91b64df96b
# Objective The `nondeterministic_system_order` example doesn't actually detect and log its deliberate order ambiguities! It should, tho. ## Solution Update the schedule label, and explain in a comment that you can't turn it on for the whole `Main` schedule in one go (alas, that would be nice, but it makes sense that it doesn't work that way).
101 lines
4.1 KiB
Rust
101 lines
4.1 KiB
Rust
//! By default, Bevy systems run in parallel with each other.
|
|
//! Unless the order is explicitly specified, their relative order is nondeterministic.
|
|
//!
|
|
//! In many cases, this doesn't matter and is in fact desirable!
|
|
//! Consider two systems, one which writes to resource A, and the other which writes to resource B.
|
|
//! By allowing their order to be arbitrary, we can evaluate them greedily, based on the data that is free.
|
|
//! Because their data accesses are **compatible**, there is no **observable** difference created based on the order they are run.
|
|
//!
|
|
//! But if instead we have two systems mutating the same data, or one reading it and the other mutating,
|
|
//! then the actual observed value will vary based on the nondeterministic order of evaluation.
|
|
//! These observable conflicts are called **system execution order ambiguities**.
|
|
//!
|
|
//! This example demonstrates how you might detect and resolve (or silence) these ambiguities.
|
|
|
|
use bevy::{
|
|
ecs::schedule::{LogLevel, ScheduleBuildSettings},
|
|
prelude::*,
|
|
};
|
|
|
|
fn main() {
|
|
App::new()
|
|
// We can modify the reporting strategy for system execution order ambiguities on a per-schedule basis.
|
|
// You must do this for each schedule you want to inspect; child schedules executed within an inspected
|
|
// schedule do not inherit this modification.
|
|
.edit_schedule(Update, |schedule| {
|
|
schedule.set_build_settings(ScheduleBuildSettings {
|
|
ambiguity_detection: LogLevel::Warn,
|
|
..default()
|
|
});
|
|
})
|
|
.init_resource::<A>()
|
|
.init_resource::<B>()
|
|
.add_systems(
|
|
Update,
|
|
(
|
|
// This pair of systems has an ambiguous order,
|
|
// as their data access conflicts, and there's no order between them.
|
|
reads_a,
|
|
writes_a,
|
|
// This pair of systems has conflicting data access,
|
|
// but it's resolved with an explicit ordering:
|
|
// the .after relationship here means that we will always double after adding.
|
|
adds_one_to_b,
|
|
doubles_b.after(adds_one_to_b),
|
|
// This system isn't ambiguous with adds_one_to_b,
|
|
// due to the transitive ordering created by our constraints:
|
|
// if A is before B is before C, then A must be before C as well.
|
|
reads_b.after(doubles_b),
|
|
// This system will conflict with all of our writing systems
|
|
// but we've silenced its ambiguity with adds_one_to_b.
|
|
// This should only be done in the case of clear false positives:
|
|
// leave a comment in your code justifying the decision!
|
|
reads_a_and_b.ambiguous_with(adds_one_to_b),
|
|
),
|
|
)
|
|
// Be mindful, internal ambiguities are reported too!
|
|
// If there are any ambiguities due solely to DefaultPlugins,
|
|
// or between DefaultPlugins and any of your third party plugins,
|
|
// please file a bug with the repo responsible!
|
|
// Only *you* can prevent nondeterministic bugs due to greedy parallelism.
|
|
.add_plugins(DefaultPlugins)
|
|
.run();
|
|
}
|
|
|
|
#[derive(Resource, Debug, Default)]
|
|
struct A(usize);
|
|
|
|
#[derive(Resource, Debug, Default)]
|
|
struct B(usize);
|
|
|
|
// Data access is determined solely on the basis of the types of the system's parameters
|
|
// Every implementation of the `SystemParam` and `WorldQuery` traits must declare which data is used
|
|
// and whether or not it is mutably accessed.
|
|
fn reads_a(_a: Res<A>) {}
|
|
|
|
fn writes_a(mut a: ResMut<A>) {
|
|
a.0 += 1;
|
|
}
|
|
|
|
fn adds_one_to_b(mut b: ResMut<B>) {
|
|
b.0 = b.0.saturating_add(1);
|
|
}
|
|
|
|
fn doubles_b(mut b: ResMut<B>) {
|
|
// This will overflow pretty rapidly otherwise
|
|
b.0 = b.0.saturating_mul(2);
|
|
}
|
|
|
|
fn reads_b(b: Res<B>) {
|
|
// This invariant is always true,
|
|
// because we've fixed the system order so doubling always occurs after adding.
|
|
assert!((b.0 % 2 == 0) || (b.0 == usize::MAX));
|
|
}
|
|
|
|
fn reads_a_and_b(a: Res<A>, b: Res<B>) {
|
|
// Only display the first few steps to avoid burying the ambiguities in the console
|
|
if b.0 < 10 {
|
|
info!("{}, {}", a.0, b.0);
|
|
}
|
|
}
|