Triggering an Event from an exclusive system fails silently

[lucidBrot]

It seems an established fact† that observers can not be exclusive systems. This issue is about the opposite.

† : #14925 and #14507

Running commands.trigger(MyEvent) silently does nothing when done from an exclusive system.

Bevy version and features

bevy 0.17.3

Relevant system information

cargo 1.92.0-nightly (f2932725b 2025-09-24)

OS: debian

$ uname -a
Linux nevermore 6.12.48+deb13-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.12.48-1 (2025-09-20) x86_64 GNU/Linux

What you did

In my project, I had some older code that I was migrating from bevy 0.16, with one part of the code relying on moonshine-save also in need of a larger rework even since before. In that code, I had an exclusive system. The newest version of moonshine-save now uses bevy observers, but adding a commands.trigger call did not do anything. I have now reproduced this with no dependencies other than bevy: Triggering an Event does nothing.

What went wrong

I was expecting commands.trigger(MyEvent) to lead to the execution of the observer that I registered with app.add_observer(my_observer). Nothing happened, the observer was not called. Until I changed my system to no longer take World as parameter.

Additional information

I have attached a small example project that demonstrates the issue.

exclusive-observers.zip

There are two systems, which cause the sun to rise and set. The observer prints when it observes something. It works fine for the sunrise, but does not observe the sun set.

[lucidBrot]

I'm also pasting the code of my example here, for easier reading:

use bevy::prelude::*;
fn main() {
    println!("Hello, world!");
    App::new()
        .add_systems(PreUpdate, cause_sunrise)
        .add_systems(PostUpdate, cause_sunset)
        .add_observer(sun_observer)
        .run();
}

#[derive(Event)]
struct SunEvent{
    text: String
}

fn cause_sunrise(mut commands: Commands){
    println!("Sunrise");
    commands.trigger(SunEvent{
        text: "Sunrise".to_string()
    })
}

fn cause_sunset(
    // Making this very complex system exclusive because that should make it easier to reason about.
    world: &mut World,

    // The actual params (non-exclusive):
    params: &mut bevy::ecs::system::SystemState<(
        Commands,
    )>,

){
    println!("Sunset");

    {
        let ( mut commands,) = params.get_mut(world);
        commands.trigger(SunEvent {
            text: "Sunset".to_string()
        })
    }

    // Actually do something exclusive with world here
    // ...
}

fn sun_observer(event: On<SunEvent>){
    println!("Observed {}", event.text);
}

Output:

/home/generic/.cargo/bin/cargo run --color=always --package exclusive-observers --bin exclusive-observers --profile dev
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.17s
     Running `target/debug/exclusive-observers`
Hello, world!
Sunrise
Observed Sunrise
Sunset

Process finished with exit code 0

[PPakalns]

In SystemState documentation example it is written that apply needs to be called manually.
https://doc.qu1x.dev/bevy_trackball/bevy/ecs/system/struct.SystemState.html#example

I myself would like that there would be some kind of alternative SystemState struct that would do this automatically so that I don't forget to call apply, which needs to be called almost always.

[lucidBrot]

Oh interesting. This is surprising behavior to me.

I would have expected that the commands get, as usual, queued "sometime later but pretty soon", to put it in the vague terms of my understanding. And of course during an exclusive system it can not run the commands, so I'd expect it to run the pending commands after the system, automatically.

It's of course nice to be able to choose to apply them earlier if one so wishes, but leftover commands could maybe still be auto-applied? Or if it has to be manual for now, is there any way I as a user could get notified in any way when I have commands I forgot to apply?
(If there were warnings, I did not notice them)