Struct Previous

pub struct Previous<T: Component + Clone, S: ScheduleLabel + Clone = Last>(pub T, _);

A component that represents the previous value of another component T. To enable previous-value-tracking for a component use PreviousPlugin. The parameter S must be the same as the one specified in PreviousPlugin, or be ommited, like with PreviousPlugin.

You don’t have to manually add Previous to your entity. This is done automatically in the specified schedule S.

Also note that queries like Query<(&T, &Previous<T>)> won’t match entities that were just created, as the may not have Previous yet.

Like with PreviousPlugin, there is a FixedMain type alias for it: FixedUpdate.

Examples

#[derive(Component, Clone)]
struct Health(pub u32);

fn main() {
    App::new()
        .add_plugins(PreviousPlugin::<Health>::default())
        .add_systems(Update, print_differences)
        .run();
}

fn print_differences(query: Query<(&Health, &Previous<Health>), Changed<Health>>) {
    for (health, previous_health) in &query {
        println!("Health reduced by {}", previous_health.0.0 - health.0);
    }
}

With custom schedule:

#[derive(Component, Clone)]
struct Health(pub u32);

#[derive(DefaultSchedule, ScheduleLabel, Debug, Clone, Hash, PartialEq, Eq)]
struct AfterGameLogic;


// create a type alias to reduce boilerplate
type Previous<T> = bevy_previous::Previous<T, AfterGameLogic>;

fn main() {
    App::new()
        .add_plugins(PreviousPlugin::<Health, AfterGameLogic>::default())
        .add_systems(GameLogic, print_differences)
        .run();
}

fn print_differences(query: Query<(&Health, &Previous<Health>), Changed<Health>>) {
    for (health, previous_health) in &query {
        println!("Health reduced by {}", previous_health.0.0 - health.0);
    }
}

Tuple Fields

0: T

Implementations

impl<T, S> Previous<T, S>

where T: Component + Clone, S: ScheduleLabel + Clone,

pub fn new(value: T) -> Self

Trait Implementations

impl<T: Clone + Component + Clone, S: Clone + ScheduleLabel + Clone> Clone for Previous<T, S>

fn clone(&self) -> Previous<T, S>

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<T: Component + Clone, S: ScheduleLabel + Clone> Component for Previous<T, S>

where Self: Send + Sync + 'static,

const STORAGE_TYPE: StorageType = bevy::ecs::component::StorageType::Table

A constant indicating the storage type used for this component.

fn register_component_hooks(_hooks: &mut ComponentHooks)

Called when registering this component, allowing mutable access to its ComponentHooks.

impl<T: Debug + Component + Clone, S: Debug + ScheduleLabel + Clone> Debug for Previous<T, S>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<'de, T, S: ScheduleLabel + Clone> Deserialize<'de> for Previous<T, S>

where T: Deserialize<'de> + Component + Clone,

fn deserialize<__D>(__deserializer: __D) -> Result<Self, __D::Error>

where __D: Deserializer<'de>,

Deserialize this value from the given Serde deserializer.

impl<T, S> From<T> for Previous<T, S>

where T: Component + Clone, S: ScheduleLabel + Clone,

fn from(value: T) -> Self

Converts to this type from the input type.

impl<T: Hash + Component + Clone, S: Hash + ScheduleLabel + Clone> Hash for Previous<T, S>

fn hash<__H: Hasher>(&self, state: &mut __H)

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl<T: Ord + Component + Clone, S: Ord + ScheduleLabel + Clone> Ord for Previous<T, S>

fn cmp(&self, other: &Previous<T, S>) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

where Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized,

Restrict a value to a certain interval.

impl<T: PartialEq + Component + Clone, S: PartialEq + ScheduleLabel + Clone> PartialEq for Previous<T, S>

fn eq(&self, other: &Previous<T, S>) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<T: PartialOrd + Component + Clone, S: PartialOrd + ScheduleLabel + Clone> PartialOrd for Previous<T, S>

fn partial_cmp(&self, other: &Previous<T, S>) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, other: &Rhs) -> bool

Tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

Tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, other: &Rhs) -> bool

Tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

Tests greater than or equal to (for self and other) and is used by the >= operator.

impl<T, S: ScheduleLabel + Clone> Serialize for Previous<T, S>

where T: Serialize + Component + Clone,

fn serialize<__S>(&self, __serializer: __S) -> Result<__S::Ok, __S::Error>

where __S: Serializer,

Serialize this value into the given Serde serializer.

impl<T: Eq + Component + Clone, S: Eq + ScheduleLabel + Clone> Eq for Previous<T, S>

impl<T: Component + Clone, S: ScheduleLabel + Clone> StructuralPartialEq for Previous<T, S>