bevy_eqchanged

A simple library for Bevy to detect true changes to components.

Why?

Bevy has the Changed<T> query filter. However, it doesn't detect true changes to components. Simply mutably dereferencing (DerefMut) a component is considered a 'change'.

EqChanged<T> will instead keep track of the 'previous value' of a component, and only pass the filter if the previous and new values are !=.

Example

Here is a crude example of where this might be useful. The game would have a debug console where cheat commands could be entered.

#[derive(Component)]
enum PlayerMode {
    Spectator,
    Builder
}

fn main() {
    App::new()
        .add_plugins(DefaultPlugins)
        .add_systems(Update, run_change_mode_cheats)
        .add_systems(Update, on_mode_changed)
        .add_systems(Update, on_mode_eqchanged)
        .run();
}

fn run_change_mode_cheats(
    mut players      : Query<&mut PlayerMode>,
    mut cheat_events : EventReader<CheatCommand>
) {
    for cheat in cheat_events.read() {
        if let CheatCommand::ChangePlayerMode(player_entity, new_player_mode) = command {
            let mut player_mode = player.get_mut(player_entity).unwrap();
            *player_mode = new_player_mode; // 'Changed' here.
        }
    }
}

fn on_mode_changed(
    players : Query<(Entity, &PlayerMode,), Changed<PlayerMode>>
) {
    for (player_entity, player_mode,) in &players {
        println!("Player {} changed mode to {}.", player_entity, player_mode);
    }
}

fn on_mode_eqchanged(
    players : Query<(Entity, &PlayerMode,), EqChanged<PlayerMode>>
) {
    for (player_entity, player_mode,) in &players {
        println!("Player {} EQchanged mode to {}.", player_entity, player_mode);
    }
}

on_mode_changed's query will hit the player every time the change player mode command is run. However, on_mode_eqchanged's query will only hit the player if the old player mode and new player mode were not equal.