Struct Mut

pub struct Mut<'w, T: ?Sized> { /* private fields */ }

Unique mutable borrow of an entity’s component or of a resource.

This can be used in queries to opt into change detection on both their mutable and immutable forms, as opposed to &mut T, which only provides access to change detection while in its mutable form:

#[derive(Component, Clone)]
struct Name(String);

#[derive(Component, Clone, Copy)]
struct Health(f32);

#[derive(Component, Clone, Copy)]
struct Position {
    x: f32,
    y: f32,
};

#[derive(Component, Clone, Copy)]
struct Player {
    id: usize,
};

#[derive(QueryData)]
#[query_data(mutable)]
struct PlayerQuery {
    id: &'static Player,

    // Reacting to `PlayerName` changes is expensive, so we need to enable change detection when reading it.
    name: Mut<'static, Name>,

    health: &'static mut Health,
    position: &'static mut Position,
}

fn update_player_avatars(players_query: Query<PlayerQuery>) {
    // The item returned by the iterator is of type `PlayerQueryReadOnlyItem`.
    for player in players_query.iter() {
        if player.name.is_changed() {
            // Update the player's name. This clones a String, and so is more expensive.
            update_player_name(player.id, player.name.clone());
        }

        // Update the health bar.
        update_player_health(player.id, *player.health);

        // Update the player's position.
        update_player_position(player.id, *player.position);
    }
}

Implementations

impl<'w, T: ?Sized> Mut<'w, T>

pub fn new( value: &'w mut T, added: &'w mut Tick, last_changed: &'w mut Tick, last_run: Tick, this_run: Tick, ) -> Self

Creates a new change-detection enabled smart pointer. In almost all cases you do not need to call this method manually, as instances of Mut will be created by engine-internal code.

Many use-cases of this method would be better served by Mut::map_unchanged or Mut::reborrow.

• value - The value wrapped by this smart pointer.
• added - A Tick that stores the tick when the wrapped value was created.
• last_changed - A Tick that stores the last time the wrapped value was changed. This will be updated to the value of change_tick if the returned smart pointer is modified.
• last_run - A Tick, occurring before this_run, which is used as a reference to determine whether the wrapped value is newly added or changed.
• this_run - A Tick corresponding to the current point in time – “now”.

impl<'w, T: ?Sized> Mut<'w, T>

pub fn into_inner(self) -> &'w mut T

Consume self and return a mutable reference to the contained value while marking self as “changed”.

pub fn reborrow(&mut self) -> Mut<'_, T>

Returns a Mut<> with a smaller lifetime. This is useful if you have &mut Mut <T>, but you need a Mut<T>.

pub fn map_unchanged<U: ?Sized>( self, f: impl FnOnce(&mut T) -> &mut U, ) -> Mut<'w, U>

Maps to an inner value by applying a function to the contained reference, without flagging a change.

You should never modify the argument passed to the closure – if you want to modify the data without flagging a change, consider using DetectChangesMut::bypass_change_detection to make your intent explicit.

// When run, zeroes the translation of every entity.
fn reset_positions(mut transforms: Query<&mut Transform>) {
    for transform in &mut transforms {
        // We pinky promise not to modify `t` within the closure.
        // Breaking this promise will result in logic errors, but will never cause undefined behavior.
        let mut translation = transform.map_unchanged(|t| &mut t.translation);
        // Only reset the translation if it isn't already zero;
        translation.set_if_neq(Vec2::ZERO);
    }
}

pub fn as_deref_mut(&mut self) -> Mut<'_, <T as Deref>::Target>

where T: DerefMut,

Allows you access to the dereferenced value of this pointer without immediately triggering change detection.

Trait Implementations

impl<'w, T> AsMut<T> for Mut<'w, T>

fn as_mut(&mut self) -> &mut T

Converts this type into a mutable reference of the (usually inferred) input type.

impl<'w, T> AsRef<T> for Mut<'w, T>

fn as_ref(&self) -> &T

Converts this type into a shared reference of the (usually inferred) input type.

impl<'w, T> Debug for Mut<'w, T>

where T: Debug + ?Sized,

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

Formats the value using the given formatter.

impl<'w, T: ?Sized> Deref for Mut<'w, T>

type Target = T

The resulting type after dereferencing.

fn deref(&self) -> &Self::Target

Dereferences the value.

impl<'w, T: ?Sized> DerefMut for Mut<'w, T>

fn deref_mut(&mut self) -> &mut Self::Target

Mutably dereferences the value.

impl<'w, T: ?Sized> DetectChanges for Mut<'w, T>

fn is_added(&self) -> bool

Returns true if this value was added after the system last ran.

fn is_changed(&self) -> bool

Returns true if this value was added or mutably dereferenced either since the last time the system ran or, if the system never ran, since the beginning of the program.

fn last_changed(&self) -> Tick

Returns the change tick recording the time this data was most recently changed.

impl<'w, T: ?Sized> DetectChangesMut for Mut<'w, T>

type Inner = T

The type contained within this smart pointer

fn set_changed(&mut self)

Flags this value as having been changed.

fn set_last_changed(&mut self, last_changed: Tick)

Manually sets the change tick recording the time when this data was last mutated.

fn bypass_change_detection(&mut self) -> &mut Self::Inner

Manually bypasses change detection, allowing you to mutate the underlying value without updating the change tick.

fn set_if_neq(&mut self, value: Self::Inner) -> bool

where Self::Inner: Sized + PartialEq,

Overwrites this smart pointer with the given value, if and only if *self != value. Returns true if the value was overwritten, and returns false if it was not.

