Add system sets and run criteria example (#1909)

If accepted, fixes #1694 .

I wanted to explore system sets and run criteria a bit, so made an example expanding a bit on the snippets shown in the 0.5 release post.

Shows a couple of system sets, uses system labels, run criterion, and a use of the interesting `RunCriterion::pipe` functionality.

Cargo.toml

@@ -294,6 +294,10 @@ path = "examples/ecs/system_chaining.rs"
 name = "system_param"
 path = "examples/ecs/system_param.rs"
 
+[[example]]
+name = "system_sets"
+path = "examples/ecs/system_sets.rs"
+
 [[example]]
 name = "timers"
 path = "examples/ecs/timers.rs"

examples/README.md

@@ -150,6 +150,7 @@ Example | File | Description
 `state` | [`ecs/state.rs`](./ecs/state.rs) | Illustrates how to use States to control transitioning from a Menu state to an InGame state
 `system_chaining` | [`ecs/system_chaining.rs`](./ecs/system_chaining.rs) | Chain two systems together, specifying a return type in a system (such as `Result`)
 `system_param` | [`ecs/system_param.rs`](./ecs/system_param.rs) | Illustrates creating custom system parameters with `SystemParam`
+`system_sets` | [`ecs/system_sets.rs`](./ecs/system_sets.rs) | Shows `SystemSet` use along with run criterion
 `timers` | [`ecs/timers.rs`](./ecs/timers.rs) | Illustrates ticking `Timer` resources inside systems and handling their state
 
 ## Games

examples/ecs/system_sets.rs

@@ -0,0 +1,163 @@
+use bevy::{app::AppExit, ecs::schedule::ShouldRun, prelude::*};
+
+/// A [SystemLabel] can be applied as a label to systems and system sets,
+/// which can then be referred to from other systems.
+/// This is useful in case a user wants to e.g. run _before_ or _after_
+/// some label.
+/// `Clone`, `Hash`, `Debug`, `PartialEq`, `Eq`, are all required to derive
+/// [SystemLabel].
+#[derive(Clone, Hash, Debug, PartialEq, Eq, SystemLabel)]
+struct Physics;
+
+#[derive(Clone, Hash, Debug, PartialEq, Eq, SystemLabel)]
+struct PostPhysics;
+
+/// Resource used to stop our example.
+#[derive(Default)]
+struct Done(bool);
+
+/// This is used to show that within a [SystemSet], individual systems can also
+/// be labelled, allowing further fine tuning of run ordering.
+#[derive(Clone, Hash, Debug, PartialEq, Eq, SystemLabel)]
+pub enum PhysicsSystem {
+    UpdateVelocity,
+    Movement,
+}
+
+/// This example realizes the following scheme:
+///
+/// ```none
+/// Physics                     (Criteria: App has run < 1.0 seconds)
+///     \--> update_velocity        (via label PhysicsSystem::UpdateVelocity)
+///     \--> movement               (via label PhysicsSystem::Movement)
+/// PostPhysics                 (Criteria: Resource `done` is false)
+///     \--> collision || sfx
+/// Exit                        (Criteria: Resource `done` is true)
+///     \--> exit
+/// ```
+///
+/// The `Physics` label represents a [SystemSet] containing two systems.
+/// This set's criteria is to stop after a second has elapsed.
+/// The two systems (update_velocity, movement) runs in a specified order.
+///
+/// Another label `PostPhysics` uses run criteria to only run after `Physics` has finished.
+/// This set's criteria is to run only when _not done_, as specified via a resource.
+/// The two systems here (collision, sfx) are not specified to run in any order, and the actual
+/// ordering can then change between invocations.
+///
+/// Lastly a system with run criterion _done_ is used to exit the app.
+/// ```
+fn main() {
+    App::build()
+        .add_plugins(DefaultPlugins)
+        .init_resource::<Done>()
+        // Note that the system sets added in this example set their run criteria explicitly.
+        // See the `ecs/state.rs` example for a pattern where run criteria are set implicitly for common
+        // use cases- typically state transitions.
+        // Also note that a system set has a single run criterion at most, which means using `.with_run_criteria(...)`
+        // after `SystemSet::on_update(...)` would override the state transition criterion.
+        .add_system_set(
+            SystemSet::new()
+                // This label is added to all systems in this set.
+                // The label can then be referred to elsewhere (other sets).
+                .label(Physics)
+                // This criteria ensures this whole system set only runs when this system's
+                // output says so (ShouldRun::Yes)
+                .with_run_criteria(run_for_a_second.system())
+                .with_system(
+                    update_velocity
+                        .system()
+                        // Only applied to the `update_velocity` system
+                        .label(PhysicsSystem::UpdateVelocity),
+                )
+                .with_system(
+                    movement
+                        .system()
+                        // Only applied to the `movement` system
+                        .label(PhysicsSystem::Movement)
+                        // Enforce order within this system by specifying this
+                        .after(PhysicsSystem::UpdateVelocity),
+                ),
+        )
+        .add_system_set(
+            SystemSet::new()
+                .label(PostPhysics)
+                // This whole set runs after `Physics` (which in this case is a label for
+                // another set).
+                // There is also `.before(..)`.
+                .after(Physics)
+                // This shows that we can modify existing run criteria results.
+                // Here we create a _not done_ criteria by piping the output of
+                // the `is_done` system and inverting the output.
+                // Notice a string literal also works as a label.
+                .with_run_criteria(RunCriteria::pipe("is_done_label", inverse.system()))
+                // `collision` and `sfx` are not ordered with respect to
+                // each other, and may run in any order
+                .with_system(collision.system())
+                .with_system(sfx.system()),
+        )
+        .add_system(
+            exit.system()
+                .after(PostPhysics)
+                // Label the run criteria such that the `PostPhysics` set can reference it
+                .with_run_criteria(is_done.system().label("is_done_label")),
+        )
+        .run();
+}
+
+/// Example of a run criteria.
+/// Here we only want to run for a second, then stop.
+fn run_for_a_second(time: Res<Time>, mut done: ResMut<Done>) -> ShouldRun {
+    let elapsed = time.seconds_since_startup();
+    if elapsed < 1.0 {
+        info!(
+            "We should run again. Elapsed/remaining: {:.2}s/{:.2}s",
+            elapsed,
+            1.0 - elapsed
+        );
+        ShouldRun::Yes
+    } else {
+        done.0 = true;
+        ShouldRun::No
+    }
+}
+
+/// Another run criteria, simply using a resource.
+fn is_done(done: Res<Done>) -> ShouldRun {
+    if done.0 {
+        ShouldRun::Yes
+    } else {
+        ShouldRun::No
+    }
+}
+
+/// Used with [RunCritera::pipe], inverts the result of the
+/// passed system.
+fn inverse(input: In<ShouldRun>) -> ShouldRun {
+    match input.0 {
+        ShouldRun::No => ShouldRun::Yes,
+        ShouldRun::Yes => ShouldRun::No,
+        _ => unreachable!(),
+    }
+}
+
+fn update_velocity() {
+    info!("Updating velocity");
+}
+
+fn movement() {
+    info!("Updating movement");
+}
+
+fn collision() {
+    info!("Physics done- checking collisions");
+}
+
+fn sfx() {
+    info!("Physics done- playing some sfx");
+}
+
+fn exit(mut app_exit_events: EventWriter<AppExit>) {
+    info!("Exiting...");
+    app_exit_events.send(AppExit);
+}