Struct MessageMutator 

pub struct MessageMutator<'w, 's, E: Message> { /* private fields */ }

Mutably reads messages of type T keeping track of which messages have already been read by each system allowing multiple systems to read the same messages. Ideal for chains of systems that all want to modify the same messages.

Usage

MessageMutators are usually declared as a SystemParam.

#[derive(Message, Debug)]
pub struct MyMessage(pub u32); // Custom message type.
fn my_system(mut reader: MessageMutator<MyMessage>) {
    for message in reader.read() {
        message.0 += 1;
        println!("received message: {:?}", message);
    }
}

Concurrency

Multiple systems with MessageMutator<T> of the same message type can not run concurrently. They also can not be executed in parallel with MessageReader or MessageWriter.

Clearing, Reading, and Peeking

Messages are stored in a double buffered queue that switches each frame. This switch also clears the previous frame’s messages. Messages should be read each frame otherwise they may be lost. For manual control over this behavior, see Messages.

Most of the time systems will want to use MessageMutator::read(). This function creates an iterator over all messages that haven’t been read yet by this system, marking the message as read in the process.

Implementations

impl<'w, 's, E: Message> MessageMutator<'w, 's, E>

pub fn read(&mut self) -> MessageMutIterator<'_, E>

Iterates over the messages this MessageMutator has not seen yet. This updates the MessageMutator’s message counter, which means subsequent message reads will not include messages that happened before now.

pub fn read_with_id(&mut self) -> MessageMutIteratorWithId<'_, E>

Like read, except also returning the MessageId of the messages.

pub fn par_read(&mut self) -> MessageMutParIter<'_, E>

Returns a parallel iterator over the messages this MessageMutator has not seen yet. See also for_each.

Example

#[derive(Message)]
struct MyMessage {
    value: usize,
}

#[derive(Resource, Default)]
struct Counter(AtomicUsize);

// setup
let mut world = World::new();
world.init_resource::<Messages<MyMessage>>();
world.insert_resource(Counter::default());

let mut schedule = Schedule::default();
schedule.add_systems(|mut messages: MessageMutator<MyMessage>, counter: Res<Counter>| {
    messages.par_read().for_each(|MyMessage { value }| {
        counter.0.fetch_add(*value, Ordering::Relaxed);
    });
});
for value in 0..100 {
    world.write_message(MyMessage { value });
}
schedule.run(&mut world);
let Counter(counter) = world.remove_resource::<Counter>().unwrap();
// all messages were processed
assert_eq!(counter.into_inner(), 4950);

pub fn len(&self) -> usize

Determines the number of messages available to be read from this MessageMutator without consuming any.

pub fn is_empty(&self) -> bool

Returns true if there are no messages available to read.

Example

The following example shows a useful pattern where some behavior is triggered if new messages are available. MessageMutator::clear() is used so the same messages don’t re-trigger the behavior the next time the system runs.

#[derive(Message)]
struct Collision;

fn play_collision_sound(mut messages: MessageMutator<Collision>) {
    if !messages.is_empty() {
        messages.clear();
        // Play a sound
    }
}

pub fn clear(&mut self)

Consumes all available messages.

This means these messages will not appear in calls to MessageMutator::read() or MessageMutator::read_with_id() and MessageMutator::is_empty() will return true.

For usage, see MessageMutator::is_empty().

Trait Implementations

impl<'w, 's, E: Debug + Message> Debug for MessageMutator<'w, 's, E>

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

Formats the value using the given formatter.

impl<E: Message> SystemParam for MessageMutator<'_, '_, E>

type State = FetchState<E>

Used to store data which persists across invocations of a system.

type Item<'w, 's> = MessageMutator<'w, 's, E>

The item type returned when constructing this system param. The value of this associated type should be Self, instantiated with new lifetimes.

fn init_state(world: &mut World) -> Self::State

Creates a new instance of this param’s State.

fn init_access( state: &Self::State, system_meta: &mut SystemMeta, component_access_set: &mut FilteredAccessSet, world: &mut World, )

Registers any World access used by this SystemParam

fn apply(state: &mut Self::State, system_meta: &SystemMeta, world: &mut World)

Applies any deferred mutations stored in this SystemParam’s state. This is used to apply Commands during ApplyDeferred.

fn queue( state: &mut Self::State, system_meta: &SystemMeta, world: DeferredWorld<'_>, )

Queues any deferred mutations to be applied at the next ApplyDeferred.

unsafe fn validate_param<'w, 's>( state: &'s mut Self::State, _system_meta: &SystemMeta, _world: UnsafeWorldCell<'w>, ) -> Result<(), SystemParamValidationError>

Validates that the param can be acquired by the get_param.

unsafe fn get_param<'w, 's>( state: &'s mut Self::State, system_meta: &SystemMeta, world: UnsafeWorldCell<'w>, change_tick: Tick, ) -> Self::Item<'w, 's>

Creates a parameter to be passed into a SystemParamFunction.

impl<'w, 's, E: Message> ReadOnlySystemParam for MessageMutator<'w, 's, E>

where Local<'s, MessageCursor<E>>: ReadOnlySystemParam, ResMut<'w, Messages<E>>: ReadOnlySystemParam,