Add Label type

crates/bevy_transform/src/lib.rs

@@ -7,8 +7,7 @@ pub mod prelude {
 }
 
 use bevy_app::prelude::*;
-use bevy_ecs::IntoSystem;
-use bevy_ecs::ParallelSystemDescriptorCoercion;
+use bevy_ecs::{IntoSystem, ParallelSystemDescriptorCoercion};
 use bevy_reflect::RegisterTypeBuilder;
 use prelude::{parent_update_system, Children, GlobalTransform, Parent, PreviousParent, Transform};
 
@@ -23,15 +22,27 @@ impl Plugin for TransformPlugin {
             .register_type::<Transform>()
             .register_type::<GlobalTransform>()
             // add transform systems to startup so the first update is "correct"
-            .add_startup_system_to_stage(StartupStage::PostStartup, parent_update_system.system())
             .add_startup_system_to_stage(
                 StartupStage::PostStartup,
-                transform_propagate_system::transform_propagate_system.system(),
+                parent_update_system.system().label("parent_update_system"),
+            )
+            .add_startup_system_to_stage(
+                StartupStage::PostStartup,
+                transform_propagate_system::transform_propagate_system
+                    .system()
+                    .label("transform_propagate_system")
+                    .after("parent_update_system"),
+            )
+            .add_system_to_stage(
+                CoreStage::PostUpdate,
+                parent_update_system.system().label("parent_update_system"),
             )
-            .add_system_to_stage(CoreStage::PostUpdate, parent_update_system.system())
             .add_system_to_stage(
                 CoreStage::PostUpdate,
-                transform_propagate_system::transform_propagate_system.system(),
+                transform_propagate_system::transform_propagate_system
+                    .system()
+                    .label("transform_propagate_system")
+                    .after("parent_update_system"),
             );
     }
 }

examples/ecs/ecs_guide.rs

@@ -294,9 +294,9 @@ fn main() {
         // However we can manually specify the stage if we want to. The following is equivalent to add_system(score_system)
         .add_system_to_stage(CoreStage::Update, score_system.system())
         // We can also create new stages. Here is what our games stage order will look like:
-        // `BeforeRound`: new_player_system, new_round_system
-        // `Update`: print_message_system, score_system
-        // `AfterRound`: score_check_system, game_over_system
+        // "before_round": new_player_system, new_round_system
+        // "update": print_message_system, score_system
+        // "after_round": score_check_system, game_over_system
         .add_stage_before(
             CoreStage::Update,
             MyStages::BeforeRound,
@@ -309,11 +309,22 @@ fn main() {
         )
         .add_system_to_stage(MyStages::BeforeRound, new_round_system.system())
         .add_system_to_stage(MyStages::BeforeRound, new_player_system.system())
-        .add_system_to_stage(MyStages::AfterRound, score_check_system.system())
-        .add_system_to_stage(MyStages::AfterRound, game_over_system.system())
-        // score_check_system will run before game_over_system because score_check_system modifies GameState and game_over_system
-        // reads GameState. This works, but it's a bit confusing. In practice, it would be clearer to create a new stage that runs
-        // before "after_round"
+        // We can ensure that game_over system runs after score_check_system using explicit ordering constraints
+        // First, we label the system we want to refer to using `.label`
+        // Then, we use either `.before` or `.after` to describe the order we want the relationship
+        .add_system_to_stage(
+            MyStages::AfterRound,
+            score_check_system.system().label("score_check"),
+        )
+        .add_system_to_stage(
+            MyStages::AfterRound,
+            game_over_system.system().after("score_check"),
+        )
+        // We can check our systems for execution order ambiguities by examining the output produced in the console
+        // by adding the following Resource to our App :)
+        // Be aware that not everything reported by this checker is a potential problem, you'll have to make
+        // that judgement yourself.
+        .insert_resource(ReportExecutionOrderAmbiguities)
         // This call to run() starts the app we just built!
         .run();
 }