hecs-hierarchy

hecs-hierarchy

Hierarchy implementation for use with the hecs ECS.

Features

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

Motivation

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.

Usage

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`
world.detach::<Tree>(child).unwrap();

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

Inspiration

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

• shipyard-hierarchy