TreeState

A hoistable state holder for what a Tree has selected and what it has open, carrying the gesture that brings one of its nodes into view.

Each path is the chain of child indices from the root, so [] is the root, [0] its first child, and [0, 2] that child's third child.

selectedPaths and expandedPaths are two-way: assigning either applies it to the tree, and the user selecting a node or opening one writes the change back here. Both are snapshot-observable, so reading one inside a composable (or a snapshotFlow collector) subscribes to the user's later changes as well.

What this state names is the composition's own and is re-applied on every pass: a tree driven by a state stands on exactly the nodes the state holds, so a state starting on the empty expansion opens nothing - start it on setOf(emptyList()) for a tree that opens on its root. A node the structure does not have is left out of the tree while it goes on being named here - a structure that has it again shows it selected, or open.

revealPath brings one node into view when the application decides to - a search hit, a node a load has just filled in:

val state = rememberTreeState(initialExpandedPaths = setOf(emptyList()))

Button("Find", onClick = { state.revealPath(search(query)) })
ScrollPane {
    Tree(root = root, children = ::childrenOf, state = state, modifier = SwingModifier.viewport())
}

rowCount, isExpanded and shownSelectedPaths answer for the tree instead of for what this state holds. Each reads the bound tree where it is called, so what it reports is what the tree stands on - which is not always what was declared, since a structure can drop a node and a closed node cannot show its descendants selected. They are not snapshot state, so reading one subscribes to nothing; a composable that has to follow the user reads selectedPaths and expandedPaths. An unbound state has no tree to answer for and reports no rows, nothing open and nothing selected.

A state drives at most one tree: passing it to a second one moves it there and leaves the first unbound.

Parameters

See also