Timing regarding hooks and observers

[gwafotapa]

I'm curious about the order things run when both a hook and an observer are scheduled to run, assuming both run commands and that the command queue is not empty. I found out the order was:

hook, observer, hook's commands, observer's commands, command queue

I thought since hooks are designed to maintain invariants, their commands would either be pushed to the end of the queue (and their use discouraged in that case) or that they would execute right after the hook itself but the observer runs in between. Is this correct ?

[Henauxg]

Since you have mutable world (DeferredWorld) access in hooks, you usually directly modify entities, components and resources, without using commands (which are deferred). If you are using commands in a hook and want to apply them before the observers are running, you may need to manually flush the command queue on the world.

EDIT: Also, notice that when we're talking about removing or de-spawning, the order of hooks & observers is not the same, observers run before the hooks.

[gwafotapa]

No I was expecting the commands from the hooks to run before the observers themselves as in :
hooks, commands from hooks, observers, commands from observers

[Henauxg]

Oh, I don't quite understand what you meant with pushed to the end of the queue then.
If you're interested, I found a past discussion on Discord about hooks & observers and why they are structured that way (with no immediate commands/mut world access). In short: allowing structural changes would cause many possibly undefined behaviors and would ensue a sizable performance loss/complexity to try and prevent/fix these.

[gwafotapa]

When a hook runs, the command queue might not be empty. When a command is encountered while running the hook, the standard behavior would have been to add that command at the end of the queue. Instead the choice has been made to make these commands jump to the front of the queue. That makes sense to me as you want all the hook logic to apply immediately. But I would have gone one step further and have these commands run before any observer as well. I was curious why this is not the case.

I'm not sure why but I can't access your link.

[Henauxg]

Understood. The link leads to the Bevy Discord server, you can find an invitation in the Bevy Community page, you might not have a Discord account ?

[gwafotapa]

I was missing the invitation. Thanks !