Objective

• Introduce the CommandCondition trait to enable fluent conditional entity mutations.
• Eliminate the need to break method chains when performing operations like insert or remove based on boolean states or lazy evaluation.

Solution

• CommandCondition Trait: Created a new trait defining the evaluate(self) -> bool method. Taking self by value allows closures to capture and move data from their environment flexibly.
• Blanket Implementations: Implemented the trait for bool (direct evaluation) and FnOnce() -> bool (lazy evaluation via closures).
• EntityCommands Extension: Added utility methods that leverage this trait:
  • insert_if: Inserts a bundle only if the condition is true.
  • insert_if_new_and: Inserts a bundle (preserving existing components) under a condition.
  • remove_if: Removes a bundle under a condition.
  • try_insert_if: A silenced version of conditional insertion for entities that might have been despawned.

Testing

• Manual Testing: Verified that components are correctly inserted when the condition is true and skipped when false.
• Compiler Validation: Confirmed that closures capturing local variables (move ||) work as expected due to the self ownership model.
• Platform: Tested on standard development environments via cargo check and cargo test.
• Reviewer Note: Pay close attention to the ownership semantics in CommandCondition; it was designed to be consumed on evaluation to support the broadest range of closure captures.

Showcase

This PR adds a new public API to make system code more declarative and readable.

fn upgrade_player_system(mut commands: Commands, player: Query<(Entity, &Stats)>) {
    for (entity, stats) in &player {
        let is_max_level = stats.level >= 100;

        commands.entity(entity)
            // Usage with a direct boolean
            .insert_if(GodMode, is_max_level)
            // Usage with a closure for lazy evaluation
            .remove_if::<Poison>(stats.health > 50)
            .insert_if_new_and(MasterBadge, stats.has_achievements());
    }
}