fn replace_if_neq(&mut self, value: Self::Inner) -> Option<Self::Inner>

where Self::Inner: Sized + PartialEq,

Overwrites this smart pointer with the given value, if and only if *self != value, returning the previous value if this occurs.

impl<'w, T> From<Mut<'w, T>> for MutUntyped<'w>

fn from(value: Mut<'w, T>) -> Self

Converts to this type from the input type.

impl<'w, T: ?Sized> From<Mut<'w, T>> for Ref<'w, T>

fn from(mut_ref: Mut<'w, T>) -> Self

Converts to this type from the input type.

impl<'w, T: 'static> From<NonSendMut<'w, T>> for Mut<'w, T>

fn from(other: NonSendMut<'w, T>) -> Mut<'w, T>

Convert this NonSendMut into a Mut. This allows keeping the change-detection feature of Mut while losing the specificity of NonSendMut.

impl<'w, T: Resource> From<ResMut<'w, T>> for Mut<'w, T>

fn from(other: ResMut<'w, T>) -> Mut<'w, T>

Convert this ResMut into a Mut. This allows keeping the change-detection feature of Mut while losing the specificity of ResMut for resources.

impl<'w, 'a, T> IntoIterator for &'a Mut<'w, T>

where &'a T: IntoIterator,

type Item = <&'a T as IntoIterator>::Item

The type of the elements being iterated over.

type IntoIter = <&'a T as IntoIterator>::IntoIter

Which kind of iterator are we turning this into?

fn into_iter(self) -> Self::IntoIter

Creates an iterator from a value.

impl<'w, 'a, T> IntoIterator for &'a mut Mut<'w, T>

where &'a mut T: IntoIterator,

type Item = <&'a mut T as IntoIterator>::Item

The type of the elements being iterated over.

type IntoIter = <&'a mut T as IntoIterator>::IntoIter

Which kind of iterator are we turning this into?

fn into_iter(self) -> Self::IntoIter

Creates an iterator from a value.

impl<'__w, T: Component> QueryData for Mut<'__w, T>

type ReadOnly = Ref<'__w, T>

The read-only variant of this QueryData, which satisfies the ReadOnlyQueryData trait.

impl<'__w, T: Component> WorldQuery for Mut<'__w, T>

When Mut<T> is used in a query, it will be converted to Ref<T> when transformed into its read-only form, providing access to change detection methods.

By contrast &mut T will result in a Mut<T> item in mutable form to record mutations, but result in a bare &T in read-only form.

SAFETY: fetch accesses a single component mutably. This is sound because update_component_access and update_archetype_component_access add write access for that component and panic when appropriate. update_component_access adds a With filter for a component. This is sound because matches_component_set returns whether the set contains that component.

const IS_DENSE: bool = <&mut T as WorldQuery>::IS_DENSE

Returns true if (and only if) every table of every archetype matched by this fetch contains all of the matched components. This is used to select a more efficient “table iterator” for “dense” queries. If this returns true, WorldQuery::set_table must be used before WorldQuery::fetch can be called for iterators. If this returns false, WorldQuery::set_archetype must be used before WorldQuery::fetch can be called for iterators.

type Item<'w> = Mut<'w, T>

The item returned by this WorldQuery For QueryData this will be the item returned by the query. For QueryFilter this will be either (), or a bool indicating whether the entity should be included or a tuple of such things.

type Fetch<'w> = WriteFetch<'w, T>

Per archetype/table state used by this WorldQuery to fetch Self::Item

type State = ComponentId

State used to construct a Self::Fetch. This will be cached inside QueryState, so it is best to move as much data / computation here as possible to reduce the cost of constructing Self::Fetch.

fn shrink<'wlong: 'wshort, 'wshort>(item: Mut<'wlong, T>) -> Mut<'wshort, T>

This function manually implements subtyping for the query items.

unsafe fn init_fetch<'w>( world: UnsafeWorldCell<'w>, state: &ComponentId, last_run: Tick, this_run: Tick, ) -> WriteFetch<'w, T>

Creates a new instance of this fetch.

unsafe fn set_archetype<'w>( fetch: &mut WriteFetch<'w, T>, state: &ComponentId, archetype: &'w Archetype, table: &'w Table, )

Adjusts internal state to account for the next Archetype. This will always be called on archetypes that match this WorldQuery.

unsafe fn set_table<'w>( fetch: &mut WriteFetch<'w, T>, state: &ComponentId, table: &'w Table, )

Adjusts internal state to account for the next Table. This will always be called on tables that match this WorldQuery.

unsafe fn fetch<'w>( fetch: &mut Self::Fetch<'w>, entity: Entity, table_row: TableRow, ) -> Mut<'w, T>

Fetch Self::Item for either the given entity in the current Table, or for the given entity in the current Archetype. This must always be called after WorldQuery::set_table with a table_row in the range of the current Table or after WorldQuery::set_archetype with a entity in the current archetype.

fn update_component_access( component_id: &ComponentId, access: &mut FilteredAccess<ComponentId>, )

Adds any component accesses used by this WorldQuery to access.

fn init_state(world: &mut World) -> ComponentId

Creates and initializes a State for this WorldQuery type.

fn get_state(components: &Components) -> Option<ComponentId>

Attempts to initialize a State for this WorldQuery type using read-only access to Components.

fn matches_component_set( state: &ComponentId, set_contains_id: &impl Fn(ComponentId) -> bool, ) -> bool

Returns true if this query matches a set of components. Otherwise, returns false.

fn set_access(_state: &mut Self::State, _access: &FilteredAccess<ComponentId>)

Sets available accesses for implementors with dynamic access such as FilteredEntityRef or FilteredEntityMut.