bevy_system_graph

This crate provides the utilities for creating strictly ordered execution graphs of systems for the Bevy game engine.

Bevy Version Supported

Starting a Graph

To start building a system graph, one or more systems must be added to the graph as root nodes. Root systems have no dependencies within the graph.

let graph = SystemGraph::new();

// Create a root system for the graph.
let root_a = graph.root(sys_a);

// Graphs can have multiple root nodes.
let root_b = graph.root(sys_b);
let root_c = graph.root(sys_c);

Using SystemLabels

Systems can still use labels to establish the ordering of systems relative to other systems outside of the graph.

let graph = SystemGraph::new();
let root_a = graph.root(
	sys_a
	   .label("Physics")
	   .before("Propagate Transforms")
);

Conversion into SystemSet

To ease adding all of the graph's systems into a Schedule, both SystemGraph and SystemGraphNode implement Into<SystemSet>.

let graph = SystemGraph::new();
let root_a = graph.root(sys_a);

// Convert into a SystemSet
let system_set: SystemSet = graph.into();

Sequential Execution

Graph nodes can be sequentially chained via SystemGraphNode::then. This creates a new node from a system and adds a "after" dependency on the original system.

let graph = SystemGraph::new();
graph
  .root(sys_a)
  .then(sys_b)
  .then(sys_c);

// Convert into a SystemSet
let system_set: SystemSet = graph.into();

Fan Out

SystemGraphNode::fork can be used to fan out into multiple branches. All fanned out systems will not execute until the original has finished, but do not have a mutual dependency on each other.

let graph = SystemGraph::new();

// Fork out from one original node.
// sys_b, sys_c, and sys_d will only start when sys_a finishes.
let (c, b, d) = graph.root(sys_a)
    .fork((
        sys_b,
        sys_c,
        sys_d,
    ));

// Alternatively, calling "then" repeatedly achieves the same thing.
let e = d.then(sys_e);
let f = d.then(sys_f);

// Convert into a SystemSet
let system_set: SystemSet = graph.into();

Fan In

A graph node can wait on multiple systems before running via SystemJoin::join. The system will not run until all prior systems are finished.

let graph = SystemGraph::new();

let start_a = graph.root(sys_a);
let start_b = graph.root(sys_b);
let start_c = graph.root(sys_c);

(start_a, start_b, start_c)
    .join(sys_d)
    .then(sys_e);

// Convert into a SystemSet
let system_set: SystemSet = graph.into();

Fan Out into Fan In

The types used to implement fork and join are composable.

let graph = SystemGraph::new();
graph.root(sys_a)
     .fork((sys_b, sys_c, sys_d))
     .join(sys_e)
     .then(sys_f);

// Convert into a SystemSet
let system_set: SystemSet = graph.into();

Cloning

Individual [graph nodes] are backed by a Rc, so cloning it will still point to the same logical underlying graph.