Struct TreeDriver 

pub struct TreeDriver {
    pub row_height: RowHeight,
    pub children_len: Box<dyn Fn(Option<u64>) -> usize>,
    pub child_token: Box<dyn Fn(Option<u64>, usize) -> u64>,
    pub expandable: Box<dyn Fn(u64) -> bool>,
    pub section_header: Box<dyn Fn(u64) -> bool>,
    pub expanded: Box<dyn Fn(u64) -> bool>,
    pub build: Box<dyn Fn(u64, RNode) -> TreeBuiltRow>,
    pub type_select_text: Box<dyn Fn(u64) -> String>,
    pub resolve_row: RowResolver,
    pub row_menu: Option<RowMenuFn>,
    pub moves: Option<TreeMovesDriver>,
}

Supplied by the tree() piece, type-erased over the item type. day-core invokes these to answer the native data-source and to build/rebind rows. All hierarchy queries are token-addressed (None = the root level); see docs/tree.md for why indices cannot address tree rows.

Fields

row_height: RowHeight

children_len: Box<dyn Fn(Option<u64>) -> usize>

How many children parent has (reads the piece’s snapshot; no tree access).

child_token: Box<dyn Fn(Option<u64>, usize) -> u64>

The i-th child of parent: the stable token every backend keys its rows by.

expandable: Box<dyn Fn(u64) -> bool>

Whether this token can hold children at all (draws or omits the disclosure).

section_header: Box<dyn Fn(u64) -> bool>

expanded: Box<dyn Fn(u64) -> bool>

The piece’s current desired expansion state for this token (untracked read of the app’s expansion signal), which is what the flattener descends into.

build: Box<dyn Fn(u64, RNode) -> TreeBuiltRow>

Build the row for token into anchor. Uses BuildCx internally, so it must be called with no with_tree borrow held. Returns the row’s scope + a rebind writer.

type_select_text: Box<dyn Fn(u64) -> String>

The row’s type-ahead string (docs/tree.md); native type-select answers from it.

resolve_row: RowResolver

Resolve a dayscript row id (the piece’s .row_id string) to its token. This is how the expand:/tree_move: steps and the mock address rows without native gestures.

row_menu: Option<RowMenuFn>

The row’s summon-time context menu (docs/menus.md), None = no row menus.

moves: Option<TreeMovesDriver>

Drag-to-move half, present when the piece is .movable(true) (docs/tree.md).