Use Bevy state API for client and server states v2 #565

Shatur · 2025-09-06T12:45:34Z

Recently I took a quick look at how backends handle system ordering, and they didn't do it the way I intended. For example, no one uses the PrepareSend sets. This isn't the authors' fault - it just means our API sucks and we need to improve it.

This PR resurrects (and extends) my previous attempt to migrate to states (#487), which solves the problem. I closed it last time because of the inevitable one-frame delay: StateTransitions runs after PreUpdate, but backend receive systems (and ours) run in PreUpdate. Since our systems rely on the state, they wouldn't run until the next frame.
But I found a simple solution: run our receive systems additionally in OnEnter(ClientState::Connected)!

Pros of this approach:

States let users to use things like StateScoped, and run systems more efficiently using OnEnter or OnExit instead of evaluating conditions every frame.
No more PrepareSend set or tricky state updates split between PreUpdate and PostUpdate. Backends just update the state in PreUpdate during ReceivePackets.
No more RepliconServer and RepliconClient. Since they only hold messages, they're now renamed to ServerMessages and ClientMessages.
The state can now be queried in observers during replication receive. Needed for bevy_rewind.

Cons:

We now depend on bevy_state, which means we have to use it everywhere in our tests since we rely on MinimalPlugins. But I think that's fine.

To make the review simpler, I split each change into a separate commit. I'd recommend reviewing them one by one. The only important change is 71e97b1. Everything else is just renames/refactors (which take most of the PR diff).

codecov · 2025-09-06T13:29:01Z

Codecov Report

❌ Patch coverage is 86.81319% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.45%. Comparing base (aca414e) to head (dc5c386).
⚠️ Report is 13 commits behind head on master.

Files with missing lines	Patch %	Lines
src/shared/backend/client_messages.rs	83.33%	7 Missing ⚠️
src/shared/event/server_event.rs	45.45%	6 Missing ⚠️
src/server.rs	78.94%	4 Missing ⚠️
src/client/event.rs	92.30%	2 Missing ⚠️
src/server/event.rs	83.33%	2 Missing ⚠️
src/client.rs	96.15%	1 Missing ⚠️
src/shared/backend/server_messages.rs	75.00%	1 Missing ⚠️
src/test_app.rs	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #565      +/-   ##
==========================================
+ Coverage   78.34%   78.45%   +0.10%     
==========================================
  Files          57       56       -1     
  Lines        3187     3133      -54     
==========================================
- Hits         2497     2458      -39     
+ Misses        690      675      -15

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This allows users to utilize things like `StateScoped` and run systems more efficiently using `OnEnter` or `OnExit` without evaluating conditions every frame. As a result, we now require `StatesPlugin` to be present. It's included by default in `DefaultPlugins`, but with `MinimalPlugins` you have to add it manually. In tests, I had to add `StatesPlugin` everywhere.

It's associated with both connected client on the server and the client itself.

Makes it more clear that it's only for clients.

Because it holds only messages now.

Shatur · 2025-09-06T14:15:10Z

Migrated bevy_replicon_renet to test the changes more:
simgine/bevy_replicon_renet#46

Works great, no logical changes to any tests needed.

UkoeHB · 2025-09-14T02:03:08Z

The PrepareSend sets would be used in renet2. I just forgot/haven't updated yet.

UkoeHB · 2025-09-14T02:21:03Z

bevy_replicon_example_backend/src/server.rs

+                    set_stopped.run_if(resource_removed::<ExampleServer>),
+                )
+                    .chain(),
                set_running.run_if(resource_added::<ExampleServer>),


Should be before receive_packets.

Not necessary, the packets can be passed into ServerMessages before we change the state.

UkoeHB · 2025-09-14T02:50:21Z

src/client.rs

+    /// System that sends [`ProtocolHash`].
    ///
-    /// Can be used by backends to add custom logic before sending data, such as transition to a disconnected or connecting state.
-    PrepareSend,


I'd like to keep this for renet2 to use.

I'm not against, but how do you plan to use it? I ported bevy_renet, and PrepareSend wasn't needed because of the states (I used it previously in bevy_renet).

https://github.com/UkoeHB/renet2/blob/fb67e8f0eca07019849525d6662144916f933557/bevy_replicon_renet2/src/client/plugin.rs#L27

No necessary anymore. Just like with the server everything can be done in ClientSet::ReceivePackets. Here is how I did it in renet:
https://github.com/simgine/bevy_replicon_renet/blob/9b45a0bb84f4d47d47cc595b5cfbb6309fc139db/src/client.rs#L23-L24

UkoeHB · 2025-09-14T02:52:53Z

src/client/diagnostics.rs

                add_measurements
                    .in_set(ClientSet::Diagnostics)
-                    .run_if(client_connected),
+                    .run_if(in_state(ClientState::Connected)),
+            )
+            .add_systems(
+                OnEnter(ClientState::Connected),
+                add_measurements.in_set(ClientSet::Diagnostics),
            )


This looks very awkward to me. Adding states means there are systems in the main schedule and in state schedules, which is an increase in complexity not a decrease.

I ran Replicon systems upon entering the schedule and within the schedule. This lets me process packets immediately after connection. It's not ideal, but I think it's worth it.

UkoeHB · 2025-09-14T02:58:55Z

src/server.rs

+    /// [`SyncRelatedAppExt::sync_related_entities`].
    ///
-    /// Can be used by backends to add custom logic before sending data, such as transition to a stopped state.
-    PrepareSend,


I'd like to keep this for use in renet2.

Shatur added 3 commits September 6, 2025 12:38

Add _fn for built system to avoid shadowing function names

d859659

Simplify system ordering

837e1bd

Add StatesPlugin everywhere

181cf74

Shatur requested a review from UkoeHB September 6, 2025 12:45

Shatur force-pushed the states-v2 branch 2 times, most recently from 92f62b6 to b8fa30d Compare September 6, 2025 12:49

Shatur marked this pull request as draft September 6, 2025 12:59

Shatur force-pushed the states-v2 branch 2 times, most recently from 50a9742 to a6b97fa Compare September 6, 2025 13:19

Shatur marked this pull request as ready for review September 6, 2025 13:32

Shatur added 6 commits September 6, 2025 16:39

Move NetworkStats to shared::backend

8c49c09

It's associated with both connected client on the server and the client itself.

Move NetworkStats from RepliconClient into a separate resource

b6286ee

Rename NetworkStats into ClientStats

b069f99

Makes it more clear that it's only for clients.

Rename RepliconClient into ClientMessages

f518137

Because it holds only messages now.

Rename RepliconServer into ServerMessages

dc5c386

Because it holds only messages now.

Shatur force-pushed the states-v2 branch from a6b97fa to dc5c386 Compare September 6, 2025 13:39

This was referenced Sep 6, 2025

Add _fn for built system to avoid shadowing function names #564

Closed

Rework tick settings #567

Merged

Remove StateTransition schedule in favor of using observers bevyengine/bevy#15133

Open

UkoeHB requested changes Sep 14, 2025

View reviewed changes

UkoeHB approved these changes Sep 15, 2025

View reviewed changes

Shatur merged commit 466ac88 into master Sep 15, 2025
10 checks passed

Shatur deleted the states-v2 branch September 15, 2025 22:41

Use Bevy state API for client and server states v2 #565

Use Bevy state API for client and server states v2 #565

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