Observers don't respect the "stack nature" of commands

[Pascualex]

Bevy version

v0.16.1

What you did

Run commands from an observer.

What went wrong

I was expecting those commands to be executed before any other observer, instead several observers ran before the "previous" commands.

The stack nature of commands is critical for certain tasks. In my case, I want to fully solve one collision and its consequences before moving to the next one since I don't want my projectiles to hit more than one target.

Order I expected:

• Observer 1
  • Command 1 from Observer 1
  • Command 2 from Observer 1
• Observer 2
  • Command 3 from Observer 2
  • Command 4 from Observer 2

Order I got:

• Observer 1
• Observer 2
• Command 1 from Observer 1
• Command 2 from Observer 1
• Command 3 from Observer 2
• Command 4 from Observer 2

Additional information

It is easy to work around, now my observers only do one thing: run a single one-shot system. This way, all my logic now runs in command-land and the stack works as expected.

Order I have now:

• Observer 1
• Observer 2
• Command A from Observer 1
  • Command 1 from Command A
  • Command 2 from Command A
• Command B from Observer 2
  • Command 3 from Command B
  • Command 4 from Command B

Maybe this could also be the solution? Making observers insert a command instead of running their system directly?

In my case the consequence was that my game was crashing unexpectedly, since I was triggering new events and despawning entities. When unexpected ordering caused me to trigger an event on a despawned entity, the game crashed.

[alice-i-cecile]

I agree that this is a bug: it makes using observers for complex logic (like turn-based games) much harder.

[notmd]

This might be hard or impossible to solve due to the deferred nature of hook/observer. Especially is the stackoverflow error.

[alice-i-cecile]

@james-j-obrien says:

[15:15]James 🦃: Can't do that without breaking the guarantee that an OnAdd observer will actually have the component
[15:15]James 🦃: (plus you could create new observers so you can't hold any iterators on the observer cache)