# HierarchyContributor
> _Since v3.9.0_
A plugin that contributes structure to the hierarchy.
Contributors never call each other: the HierarchyPlugin runs every attached
contributor on each natural sibling list — `structure` contributors first
(discover children, order siblings), then `transform` contributors
(re-shape the list, e.g. insert group nodes).
## Properties
| Property | Type | Description |
| -------- | ---- | ----------- |
| `hierarchyStage` | structure | transform | `structure` contributors run before `transform` contributors on every list. |
## Methods
### beginHierarchy()
Called once per row-model rebuild with the root rows. Return `false` to sit
out this rebuild (e.g. the data is not a tree).
```ts
beginHierarchy(rows: readonly unknown[]): boolean
```
#### Parameters
| Name | Type | Description |
| ---- | ---- | ----------- |
| `rows` | readonly unknown[] | |
***
### processSiblings()
Re-shape one natural sibling list. Return the input array when nothing changes.
```ts
processSiblings(nodes: HierarchyNode[], context: HierarchySiblingContext): HierarchyNode[]
```
#### Parameters
| Name | Type | Description |
| ---- | ---- | ----------- |
| `nodes` | HierarchyNode[] | |
| `context` | HierarchySiblingContext | |
***
### endHierarchy()
Called once per rebuild (only if beginHierarchy returned `true`) with the flattened
rows. `positionOf` returns a row's final position — its depth can exceed the
`context.depth` seen in processSiblings when a transform inserted levels above it.
Return `false` when this rebuild produced no structure from this contributor (e.g. no
list was grouped). When every contributor returns `false`, the rows are announced as a
flat `grid` — no `treegrid` role, no `aria-level` / `aria-setsize` / `aria-posinset`.
```ts
endHierarchy(rows: readonly unknown[], positionOf: (row: unknown) => HierarchyRowMeta | undefined): boolean
```
#### Parameters
| Name | Type | Description |
| ---- | ---- | ----------- |
| `rows` | readonly unknown[] | |
| `positionOf` | (row: unknown) => HierarchyRowMeta | undefined | |
***