Add RunIfExists command for only executing if a given entity exists.

src/commands.rs

@@ -0,0 +1,22 @@
+use bevy::{
+    ecs::{system::Command, world::EntityMut},
+    prelude::*,
+};
+
+struct RunIfExistsCommand<F>(Entity, F);
+
+impl<F: FnOnce(EntityMut) + Send + Sync + 'static> Command for RunIfExistsCommand<F> {
+    fn write(self, world: &mut World) {
+        world.get_entity_mut(self.0).map(self.1);
+    }
+}
+
+pub trait RunIfExistsExt {
+    fn run_if_exists(&mut self, entity: Entity, f: impl FnOnce(EntityMut) + Send + Sync + 'static);
+}
+
+impl RunIfExistsExt for Commands<'_, '_> {
+    fn run_if_exists(&mut self, entity: Entity, f: impl FnOnce(EntityMut) + Send + Sync + 'static) {
+        self.add(RunIfExistsCommand(entity, f));
+    }
+}