rework example

Author: NathanSWard

examples/ecs/command_error_handling.rs

@@ -1,51 +1,72 @@
+use bevy::core::FixedTimestep;
 use bevy::prelude::*;
 
 fn main() {
     App::build()
-        .add_startup_system(handle_command_error.system())
+        .add_startup_system(setup.system())
+        .add_system_set(
+            SystemSet::new()
+                .with_run_criteria(FixedTimestep::step(1.0))
+                .with_system(despawn_all_entities.system()),
+        )
+        .add_system(insert_components.system())
         .run();
 }
 
 #[derive(Debug)]
-struct AComponent(usize);
-
-fn handle_command_error(mut commands: Commands) {
-    let e = commands.spawn().id();
-
-    // Immediately despawn the entity.
-    commands.entity(e).despawn();
-
-    // This `despawn` command will fail because the entity was already despawned!
-    // If no error handler is specified, the error will be logged.
-    commands.entity(e).despawn();
-
-    // Optionally, `on_failure` allows you to provide a custom error handler!
-    commands
-        .entity(e)
-        .insert(AComponent(0))
-        .on_failure(|CommandError { error, world, .. }| {
-            // You'll notice that the `error` will also give you back the component you
-            // attempted to insert on the entity.
-
-            println!(
-                "Sadly our component '{:?}' for entity '{:?}' didn't insert... :(",
-                error.component, error.entity
-            );
-
-            // error handlers have mutable access to `World`
-            world.insert_resource("🐦");
-        });
-
-    // Some nice things:
-    // - You can still chain commands!
-    // - There are a slew of built-in error handlers
-    commands
-        .entity(e)
-        .insert(AComponent(1))
-        .ignore() // `ignore` will neither log nor panic the error
-        .insert(AComponent(2))
-        .log_on_failure(); // `log_on_failure` is the default behavior, and will log the error
-
-    // Uncomment the below line to see the command error cause a panic due to `panic_on_failure`
-    // commands.entity(e).despawn().panic_on_failure();
+struct A(usize);
+
+#[derive(Bundle, Default)]
+struct B {
+    value: usize,
+}
+
+struct FailedDespawnAttempts(usize);
+
+fn setup(mut commands: Commands) {
+    for i in 0..3 {
+        // Note that `insert` is a fallible function and could fail if the entity doesn't exist.
+        // If no error handler is specified, the default behavior is to log the error, and continue.
+        // This call to `insert` will not fail, since the entity is valid.
+        commands.spawn().insert(A(i));
+    }
+
+    commands.insert_resource(FailedDespawnAttempts(0));
+}
+
+fn despawn_all_entities(mut commands: Commands, query: Query<Entity>) {
+    for e in query.iter() {
+        // `on_failure` allows you to provide a custom error handler!
+        commands
+            .entity(e)
+            .despawn()
+            .on_failure(|CommandError { error, world, .. }| {
+                // You'll notice that the `error` will also give you back the entity
+                // you tried to despawn.
+                let entity = error.entity;
+
+                println!("Sadly our entity '{:?}' didn't despawned... :(", entity);
+
+                // error handlers have mutable access to `World`
+                if let Some(mut failed_despawns) = world.get_resource_mut::<FailedDespawnAttempts>()
+                {
+                    failed_despawns.0 += 1;
+                }
+            });
+    }
+}
+
+fn insert_components(mut commands: Commands, query: Query<Entity>) {
+    for (i, e) in query.iter().enumerate() {
+        // Some nice things:
+        // - You can still chain commands!
+        // - There are a slew of built-in error handlers
+        commands
+            .entity(e)
+            .insert(A(i))
+            .ignore() // `ignore` will neither log nor panic the error
+            .insert_bundle(B::default())
+            .log_on_failure(); // `log_on_failure` is the default behavior, and will log the error.
+                               // `panic_on_failure` is another alternative which will panic on the error.
+    }
 }