouse wheel scrolls the tree by 1 row without changing selection.

Styling

TreeViewStyle

TreeViewStyle controls:

• indentation and spacing:
  • IndentSize
  • SpaceBetweenGlyphAndText
• hierarchy lines:
  • HierarchyLines : LineGlyphs? (default: Single; null disables)
  • HierarchyLineStyle : Style? (default: theme border style with dim)
  • convenience variants: NoLines, HeavyLines, DoubleLines
• glyphs:
  • ExpandedGlyph, CollapsedGlyph (non-ASCII defaults; override if needed)
  • FocusMarkerGlyph (non-ASCII default; this is the selection marker)
• icons:
  • IconResolver : Func<object?, Rune?, Rune>?
  • node icon fallback uses TreeNodeIcons.DocumentGlyph
    • defaults are emoji glyphs (e.g. folder/file/document); see TreeNodeIcons for the code points
• row styles:
  • Item, SelectedFocused, SelectedUnfocused, Disabled
  • ResolveItemStyle(theme, enabled, selected, focused) provides theme-driven defaults when overrides are not set.

Tests and demos

• TreeViewTests cover:
  • expanding nodes and showing children
  • hierarchy line rendering (default and disabled)
  • root sibling connector rendering
  • scrolling to keep selection visible when hosted in a ScrollViewer
• TreeViewScrollViewerTests cover:
  • ScrollViewer driving the tree scroll offsets
  • mouse wheel scrolling
  • horizontal extent reporting for long content (horizontal scrolling)
• ControlsDemo shows:
  • style variants for hierarchy lines
  • a larger tree hosted in a ScrollViewer

Future / v2 ideas

• Viewport virtualization (only arrange/measure headers in the viewport plus overscan).
• Type-to-search or incremental search on node labels.
• Per-node enabled state and callbacks (activate node, selection changed events).
• Lazy child loading (async-friendly) as a separate model layer or helper API.