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Cargo Documentation LICENSE

Hierarchy implementation for hecs ECS.


  • Iterate children of parent
  • Lookup parent of child
  • Traverse hierarchy depth first
  • Traverse hierarchy breadth first
  • Traverse ancestors
  • Detach child from hierarchy
  • Reverse iteration
  • Sorting
  • (Optional) associated data to relation

Getting Started

Include both hecs and hecs-hierarchy as dependencies in your Cargo.toml.

hecs-hierarchy does not re-export hecs

hecs = 0.5
hecs-hierarchy = 0.1


An ECS is a fantastic design principle for designing software which allows a data oriented design. Most of the time, the ECS is flat with maybe a few components referencing each other via Entity ids. Sometimes however, the need to create and manage proper, well behaved graphs, arises.

This is were hecs-hierarchy comes in and gives the ability to manage directed graphs that can connect entities. This is very useful when developing a UI library using the ECS design pattern, or purely for grouping entities together from the same model.


Import the Hierarchy trait which extends hecs::World

The trait Hierarchy extends hecs::World with functions for manipulating and iterating the hierarchy tree.

The hierarchy uses a marker type which makes it possible for a single entity to belong to several hierarchy trees.

See the documentation, more specifically the Hierarchy trait

Example usage:

use hecs_hierarchy::*;

// Marker type which allows several hierarchies.
struct Tree;

let mut world = hecs::World::default();

// Create a root entity, there can be several.
let root = world.spawn(("Root",));

// Create a loose entity
let child = world.spawn(("Child 1",));

// Attaches the child to a parent, in this case `root`
world.attach::<Tree>(child, root).unwrap();

// Iterate children
for child in world.children::<Tree>(root) {
    let name = world.get::<&str>(child).unwrap();
    println!("Child: {:?} {}", child, *name);

// Add a grandchild
world.attach_new::<Tree, _>(child, ("Grandchild",)).unwrap();

// Iterate recursively
for child in world.descendants_depth_first::<Tree>(root) {
    let name = world.get::<&str>(child).unwrap();
    println!("Child: {:?} {}", child, *name)

// Detach `child` and `grandchild`

let child2 = world.attach_new::<Tree, _>(root, ("Child 2",)).unwrap();

// Reattach as a child of `child2`
world.attach::<Tree>(child, child2).unwrap();

world.attach_new::<Tree, _>(root, ("Child 3",)).unwrap();

// Hierarchy now looks like this:
// Root
// |-------- Child 3
// |-------- Child 2
//           |-------- Child 1
//                     |-------- Grandchild


This project is heavily inspired by Shipyard’s hierarchy implementation and exposes a similar API.


Component of a child entity in hierarchy tree T. Children represent a circular linked list. Since Parent and child is generic over a marker type, several hierarchies can coexist.

Iterates children along with Query Q. Children who do not satisfy Q will be skipped. Count is known in advanced and will not fold iterator.

Component of a entity with descendents in hierarchy tree T. Children represent a circular linked list. Since Parent and child is generic over a marker type, several hierarchies can coexist.

Ergonomically construct trees without knowledge of world.

Cloneable version of the crate::TreeBuilder


Exported error types. Some of these errors map the hecs errors but provide better context such as the entity.


Non mutating part of hierarchy

A trait for modifying the worlds hierarchy. Implemented for hecs::World>

Type Definitions

A query for defininig a compatible subworld for Hierarchy