always insert Swarm before any systems can start running

azalea-client/src/client.rs

@@ -1,7 +1,7 @@
 use std::{
     collections::HashMap,
     fmt::Debug,
-    io,
+    io, mem,
     net::SocketAddr,
     sync::Arc,
     thread,
@@ -130,7 +130,8 @@ impl<'a> StartClientOpts<'a> {
         let mut app = App::new();
         app.add_plugins(DefaultPlugins);
 
-        let ecs_lock = start_ecs_runner(app);
+        let (ecs_lock, start_running_systems) = start_ecs_runner(app);
+        start_running_systems();
 
         Self {
             ecs_lock,
@@ -663,12 +664,14 @@ impl Plugin for AzaleaPlugin {
     }
 }
 
-/// Start running the ECS loop!
+/// Create the ECS world, and return a function that begins running systems.
+/// This exists to allow you to make last-millisecond updates to the world
+/// before any systems start running.
 ///
 /// You can create your app with `App::new()`, but don't forget to add
 /// [`DefaultPlugins`].
 #[doc(hidden)]
-pub fn start_ecs_runner(mut app: App) -> Arc<Mutex<World>> {
+pub fn start_ecs_runner(mut app: App) -> (Arc<Mutex<World>>, impl FnOnce()) {
     // this block is based on Bevy's default runner:
     // https://github.com/bevyengine/bevy/blob/390877cdae7a17095a75c8f9f1b4241fe5047e83/crates/bevy_app/src/schedule_runner.rs#L77-L85
     if app.plugins_state() != PluginsState::Cleaned {
@@ -686,14 +689,15 @@ pub fn start_ecs_runner(mut app: App) -> Arc<Mutex<World>> {
 
     // all resources should have been added by now so we can take the ecs from the
     // app
-    let ecs = Arc::new(Mutex::new(std::mem::take(app.world_mut())));
+    let ecs = Arc::new(Mutex::new(mem::take(app.world_mut())));
 
-    tokio::spawn(run_schedule_loop(
-        ecs.clone(),
-        *app.main().update_schedule.as_ref().unwrap(),
-    ));
+    let ecs_clone = ecs.clone();
+    let outer_schedule_label = *app.main().update_schedule.as_ref().unwrap();
+    let start_running_systems = move || {
+        tokio::spawn(run_schedule_loop(ecs_clone, outer_schedule_label));
+    };
 
-    ecs
+    (ecs, start_running_systems)
 }
 
 async fn run_schedule_loop(ecs: Arc<Mutex<World>>, outer_schedule_label: InternedScheduleLabel) {

azalea/src/swarm/mod.rs

@@ -396,7 +396,7 @@ where
 
         let main_schedule_label = self.app.main().update_schedule.unwrap();
 
-        let ecs_lock = start_ecs_runner(self.app);
+        let (ecs_lock, start_running_systems) = start_ecs_runner(self.app);
 
         let swarm = Swarm {
             ecs_lock: ecs_lock.clone(),
@@ -420,6 +420,10 @@ where
             ecs.clear_trackers();
         }
 
+        // only do this after we inserted the Swarm and state resources to avoid errors
+        // where Res<Swarm> is inaccessible
+        start_running_systems();
+
         // SwarmBuilder (self) isn't Send so we have to take all the things we need out
         // of it
         let swarm_clone = swarm.clone();